HPM7177 ADC from CERN - Page 1
Forum section: electronics / general

author: jaromir
author_rank: Unknown
timestamp: February 05, 2020, 09:10:53 pm
content: Not sure if this was discussed before, but CERN released some fresh details of one of their projects - HPM7177 ADC, including preliminary testing and paper comparing commercially available on-chip ADCs. As name suggests, this one is built around AD7177. I can see some unclear points there, but overall it's nice design with a few details to take inspiration from.https://www.ohwr.org/project/opt-adc-10k-32b-1cha/wikis/home

author: niner_007
author_rank: Frequent Contributor
timestamp: February 06, 2020, 09:19:44 am
content: Correct me if I’m wrong, but this is not really an ADC, but more like a DAQ system, they are using an off the shelf ADC

author: guenthert
author_rank: Frequent Contributor
timestamp: February 06, 2020, 05:28:02 pm
content: Perhaps noteworthy is that their claimed INL error specs are lower than the published one of the ADC. Did CERN find that AD is overly conservative, are those selected parts or do they some post-acquisition data transformation?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2020, 05:50:51 pm
content: The actual INL can be effected by the input and reference buffering. They may have improved there over the AD reference design used for the DS.Another point is using a nominal range that does not extend all the way to the extremes. Much of the INL looks like to be similar to an U³ part and there it helps to reduce the range a little and get some extra over-range where the INL is larger.Driving the differential signal to the ADC is a main part of the circuit. Some of the INL error seen by AD could also come from this part and not just the ADC itself. The extra scaling (e.g. 7V from LTZ to some 5 V ref for the ADC chip and +-10/13 V input signal to a +-5 V range differential signal is quite some effort with likely custom resistor arrays.It looks like they tested only few samples - they could have been lucky.There can also be some selection check, to through out bad units.

author: jaromir
author_rank: Unknown
timestamp: February 06, 2020, 06:47:38 pm
content: Quote from: guenthert on February 06, 2020, 05:28:02 pmThey are selecting both ADCs and LTZ references (hundreds of pieces of HPM7177 to be built) to achieve needed parameters.

author: DaJMasta
author_rank: Super Contributor
timestamp: February 06, 2020, 07:02:03 pm
content: Quote from: Kleinstein on February 06, 2020, 05:50:51 pmBut every device operating in a class near than this is hand-selected (or at least, machine selected based on testing) based off performance, right? That seems like a given to me.23 effective bits at 10kS/s is really impressive. Even if it's a complete module/system with some arbitrary pricetag, getting something like that outside of cryo conditions sounds like we could have some neat additions to the next generation of equipment's highest end.

author: Castorp
author_rank: Regular Contributor
timestamp: February 06, 2020, 08:58:21 pm
content: Hi guys,I'm newly registered here, but I've been reading posts for a long time. Now there's finally a good excuse to dive in - I'm the developer of HPM7177We do plan to select LTZ1000s for low 1/f noise. We also do burn-in to accelerate their ageing. That was already done for LTZs used in the older digitizer, which is a 3rd order, single-bit Sigma-Delta built of op amps and discrete parts. These devices have been in use in the LHC for over a decade already.About the INL - Kleinstein is right. It's better than the AD7177-2 datasheet spec because we use part of the range. The driving of the inputs and the Vref pins is also quite critical. It seems that half of the INL curve is very repeatable - it looks the same for those 4 tested "full prototypes", but I've measured something similar on earlier test boards. Luckily, for our most demanding application we only need unipolar range. The digitizer is still bipolar, because it will be used also in another (lower) accuracy class for magnets powered with bipolar current.So far the plan is not to select ADC chips for good INL. Noise seems to be very much repeatable (both white and 1/f at zero). The temperature controller brings T drift down to pretty low levels. And in any case, the devices in the highest accuracy class are placed in temperature-controlled racks, so normally they shouldn't see more than 0.5 degree delta T.I'm planning to add a lot more information in the OHWR wiki page. Also, the design documentation will be updated. There are a few small things to polish up here and there.I'll be happy to answer all sorts of questions. Criticism is also welcomeCheers,Nikolai

author: jaromir
author_rank: Unknown
timestamp: February 06, 2020, 09:20:41 pm
content: Oh, it's great to see you here.Quote from: Castorp on February 06, 2020, 08:58:21 pmThis is interesting - how is the burn-in procedure performed?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 06, 2020, 09:25:22 pm
content: The noise performance is impressive. If my calculation is right the SNR of the ADC alone is about a 6-9 dB better than the 3458. With the extra noise from scaling the module noise is still slightly better. However there are also limitations: there is some 1/f noise with a cross over at around 0.01 Hz and there may be variations in the gain factor, not only from temperature, but also from random effects (maybe EMI). The other point is that the INL is good but still possibly a weak point, especially for slower conversions.With the classical multi-slope ADC in the 3458 one has to expect higher INL errors for faster conversions - I would expect more like ~ 10 ppm INL for the 3458 at 10 kSPS. The fast mode does not look like high linearity, but even just going from 10 PLC to 1 PLC amplifies some INL contributions.For the performance they also need critical resistor array(s) - possibly the most expensive part of the module.

author: TiN
author_rank: Super Contributor
timestamp: February 07, 2020, 12:09:26 am
content: CastorpWelcome to the forum. It's great to see actual practical high-performance ADC and LTZ1000 reference use. Many of us here playing with LTZ-based refs just for hobby purposes only. I looked at HPM7177 back when it was just few documents and preliminary initial test module results and even picked few pieces from that design into my own experiment but with ADS1262. Also would be cool to hear about bias currents from such a design, if that was validated and tested in detail as well?One of the questions - Looked on schematics again, but couldn't find the part that used to drive TEC. It would be particulary interesting to hear about temperature stability validation and if there were any issues with induced noise/EMI from running high-power TEC element next to highly sensitive analog circuit.I have developed few small benchtopTEC setupsusing off the shelf PID controller (Keithley 2510 or ILX 5910B) to evaluate temperature stability of the various LTZ1000 designs, precision resistors. But never tried to use TEC as onboard thermostat yet, however it would be a good idea for those looking at best possible stability without spending hundeds of hours to select references, chips and resistors for lowest TC.Also do you get 20-30 week leadtimes for those beautiful custom CerDIP VPG networks, or sourcing BMF resistors is actually better for CERN than individuals like myself? Over last 9 years I had placed few orders (50-100pcs each) for VPG resistors and everytime customer support was rock bottom bad, even without considering money paid. If you can't comment on this, I'd understand, just curious to hear the difference.

author: Berni
author_rank: Super Contributor
timestamp: February 07, 2020, 06:29:06 am
content: Oh neat, didn't know you can just download the entire Altium Designer project and everythyng.But yeah its just a LTZ1000 reference an AD7177 ADC and some analog front end stuff put together. Still interesting to look at tho.

author: Castorp
author_rank: Regular Contributor
timestamp: February 07, 2020, 10:58:22 am
content: I tested some of the Linear SAR ADCs, and they all had considerably higher 1/f noise than AD7177-2:Those are measurements taken under completely identical conditions: shorted inputs, stable temperature, Vref from a 10 V standard.I didn't test the LTC2508-32, because I thought it was just like the 24-bit one, but with built-in digital filters. Actually, we use the LTC2378-20 in another digitizer. We do the filtering and decimation externally in an FPGA. The digitizer itself is not a standalone unit like the HPM7177. It's a mezzanine board that plugs into an over-crowded controller. It's very widely used for lower accuracy applications.HPM7177 is definitely less noisy than the 3458A, but I don't have the exact numbers at the moment.INL is not our toughest spec, because in the end we don't really deal with dynamic signals. For us it's always about measuring a slowish ramp, and then some DC level which may or may not be around the full scale. In a way, absolute accuracy is also not very important, as the machine operators fine-tune each powering circuit based on the feedback they get from beam measurements. We care mostly about the short- and mid-term stability of our measurement.EMI is a big concern here, that's why we take good care. For instance, for some 2 kA corrector magnet supplies, the digitizers would be sitting in the power converter rack. That means kiloamps switching not so far, plus big fans and other unpleasant neighbours. I should soon book some time for EMC-testing the HPM7177 together with another system. We have a good EMC lab here.The resistor networks are not super special, compared to the other stuff we buy from VishayI checked - last time I paid <$100 per unit for 25 of them. There are 2 arrays on each ADC mezzanine board, so that's probably about 1/5 to 1/4 of the material cost for the entire digitizer.The driver for the TEC is not in this unit. It will be in the power supply. I decided to keep them separate - first of all, to keep some of the heat and EMI away, and to make this whole thing easier to debug/manufacture/maintain. Our old digitizer is an "all in one" box and it's just pain.For now I don't have a release version of the PSU, but I have a working prototype. I'm waiting for an ambient temperature spec from the power converter colleagues before I settle on the final design. What I'm using now is a simple linear voltage-current converter. The setting voltage comes from the HPM7177. It's just filtered PWM from the FPGA. What comes back to the TEC is bipolar current, 1A/1V.About the burn-in - we do it for 3-4 weeks by stepping the temperature between 80 and 120 degrees C. It's done in a very simple way - the units sit in an oven at 80 degrees, and we switch on and off their heaters to add those 40 degrees. The cycle lasts a few hours, if I remember correctly. The recipe comes from John Pickering (Metron Designs, ex-Datron). Ideally it should be done like this:https://patents.google.com/patent/US5369245A/enThe voltage reference sub-circuit of HPM7177 (excluding the division down to 5 V) also comes from John. It's basically the same circuit that was used in the old digitizer.

author: TiN
author_rank: Super Contributor
timestamp: February 07, 2020, 02:27:14 pm
content: Quote from: CastorpDo you have control unit reference without burn-in, to check how does this thermal cycling procedure actually affects reference stability?

author: dietert1
author_rank: Super Contributor
timestamp: February 07, 2020, 04:47:04 pm
content: Temperature cycling as proposed by Pickering lasts about two hours. At least the text mentions 15 * 500 = 7500 seconds. The method mentioned above (1 month or so) appears to be something else.Regards, DieterPS: As far as i understand, Mr. Pickering wanted the circuit implemented into the reference board, in order to execute the two hour initialization each time the reference gets turned on.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: February 07, 2020, 07:11:13 pm
content: 3roomlab:QuoteYou can't average away 1/f noise! It's not gaussian, unlike white noise.https://www.eevblog.com/forum/metrology/should-the-number-of-measurements-be-included-in-the-measurement-uncertainty/msg2868714/#msg2868714"Low Noise Electronic Design" by Motchenbacher & Connolly:"A fact to remember concerning a 1/f noise-limited dc amplifier is thatmeasurement accuracy cannot be improved by increasing the length of themeasuring time. In contrast, when measuring white noise, the accuracyincreases as the square root of the measuring time."You can find other sources as well, e.g. "Signal Recovery from Noise" by Wilmshurst.

author: Andreas
author_rank: Super Contributor
timestamp: February 07, 2020, 07:52:38 pm
content: Quote from: Castorp on February 07, 2020, 10:58:22 amHello,So I am wondering why you dont use a capacitor between base and emitter of temperature sensing transistor of the LTZ like in the datron design.See also what I call C11 in my schematic:https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/?action=dlattach;attach=886392After my measurements this is the most sensitive pin in the whole cirquit.After I have recognized that the metal housing of the LTZ is connected to the substrate of the chip, but to no other pin directly, I am thinking about wether a additional metal shield around the LTZ can also bring some improvement.with best regardsAndreas

author: Castorp
author_rank: Regular Contributor
timestamp: February 07, 2020, 08:29:10 pm
content: I haven't tried measuring long-term drift with and without burn-in. Sounds like a very tedious thing to doAs far as I know, both methods come from John Pickering. The one with decreasing amplitude seems like a better way to get rid of thermal hysteresis effects mediated by mechanical stress in the package and the leads. Still, the main goal is to relieve the tension in the epoxy that fixes the die to the package. I really don't know whether it's better to do it quickly or slowly.1/f noise is Gaussian, in the sense that it has Gaussian PDF. However, you can't decrease its variance by averaging when you're dealing with a fixed number of samples. Let's say you have 1000 samples taken at 100 SPS, completely in the 1/f region. That means you're looking at 3 frequency decades of 1/f noise. Now, if you downsample by factor of 10, you have 10 SPS. You have narrowed your BW by a decade - you now have 100 samples and you're looking at 2 decades of 1/f noise. Your variance will be sqrt(3/2) lower because you've thrown away one decade. However, if you take a 10 times longer record, you push one decade down in frequency, you get your 1000 samples again (now at 10 SPS), and you're back at your initial variance. There's equal noise power per log(f), no matter if that's a decade or octave or another base. It's easy to see this in an Allan variance plot, where 1/f is a flat line.From top of my head, I measured the SAR ADCs sampling at 1 MSPS. When it comes to broadband white noise, their behaviour is pretty straightforward. There's a fixed rms noise value, so you get the lowest density when you run at the highest sampling rate. In other words, you spread this noise over a wider bandwidth. I guess that's the kT/C noise of the sampling capacitor spread over a very wide bandwidth, which aliases multiple times in the Nyquist band. I just checked the datasheet of LTC2378-20 - it states a -3 dB analog bandwidth of 34 MHz. That's way over the maximum sampling rate.The behaviour of the Sigma-Delta ADCs is pretty different. There you typically don't have much control over the operation of the modulator. The digital filter is there mostly to remove the high out-of-band quantization noise. In between the quantization noise at high frequencies (which rises at N*20 dB/dec, where N is the modulator order) and the 1/f noise at low frequencies (rising at 10 dB/dec), there's a mid-band where the noise floor is flat. That's where you'll see "best noise performance", if you look in terms of noise density. But of course, that's not the full story. If you extrapolate from there, you're likely to get overly optimistic LF noise estimates.I encountered the many problems of trying to pull this information from datasheets when I produced Fig. 2 in the 2018 I2MTC paper:https://www.researchgate.net/publication/325285614_Analog-to-digital_conversion_beyond_20_bits_Applications_architectures_state_of_the_art_limitations_and_future_prospectsEdit: I corrected the sqrt(3). If you go from 3 decades to 2, it's sqrt(3/2). The sqrt(3) improvement would be if you throw away 2 decades and you remain with 1. In the end, it doesn't matter where these decades are. It's the same rms noise from 0.1 to 1 Hz, then from 1 to 10 Hz, etc... as long as it's all in the 1/f region.

author: splin
author_rank: Frequent Contributor
timestamp: February 08, 2020, 02:56:42 am
content: Why aren't you using the AD7175-2?I'm pretty sure it's exactly the same chip apart from some pointless, but expensive tweeks to the digital filter to produce an 8 extra bits of additional random noise. The noise performance specs are identical for 10kSPS and below (and not surprisingly, not specified for the the 7177-2 for higher rates).

author: splin
author_rank: Frequent Contributor
timestamp: February 08, 2020, 03:07:09 am
content: Quote from: jaromir on February 06, 2020, 06:47:38 pmPerhaps you'd like to repeat after me: "I'm sorry but I was talking complete b***ocks. Please accept my sincere apologies for attempting to mislead readers here that I had any sort of inside imformation. I didn't, and was simply conjecturing without disclosing that fact. I apologise to anyone that I mislead as a result".

author: TiN
author_rank: Super Contributor
timestamp: February 08, 2020, 04:47:44 am
content: splinGood one.Also thermocycling receipe used by John in Datron/Wavetek and later renamed into Fluke reference was NOT few hours, but8.5 hours. But that's outside of the topic here, so let's keep it civil.

author: Castorp
author_rank: Regular Contributor
timestamp: February 08, 2020, 07:35:06 am
content: Quote from: 3roomlab on February 08, 2020, 01:28:29 amYeah, I think they are pretty similar in terms of white noise. But at 1 KSPS you wouldn't get almost any 1/f conribution. It really starts to play a role if you downsample a lot and you get to the sub-Hz frequencies. That's for the AD7177. For 2508 I expect it's somehow higher (but you still have to be way down in frequency).Quote from: splin on February 08, 2020, 02:56:42 amYes, I agree - it's the same chip with minor differences in the digital filters. Indeed, at 10 KSPS those extra 8 bits are pure noise. Even a few of the higher bits are noise. I vaguely remember that 3 years ago I couldn't obtain 7175 for some reason. In any case, for our purposes 10 KSPS is more than enough.By the way, AD7176-2 has the same Sigma-Delta modulator, but a different input section with much worse INL and much higher 1/f noise.

author: iMo
author_rank: Super Contributor
timestamp: February 08, 2020, 08:09:47 am
content: Happy to see that building an ADC with "comparable parameters" to 3458A with SD (or SAR) chips is doable. The usage of monolithic ADCs has been discussed here many times, except TiN's measurements with ADS1263 this is, imho, the first practical example it is doable..

author: Castorp
author_rank: Regular Contributor
timestamp: February 08, 2020, 08:34:21 am
content: The INL is not as good of course, but as I mentioned earlier - luckily it's not the biggest concern for our application.Yesterday someone asked me about the Vishay arrays and I checked my last order. Actually, the lead time was not so bad - it was 10 weeks. The 6-month lead time was for another item, a VHD200 divider. We use those in the older digitizer. We buy them as tempco-matched pairs, so they also cost quite a bit of extra $$$.

author: ScoobyDoo
author_rank: Regular Contributor
timestamp: February 08, 2020, 09:51:27 am
content: Hello Castorp - I have a question (concern) about these Vishay array networks ... - there are certain advantages that speak in favor (CDIP - hermetically sealed) of these 144X series, however these consist ofindividual resistorelements - it is not a single chip array. So did you check Zero TC ratio matching is as good (or better) than the thin film arrays as used in Datron series ? In order to create a stable ratio divider is a thin film array not better than multi-chip foil technology ? I hope Vishay uses dry and Nitrogen fill before sealing - something to think about ...Herzliche Grüße/Meilleures salutations/Best regardsScoobyDoo

author: Castorp
author_rank: Regular Contributor
timestamp: February 08, 2020, 10:54:57 am
content: Quote from: Andreas on February 07, 2020, 07:52:38 pmThanks for the tip, Andreas! I must admit - I haven't considered that issue. When I was talking about EMI and being super careful about it, I mostly meant how we deal with it on the system level. All this stuff with optic fibres, floating GND, transmitting the voltages differentially (even though they are always GND-referenced at the source), the multiple shields, etc.Quote from: ScoobyDoo on February 08, 2020, 09:51:27 amScoobyDoo - you're right, the chips inside are separate. The bulk metal foil types are either V5X5 or V15X5. Perhaps I should finally Dremel into a broken array to see which type I've got. Anyway - so far they're all within the TCR tracking spec, which is 1 ppm/K for all elements. It should be possible to do significantly better using TaN, but for us 1 ppm is good enough since we apply brute force (temperature regulation). We need the temperature regulation in any case, since the ADC gain drift is never going to be good enough.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2020, 11:07:01 am
content: Thin film resistors can be noisy. So even if they are low TC the noise can be a problem. The TDP1603 series only specifies <-30 dB of excess noise. This would be too much for a low noise voltmeter. There are a few thin film resistor arrays with lower noise specs, but not that many. Going by the data sheets the LT5400 arrays may be an option in a few cases, if suitable values are available.Even if the resistors inside the BMF arrays are individual substrates, they are very likely from the same batch and thus still very similar. The thermal coupling is not as good, but this is compensated by a much better absolute TC.With temperature control the TC is not that important any more. Though it could still effect the linearity by local self-heating.

author: jaromir
author_rank: Unknown
timestamp: February 08, 2020, 11:29:06 am
content: Quote from: splin on February 08, 2020, 03:07:09 amI do have inside info - and this topic started as a result; but apparently it was partially wrong, I apologize to anyone offended. LTZs are selected, indeed. ADCs are not. Errare humanum est.Fortunately Castorp explained in more detail what they are doing and I'm grateful for that. Instead of personal attacks I'd prefer keeping it civil and perhaps focus on a topic.

author: jaromir
author_rank: Unknown
timestamp: February 08, 2020, 11:32:15 am
content: Castorp: I'm intrigued about opamps in analog frontend. Did you try any other comparable autozero opamps? What about AZ switching noise? It can be problematic, depending on input impedance. Perhaps classic (as not AZ) opamps would be OK since the whole unit is temperature regulated?I'm sure your choice was optimal for this project, but I'm curious about thinking and reasons behind it.

author: Theboel
author_rank: Frequent Contributor
timestamp: February 08, 2020, 12:59:32 pm
content: Hi all,Sorry If my question not appropriate in this thread.Castorp can you described a little about Your temp management in rack and in field.is that also CERN design etc etc

author: Andreas
author_rank: Super Contributor
timestamp: February 08, 2020, 03:05:57 pm
content: Another question to the cirquit:Do you really use a MMBFJ202 for T2?According to the data sheet it will deliver between 0.9 and 4.5 mA at zero Gate voltage which is too low for the 4-5 mA Zener + the voltage dividers (temperature setpoint + 5V divider).I am asking because I used the BF245C/BF545C up to now. (12-25 mA zero gate voltage current).But I have recognized that it is obsolete now and I am looking for a replacement.With best regardsAndreas

author: edavid
author_rank: Super Contributor
timestamp: February 08, 2020, 04:40:15 pm
content: Quote from: Andreas on February 08, 2020, 03:05:57 pmMMBF4392 and MMBF4393 are still in production.

author: Castorp
author_rank: Regular Contributor
timestamp: February 08, 2020, 04:43:28 pm
content: Quote from: Kleinstein on February 08, 2020, 11:07:01 amIndeed, NI of -30 dB would be catastrophic. Once I tried to measure the excess noise of Vishay bulk metal foil resistors and concluded that it was below the detection capabilities of my setup. That was done with a Wheatsone bridge biased with 40 V.There is a bit of self-heating effect in the array. I see it as a slight thermal tail when I step the input voltage from 0 to 10 V. It's on the order of 0.1 ppm with time constant in the tens of seconds. I know it's thermal, because I can see the reaction of the temperature controller correlated with it. My conclusion was that it's a small effect that could be ignored.Quote from: jaromir on February 08, 2020, 11:32:15 amInitially I used ADA4522-1. So far I haven't seen any hint of switching noise with those modern auto-zero amps. It could be due to the fact that we always have low source impedance. Or it's just way out of the signal band, or both.I wanted something with lots of open-loop gain and high CMRR, so that it makes a good buffer where I don't have to worry about linearity. Also, I checked that they don't exhibit thermal tails. That's something I discovered with another design, where the buffers were built of AD8512. The tail was on the order of 1-2 ppm, time const. 20-30s, for a 10 V step.Quote from: 3roomlab on February 08, 2020, 12:49:40 pmI'll double-check. I did those measurements 2 years ago, which almost feels like a previous life... because I have a small kidQuote from: Theboel on February 08, 2020, 12:59:32 pmIt's a totally appropriate question.Those cabinets are not CERN design - they are commercial. They keep the temperature at 23 degrees C. The temperature down in the LHC tunnel doesn't vary with time of day or seasons, but there's a lot of equipment that dissipates lots of power, so locally it could vary by 5 degrees or more. It's correlated with the machine cycle - the power converters output max current when the beams reach their nominal energy.Those LTZ1000s certainly have a good life thereFirst of all, they are powered all the time. And then, there's the on-chip heater and its loop, the Peltier-stabilized module, and then all that's within the large cabinets. All in all, 3 nested control loops.Quote from: Andreas on February 08, 2020, 03:05:57 pmYes, that's what we use. I guess it operates with a bit of -Vgs to source those milliamps - maybe a volt or so.Unfortunately, lots of discrete JFETs are becoming obsolete these days...

author: Kleinstein
author_rank: Super Contributor
timestamp: February 08, 2020, 07:53:30 pm
content: With a reasonable low impedance source and a slow circuit the AZ OPs are probably OK. I can imagine that it could still help to have some filtering at the input, like pi type low pass with something like 100pF , 1 K (and/or ferrite), 100pF. Some of the AZ OPs may react to high frequency components emitted from the OP that come back from a not properly terminated line.However there could be a slight problem if used not as a single OP, but in combination with a very fast OP, like the ADA4522 + AD8065 combination for the reference drivers. The fast OP may interact with the fast spikes from the AZ OP.

author: edavid
author_rank: Super Contributor
timestamp: February 08, 2020, 08:05:36 pm
content: Quote from: Castorp on February 08, 2020, 04:43:28 pm-VGSwould reduce the drain current below IDSS, which is already too low

author: Andreas
author_rank: Super Contributor
timestamp: February 08, 2020, 09:04:43 pm
content: Quote from: edavid on February 08, 2020, 08:05:36 pmHello,Yes that is what I fear. In this case the gate source diode will be forward biased to get enough current through the zener.Quote from: edavid on February 08, 2020, 04:40:15 pmThanks for that.I think the MMBF4392 will be a good option for my LTZ1000 circuit.Unfortunately I have a 2nd use of the BF545C/BF245C as 3-5V up to 3 mA voltage regulator with zero idle current:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/?action=dlattach;attach=402884Here the MMBF does not fit with the pinch off voltage.In the mean time I have found the MMBFJ310 which seems to be suitable for both applications.with best regardsAndreas

author: Castorp
author_rank: Regular Contributor
timestamp: February 09, 2020, 07:03:24 am
content: Gentlemen, you're right. There's something fishy here. I'll investigate, as soon as I get back to the lab. Thanks for spotting it, and thanks for the suggested replacements!I believe this transistor is already a BOM change for an obsolete part. However, I don't know when it was done.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 09, 2020, 01:09:39 pm
content: Quote from: imo on February 08, 2020, 08:09:47 amTell me in what topic was it?

author: jaromir
author_rank: Unknown
timestamp: February 09, 2020, 03:49:45 pm
content: Quote from: MegaVolt on February 09, 2020, 01:09:39 pmhttps://xdevs.com/review/ti_ads1262_p1/

author: Castorp
author_rank: Regular Contributor
timestamp: February 09, 2020, 04:46:40 pm
content: Quote from: Kleinstein on February 08, 2020, 07:53:30 pmSo far I haven't seen evidence of that. Actually, that composite amplifier configuration was suggested to me by engineers from Analog Devices. The fast amp was originally something else that was hard to get.Anyway - I've probed these circuits a lot with various wideband active probes. I've also looked with a 10 MHz FFT analyser (89441A). I didn't see anything funny going on. Then again - it could have been due to circuit conditions (source/load impedances, etc.)...@3roomlab - I know what you mean. I also got cross-eyed with those datasheets. And that was not even done for fun, it was workAt some point I also considered testing ADS1262, but right now I can't remember why it didn't make the short list. The information is probably buried somewhere in my lab notebooks. Too bad I don't have the time or patience to document all these things better.By the way, we use ADS1281 in yet another system - a radiation-tolerant controller. They actually tested the part and qualified it as rad-tolerant:https://www.researchgate.net/publication/291205919_Development_of_radiation_tolerant_components_for_the_Quench_Protection_System_at_CERNWe read the bare modulator bitstream, which is available on some pins, and then do the filtering externally. The older Sigma-Delta digitizer I've referred to several times in this thread also outputs bare, undecimated bitstream, which goes over an optic fibre to the controller where the FPGA does the filtering and decimation.

author: branadic
author_rank: Super Contributor
timestamp: February 09, 2020, 06:27:03 pm
content: QuoteInteresting, ADS1256 was tested for radiation hardness before and succeeded as well. TI seems to have a decent fabrication process.-branadic-

author: iMo
author_rank: Super Contributor
timestamp: February 09, 2020, 06:48:28 pm
content: I've never been in CERN but spent several hours walking inside DESY tunnel touching the great stuff there (the beam was off).While asking them on issues with early experiments I was told by the guys there the synchrotron radiation was so strong the optical fibres (running nearby the tube) got grey cataract, thus they had to replace them by copper. Even radhard, the location of the boxes inside the machine is therefore critical, I guess

author: Castorp
author_rank: Regular Contributor
timestamp: February 10, 2020, 03:38:32 pm
content: Quote from: 3roomlab on February 08, 2020, 12:49:40 pmI found my old notes. Indeed, LTC2380-24 was running at 500 kSamples/s. I made a brief note about some timing issue in the digital interface, but I'm not sure if that's the explanation for the sampling rate.So, at 500 kS/s, the RMS noise was 32 uV. 500 kS/s means 250 KHz Nyquist band, so the noise density is 64 nV/sqrtHz. When I compared it with the other tested ADCs, I multiplied it by 1.25 to account for the lower full-scale range (I used the "digital gain compression" feature). That leads to the 80 nV/sqrtHz visible on the plot.Now, I knew at this point that operating at higher sampling rate would give better white noise floor. In theory it can go up to 2 MSa/s, so twice lower white noise floor. However, I noted that operating at lower sampling rates (250 kS/s and 125 kS/s) yielded the same 1/f noise. And since it was 3-4 times higher than in AD7177-2, I decided not to do any further tests and abandon the part.Quote from: branadic on February 09, 2020, 06:27:03 pmYes, there are possibly many ICs that fall in the gray area of "radiation tolerance". That's good enough for many of our purposes. Rad-hard parts are a different story - they are specially packaged and much more expensive.Quote from: imo on February 09, 2020, 06:48:28 pmFor us synchrotron radiation is not the issue, it's losses of high-energy protons that hit the walls and activate all sorts of materials. The LHC is pretty good in this sense - there aren't so many losses. So these rad-tolerant controllers can be placed very close to the small corrector magnets, right under the beamline. They do experience single-event upsets sometimes, but it's tolerable.There are very radioactive regions in the smaller accelerators, for instance in the PS (Proton Synchrotron) and PS-Booster. Imagine dose rates of 1 Gy/minute. A person standing there would be dead in 5 minutes. It's no place for electronic devices either.

author: Castorp
author_rank: Regular Contributor
timestamp: February 11, 2020, 09:24:06 am
content: 3roomlab - just to double-check - by FS you mean the unipolar full scale? The ADC AIN tab in your table?In principle, you should get the same noise floor estimate regardless of the digital filter type. That is - if you know precisely the "effective bandwidth" of the filter.In my tests, the VREF was definitely not noisy. It came from a 10 V travelling standard, divided down to 5 V using a 10K SMN array. Then buffered with a decent low-noise op amp. Plus - those measurements were done at zero differential voltage, with physically shorted inputs, biased at common-mode voltage of 2.5 V (again derived from the clean 10 V). Supply voltages, etc., were also good.…Andreas - you were right. I probed the voltages around T2 - the junction is forward-biased. Thanks again for spotting this!

author: splin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 09:39:27 pm
content: Quote from: jaromir on February 08, 2020, 11:29:06 amSorry if I was a bit harsh, given your statement was actually in good faith. It is extremely frustrating and annoying though to take on board someone's confidently stated information only to later find out it was wrong. Life's too short to fact check every interesting data point found on the net.

author: splin
author_rank: Frequent Contributor
timestamp: February 11, 2020, 09:49:35 pm
content: Quote from: Andreas on February 08, 2020, 09:04:43 pmI don't understand - the 4392 has a max cutoff of -5V, the J310 is -6.5V. The 4393 is only -3V but IDSS min is only 5mA. Should be fine for most LTZ1000 applications. What am I missing?

author: Andreas
author_rank: Super Contributor
timestamp: February 11, 2020, 10:09:39 pm
content: Quote from: splin on February 11, 2020, 09:49:35 pmFor the LTZ1000 all is ok.But I am using the BF545C often as cheap 4V (+/-1V) voltage regulator with zero quiescent current. Here I need a corresponding (higher) pinch off voltage. See picture on the left side (T1). (A alternative would be a XC6216 but with 3-4uA quiescent current which is rather high against 10uA average consumption of the processor)Sorry had initially linked the wrong schematics:https://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/?action=dlattach;attach=402886;imagehttps://www.eevblog.com/forum/metrology/ad587lw-10v-precision-travel-standard/msg1449488/#msg1449488with best regardsAndreas

author: Castorp
author_rank: Regular Contributor
timestamp: February 12, 2020, 01:01:03 pm
content: I think something is still off.Do you have a column somewhere with the noise bandwidth? For a given filter type it's a fixed fraction of the ODR.Assuming white noise, the density is: rms noise/sqrt(bandwidth) [V/sqrtHz]. Or (rms noise)^2/bandwidth [V^2/Hz]. That's one step away from having a scaling relative to the FS.About the dynamic range - is it for a full-scale sine wave? That's what they usually give in datasheets. It's the theoretical maximum SNR.

author: splin
author_rank: Frequent Contributor
timestamp: February 12, 2020, 03:43:32 pm
content: 3roomlab, you've got a few errors in your table. The first column is the input referred noise level which is generally independent of reference voltage and input voltage range. Yet for the LTC2508-32 your numbers are all half of the datasheet values. I guess you have adjusted the values for some reason but you shouldn't be doing it here. Similarly, the LTC2380-24 also look wrong, but I can't see where the numbers come from just now. The 1.5MSPS value however should be twice your value, based on the transition noise spec of 55.7LSBs, rms. 33uVrms = 55.7/2^24 * 10Vpp input range.The second major problem, as hinted at by Castorp, is that you are not comparing like for like - you have picked the lowest noise value for a given ODR (where there is a choice of filter, such as the 2508-32). But ODR by itself doesn't mean anything - you can take any ADC output and increase the sampling rate through interpolation but it won't improve it's noise performance.To compare ADCs fairly, you have at least two options: normalise to the fully settled sampling rate, or to the input bandwidth (but the difference will be small). The AD7175 and AD7177-2 output samples are fully settled at or below 10KSPS for the Sinec1 + 5 filter whereas the ADS127 has a fully settled output data rate of 1/2 to 1/5 the ODR for the low latency filter, depending on the OSR.For the LTC2500-32 the averaging filter, unlike the SSINC filter values you selected, are all fully settled at the ODR so can be compared directly to the AD7177-2 numbers. The tables also show the filter bandwidths so you can normalise the noise to bandwidth easily.

author: Castorp
author_rank: Regular Contributor
timestamp: February 12, 2020, 04:27:28 pm
content: Quote from: splin on February 12, 2020, 03:43:32 pmThat's pretty close to what I measured.I remembered another detail. My setup for LTC2380-24 and for LTC2378-20 was completely identical, down to the test PCB. These two parts are pin-compatible, so it was not even a different executions of the board - it was the same board with different ADC chips populated. The rest was just a tweak in the FPGA that read out the digital interface to get 24 bits per conversion instead of 20. I did conclude that the 24-chip is better in terms of 1/f noise, but not that much better. And it chilled my enthusiasm for the "32-bit" SAR.splin, I agree with the rest of your explanations and reasoning.

author: splin
author_rank: Frequent Contributor
timestamp: February 12, 2020, 05:38:50 pm
content: Quote from: Castorp on February 12, 2020, 04:27:28 pmYes, I was going to point out that Linear sell loads of variants of just a few core ADCs but with varying extras, principally built in filtering, for higher prices. But I got bored editing on a tablet.The LTC2500-32 seems to be an LTC2378-20 at heart - identical noise and linearity specs. I can't imagine how/if they managed to get any worthwhile sales volumes of the expensive variants given most designs are going to include an MCU capable of all the filtering required with much more flexibility (or at most with the addition of a $.50 MCU). The 2500-32 for example, unlike the 2378-20, doesn't support 1MSPS which can be very useful.I can possibly see some niche markets, where the product developers want to sell on the basis of a state of the art, high-performance 32 bit ADC, but the price differential would, I expect, exclude most volume applications.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2020, 06:04:10 pm
content: Price wise there is not much difference between the LTC2500-32 and the LTC2378-20. With the filtering done in the ADC chip one can get away with a slower interface. This can simplify things quite a bit and lower EMI issues.The INL specs are quite good compared to most of the SD ADCs.The nice point with a fast SAR ADC is that one knows where the difficult points are and one can thus do an relatively easy test on each sample.Another point with SD ADCs is that the noise level can be higher at some evil levels: SD ADC tend to be good on average or most of the time, but can have trouble with idle tones at a few points. Reducing those idle tones is where the art starts.

author: Castorp
author_rank: Regular Contributor
timestamp: February 12, 2020, 07:36:07 pm
content: Quote from: Kleinstein on February 12, 2020, 06:04:10 pmYes, it's all very true.I'd say both types have advantages and disadvantages, but the lines are getting blurry. There was an old wisdom that 1-bit Sigma-Delta had to be inherently more linear, because the 1-bit DAC inside is linear by definition. Well, that's no longer true. These SARs have ppm and sub-ppm linear multi-bit DACs inside. Also, both types are realized using switched capacitors, so there are dynamic charge effects at the inputs and Vref pins (unless there are built-in buffers).As for the idle tones - I really don't know how, but some designers have found a way to get rid of them in single-bit SD, or at least to suppress them way below the noise floor. I haven't noticed any of them in AD7177-2 (despite looking very carefully). In ADS1281 (4th order single-bit SD) they are present. I think they are even mentioned in the datasheet, in a rare display of honesty. In the old CERN Sigma-Delta they were known and famous. Every once in a while they re-appeared in different forms. Various types of dithering helped, but never fully removed them. It was an endless whack-a-mole game

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 13, 2020, 10:07:06 am
content: Quote from: splin on February 12, 2020, 05:38:50 pmWhere did you get that? The data sheet says differently and my lt2500-32 did not note it,running all day long at 1 MSPS and 100 MHz SPI.Data sink is a Beaglebone Black, collecting up to 3 such SPI ports in real time througha PRU with some 32 bits -> 4 bytes conversion in the coolrunner2.Analog source is a chopper with pV/rtHz noise and GaN switches.Are there any 1/f data for LT2500-32?Cheers, Gerhard

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 13, 2020, 11:08:54 am
content: Quote from: Gerhard_dk4xp on February 13, 2020, 10:07:06 amCould you say a little more about this source? What elements are used in it?

author: splin
author_rank: Frequent Contributor
timestamp: February 13, 2020, 11:31:24 am
content: Quote from: Gerhard_dk4xp on February 13, 2020, 10:07:06 amMy mistake, the DS says minimum DF = 4 but I'd forgotten about the no-latency mode that outputs raw 20 bit ADC data - it's been several years since I last looked at it in any detail.QuoteCool. So why did you choose the LTC2500-32 over the LTC2378-20? It's quite a bit more expensive if you aren't going to use the digital filtering and has no other advantages that I can see, with the potential for additional internal noise if the DF can't be turned off completely (but you'd hope that it can be).QuoteThe datasheet almost certainly shows it in Table 2, best seen in the SSINC entries: noise drops linearly with SQRT(downsampling factor) until the filter bandwidth reaches 60.28Hz where increasing 1/f noise diminishes the noise reductions from further bandwidth reductions. Of course that could be down to factors other than 1/f noise but that seems to be much the most likely explanation.But since you have the setup anyway, haven't you measured the noise spectrum yourself with shorted inputs?

author: Castorp
author_rank: Regular Contributor
timestamp: February 13, 2020, 01:25:33 pm
content: Quote from: splin on February 13, 2020, 11:31:24 amI'd be interested in those results too.The 1/f corner must be somewhere in the low Hz. If you look how noise goes down from 30.86 Hz to 7.53 Hz bandwidth ( factor of 4) - it's almost factor of 2. It goes from 270 nV RMS to 150 nV RMS.Better yet - let's take factor of 16 difference. That's 460 nV RMS at 120.55 Hz bandwidth. Now, 460/4 = 115. We have 150 instead. It means if it's 1/f, it must contribute 96 nV RMS in the 7.5 Hz bandwidth (sqrt(96^2 + 115^2) = 150).Doesn't look stellar if those numbers are right.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 13, 2020, 01:27:22 pm
content: >> Analog source is a chopper with pV/rtHz noise and GaN switches.> Could you say a little more about this source? What elements are used in it?Something like this:https://www.digikey.de/product-detail/de/epc/EPC2038/917-1138-1-ND/5774048The +point is the small capacitance, leading to less charge injection than thoseanalog switches.The minus point: needs at least 3V Vgs to really switch on and just 6 V Vgs to really die.And soldering them onto the board was an adventure at 1 mm total size.> Cool. So why did you choose the LTC2500-32 over the LTC2378-20? It's quite a bit more expensive> if you aren't going to use the digital filtering and has no other advantages that I can see, with> the potential for additional internal noise if the DF can't be turned off completely (but you'd hope that it can be).you can get 24 Bit magnitude + 7 bit offset for every 1 MSPS clock on the SPIand on the second port the filtered and decimated data in 32 bit format when they are "ripe".At 1 MSPS, the first 660 nsec are for aquisition and communication should be avoided.The other 340 nsec are just enough to fetch the 32 bits at 100 MHz.I simply shoot them into a 32 bit shift register in the coolrunner and read that as 4 bytes into the BBB.> But since you have the setup anyway, haven't you measured the noise spectrum yourself with shorted inputs?I'm not yet there. This is a voyage through pV/rtHz analog electronics, VHDL in the Coolrunner,low level C programming in the BBB PRU processor, FFTW on the ARM and the GPIB/488-like server in Debian Linux,socket communication on the LAN, the app on the laptop under Linux Mint and Gnuplot for the result pics.I can control the ADC over the chain < laptop-app / LAN / ARM-server / PRU / ADC > and get time seriesof a few ksamples in the reverse direction, but the ping-pong buffer for large transfers works only for ping,not for pong :-). And sometimes I also have to do paid work.And second opinions are always interesting.Cheers, Gerhard

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 13, 2020, 01:34:12 pm
content: > The 1/f corner must be somewhere in the low Hz.For CMOS, that would be a reason for jubilation!(I'm used to MUCH worse from my Agilent 89441A)

author: Castorp
author_rank: Regular Contributor
timestamp: February 13, 2020, 01:44:14 pm
content: Quote from: Gerhard_dk4xp on February 13, 2020, 01:27:22 pmAh, ok, those pV/sqrtHz are input-referred. I wonder (for academic reasons) where's the 1/f corner, and whether the part works when cryocooled.CMOS and low 1/f are not mutually exclusive, if it's the design objective. Like in autozero op amps, or some of these new high-resolution ADCs. It's no longer about having super-duper input stage transistors, it's more about how you do the chopping, autozeroing, correlated double sampling, or whatever trick it is.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 13, 2020, 09:51:36 pm
content: Quote from: Gerhard_dk4xp on February 13, 2020, 01:27:22 pmAnd what was the source of the signal with an ultra-low noise level?

author: Castorp
author_rank: Regular Contributor
timestamp: February 14, 2020, 08:20:18 am
content: Gerhard - sorry, I didn't clarify that I was talking about the part you use for amplification, not the switches.Yesterday I gave a talk on the HPM7177. It's a bit more pedagogical and oriented towards CERN-specific matters, but it does contain a bit of new characterization data from recent measurements, so it may be interesting for some:https://edms.cern.ch/ui/file/2323037/1/tcc_1302_beev.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2020, 09:29:44 am
content: It is an interesting series of slides. It is a good point that the HPM7177 can be reference limited when measuring higher voltages.The simple picture is to separate the noise in an additive part from the ADC itself and a multiplicative part, from the references. However it is not for sure that the ADC itself may not also have multiplicative noise, or higher noise at higher voltage levels. The simplest example would be noise of the reference scaling and buffer, but there could also be additional parts inside the chip.I kind of miss a test if the ADC reading it's own reference (the 7 V from the LTZ) over a longer time. I would expect a little more than the noise near zero.The increase in the 1/f noise level from measuring a short to the 10 V sources (by more than a factor of 1.5) also suggests that the extra noise may be more than from the reference alone. With equal noise (both are LTZ1000 based) from the external and internal reference one would expect a factor of 1.4 .

author: Castorp
author_rank: Regular Contributor
timestamp: February 14, 2020, 09:53:15 am
content: Quote from: Kleinstein on February 14, 2020, 09:29:44 amYou are absolutely right. There is a bit of "gain noise" - either from the ADC itself, or from the Vref scaling. I have measurements of own Vref (5 V, 7.1 V and 10 V - which is just the 5V amplified by 2x). I did not include them in the characterization report or in the talk, because the information is way too specific for people who are not deeply into the inner workings of these devices. Once I have a bit more free time (not sure when), I'll process all this data and I'll put it up somewhere - probably in the OHWR site.In any case, this extra noise contribution is small, compared to the LTZ1000 1/f noise measured at 10 V.I also have estimates of the LTZ1000-based PBC (John Pickering's 10mA/10V unit that we use here) and the Fluke 732B, obtained by calculating the cross-PSD of two digitizers that sample simultaneously. Once again - it's part of the tons of information that I presently keep for my own use. Not because it's secret, but because it takes time and effort to present and explain it.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 14, 2020, 10:21:15 am
content: > And what was the source of the signal with an ultra-low noise level?Could be anything where you need low noise with low source impedance:power supplies, volt. references, control loops.@ CastorpThat could be sth. like this:<http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf>The design in that paper was handicapped by its too smallinput coupling capacitor. That has been remedied with awet slug tantal. Now I get a clean 1/f response again alsowith AC coupling. The noise of the op amp bias must beshorted through the low impedance DUT, so the capacitormust be MUCH larger than needed for f-3dB.20 op amp inputs are a lot of shoulders to carry the loadof some abuse, but 4700uF++ @12V stores a lot of energy,which requires some protection circuitry.Sequencing takes some care, so there is an opticallyisolated SPI interface now that also adjusts input, gainand bandwidth via analogue switches.I noted a rise of the equiv. input noise voltage above 500 KHzwhich turned out to be skin effect in the U-shaped routing.Updating that with a wire mesh removed that.In a chopper that 1/f problem and the big capacitors do notplay a role. I have also tested a non-differential version ofthe ribbon preamp in Art Of Electronics V3. That works aspromised and delivers 70 pV/rt Hz. There is a small smd versionalso. The LTspice simulation is somewhat optimistic.When you do cross correlation measurements, the inputnoise current produces a drop over the DUT resistance thatis common to both inputs and that does not average away.Therefore I now prefer FETs. But the continuous reversal doublesthe input C for the apparent doubling of the input voltageand the switching produces a large inrush current.In a chopper, BJTs are probably better.regards, Gerhard

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 14, 2020, 11:32:40 am
content: Quote from: Gerhard_dk4xp on February 14, 2020, 10:21:15 amI find it difficult to recall at least one source of constant voltage with noise less than nV. Did you use a low impedance divider?

author: Castorp
author_rank: Regular Contributor
timestamp: February 14, 2020, 12:15:45 pm
content: Thanks, Gerhard! That's some good work again.Quote from: MegaVolt on February 14, 2020, 11:32:40 amThese are usually very cold - Josephson junctions, SQUIDs, superconducting detectors. At room temperature - possibly some batteries. Some time ago I spotted some papers in which they used AD7177-2 for LF noise measurement of Lithium batteries. I can dig up the reference if someone's interested.Another thing could be some narrowband matched circuit that transforms a higher impedance into a much lower one. There are such resonant detectors for image charges in Penning trap experiments. The people who work on them can usually afford good cryo-MOSFETs. Of course, they also don't care about DC and LF.It's all pretty exotic in the end. After all - 1 Ohm at room temperature gives you 130 pV/sqrtHz. In liquid nitrogen (4 times colder) it's half of that.

author: maat
author_rank: Regular Contributor
timestamp: February 14, 2020, 12:57:21 pm
content: Quote from: Castorp on February 14, 2020, 12:15:45 pmUnfortunately superconducting stuff has a rather large output impedance. We usually use a MESFET buffer, but at higher frequencies though, because their LF noise is horrible.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: February 14, 2020, 01:01:45 pm
content: Quote from: Castorp on February 14, 2020, 12:15:45 pmThanks for the clarifications!!!Link is very interesting. If not difficult, share it.

author: Castorp
author_rank: Regular Contributor
timestamp: February 14, 2020, 01:21:11 pm
content: Quote from: maat on February 14, 2020, 12:57:21 pmWhat kind of device are you talking about?It's true that "superconducting" doesn't mean it's a zero-ohm source. Some of them are close though. Single SQUIDs for instance - the only amplifier that's noise-matched to them is another SQUID. To clarify that - the input of a SQUID amplifier is a superconducting coil. I've worked on such a setup, with one SQUID at <100 mK and a large SQUID array at 4K. Eventually they demonstrated nearly quantum-limited energy resolution, i.e. a few times Planck's constant.Another example is transition-edge sensors (TES). They can be either calorimeters or bolometers - depending on whether they measure the energy of single photons, or average radiation power. They are very low-resistance and very low-noise. It's basically a small resistance biased on the superconducting transition. It's a steep transition from 0 to a few Ohms, so they have lots of gain on that setpoint. Once again - you need an amplifier suited for very low impedances, e.g. a SQUID.Sorry about drifting the topic far from ADCs. It just brings back some fond memoriesHere's one of those battery LF noise papers (seems the same author has many):https://www.researchgate.net/publication/338209069_Electrochemical_noise_measurement_methodologies_of_chemical_power_sources

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: February 14, 2020, 01:58:40 pm
content: I did also some measurements on batteries.<http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf>inspired from this NIST report:<https://www.researchgate.net/publication/3622215_Measurement_of_voltage_noise_in_chemical_batteries/link/555b4cc508ae6aea0816a420/download>It used to be on the NIST server of the timefreq group, but it seems it is moved around on a regular base.The NIST people don't dare to disclose / endorse commercial products, but I wanted data on things I canactually buy. "I found a NiCd cell in my drawer" is not precise enough.My preamp was better, but they had cross correlation, so they are better by a few dB over all.The pretty amplifier from some posts above released its black magical smoke btw.when measuring 4 Lithium cells.You see that wire from the op amp inputs over the DK4XP text to the "wrong" sideof the wet slug tantalum location. That was OK for verifying the skin effect, but wasforgotten to be removed before flight."Opfer müssen halt gebracht werden" -- Otto Lilienthal, early aviation pioneer("Sometimes sacrifices are necessary")@Megavolt: A LT3042 features 2 nV/rtHz noise density. I you want to verify that,it's better to have >10 dB margin in your setup.

author: splin
author_rank: Frequent Contributor
timestamp: February 14, 2020, 04:04:44 pm
content: Quote from: Gerhard_dk4xp on February 14, 2020, 01:58:40 pmIt would be good if you could either rerun the tests or at least add a disclaimer at the start of the article that the results are all wrong below 100Hz due to the 1/f noise of the amplifier used.https://www.eevblog.com/forum/projects/low-frequency-very-low-level-dc-biased-noise-measurements/msg655965/#msg655965The results are interesting data for > 100Hz; below 100Hz they are still very useful showing the relative noise performance of the various batteries. Subtracting the estimated 1/f noise of the amplifier used gives a reasonable estimate of the actual low frequency noise of the batteries so all useful data.I saw that you spotted the 1/f noise problem of the amp long ago but it's a shame you didn't get round to correcting your web pages detailing the amplifier and its performance.

author: maat
author_rank: Regular Contributor
timestamp: February 14, 2020, 07:04:15 pm
content: Quote from: Castorp on February 14, 2020, 01:21:11 pmLast bit of off-topic from me. You don't want to draw any current from the superconductor. You are only interested in a voltage fluctuation due an external exitation/disturbance. The superconductor itself has of course 0 resitance (exlcuding type II + magnetic field here), but any electron, that you remove from the sensor will disturb the measurement, hence add noise. Therefore your amplifier should ideally have infinite input impedance.

author: Castorp
author_rank: Regular Contributor
timestamp: February 16, 2020, 12:47:32 pm
content: I'm still wondering what kind of device you're talking aboutBecause the term "superconductor" is about as general as "semiconductor".Sometimes you draw current, e.g. in a voltage-biased SQUID. Another example that should be familiar to the audience here is a microwave-irradiated Josephson junction, biased at a Shapiro step. It should be obvious why you don't want to buffer this voltage with any transistor.

author: Castorp
author_rank: Regular Contributor
timestamp: February 21, 2020, 12:34:23 pm
content: Kleinstein's post finally provoked my curiosity to look more carefully at the extra 1/f noise. Here's what I've got.First - there's a multiplexer inside the HPM7177 that allows you to couple various internal voltages to the ADC. Those can be the +5V (which also goes to Vref), the bare unscaled LTZ1000 voltage (ca. 7.1 V), and a 10 V which is Vref*2. Here's how their noise spectra look like:Now here's the integrated noise within one decade. I chose 1 to 10 mHz, as it seems to be fully in the 1/f region (also for 0V input). One plot line shows the noise on the internal voltages, the other one is with external 5V and 10V. Those come from 10 sources in parallel, feeding either into a 100 Ohm or 50 Ohm load. The external contribution is small (60 nVRMS/decade at 10 V, half of that at 5V), but I rms-subtracted it anyway. The result:Now, one step further. I RMS-subtracted the 0V component. So what should be shown is just the voltage-dependent excess 1/f noise:At the moment I don't have the time and patience to trace it all down, but a rough estimate tells me that the Noise Index of the Vishay bulk metal foil resistors should be somewhere between -40 and -50 dB, i.e. less than 10 nVRMS/dec/V, but more than 3 nVRMS/dec/V. The datasheet spec is for -40 dB. If they are actually much better, i.e. better than -50 dB, then this extra noise must come from the AD7177-2.It's actually quite a jigsaw puzzle to separate the components. There's one part in the input attenuator and another in the Vref scaling. When you measure the +5V internal voltage, the component from Vref scaling won't show. That could possibly explain why the excess noise doesn't scale linearly with voltage.Anyway - it looks like it's not as negligible as I thought before. I think I had a wrong number stuck in my head - perhaps I confused the 7.1 V measurement for 10 V. Or maybe I hadn't subtracted the contribution of the 10 V external source that I used before. In any case - the results are still pretty good.Edit:For completeness, here are the spectra with external 5V and 10V. Some plots look thinner than others because they were averaged over much longer time (e.g. one week instead of overnight). Also the EMI pickup varies due to the cabling, etc. It doesn't matter for this particular study of 1/f noise. Also, the 50 Hz spike may look big, but actually it isn't. It's the FFT bin width which is very small (0.001 Hz), which results in a large processing gain for narrowband signals. Anyway, those details are not relevant.Once I tried to measure the excess noise in bulk metal foil resistors. Back then I concluded it must be better than -40 dB NI, which was my measurement limit, and it was sufficient at that time to point my attention elsewhere. It would be nice to solve this mystery eventually.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 02:47:14 pm
content: AFAK some of the AZ OPs also have a little bit of 1/f noise. So it is not only the AD7177 and the resistors as possible sources. I would expect the foil resistors to be much better than -40dB noise index. I would be more surprised if the resistor contribution is significant at all.Having more noise from the ADC and scaling means the noise contribution from the LTZ1000 reference is a little smaller. The overall noise has not changed, only a slight change in where the noise comes from. The reference still is the largest contribution.For the really critical parts it would be more useful to have 2 completely separate units in parallel and not use one ADC with 2 x LTZ1000 for the reference. Looks like this is the plan anyway.

author: Castorp
author_rank: Regular Contributor
timestamp: February 21, 2020, 03:01:28 pm
content: Quote from: Kleinstein on February 21, 2020, 02:47:14 pmIf it's 1/f noise (voltage/current) from the AZ op amps, I wouldn't expect it to scale like this with voltage. It would be the same at zero and elsewhere, right?I also thought the NI should be much better. If LTC5400 has -55 dB and that's thin film, then Vishay are almost certainly under-specifying. However, now I have some doubts again.You're right, it's still mostly the LTZ1000. That extra bit from the amps/resistors/ADC is less than 30% at 10 V.And yes - the plan is to have 2 completely separate units. It's always like that. In normal operation the controller uses the average of the two, so we're always sqrt(2) better in terms of noise. This fact is not reflected in the specs, we keep it as a safety margin.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 21, 2020, 04:17:59 pm
content: The OP at the input would add noise independent of the voltage. The OP at the reference would add noise to the reference, and depending on where the 5 V is actually taken to the test input this may give voltage dependent noise.For the resistor noise, i can imagine the measurement gets tricky much below -50 dB. At some point that can be just temperature variations that cause variation in the resistor ratios. Here the LT5400 can have a slight advantage because of very good thermal coupling and matching. in the extremes, combining resistors with positive and negative TC is not as good as one material with a low TC everywhere.It is not such a surprise that the ADC itself has some extra 1/f noise also for the reference path. The internal switches also have some resistance and these can have excess noise (I expect the MOSFET ON resistance to have a rather poor noise index). I don't know the details of the switched capacitor SD converters, but in the continuous time version and multi-slope ADC with a high input voltage there is more uneven weight to the resistors if the voltage is large and thus more excess noise from the resistors gets visible. This is what I see in my version. A can imagine a similar effect with the switched capacitor version too.

author: Castorp
author_rank: Regular Contributor
timestamp: February 21, 2020, 07:29:56 pm
content: Well, at least I'm sure that these in situ measurements are not affected by temperature fluctuations, turbulent air flow or thermal EMFs. At the moment I really can't pinpoint the origin. I'll dig deeper if I get more time, even if it's just for academic reasons.

author: Castorp
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:24:21 am
content: I have a question - is anyone aware of a good and reliable reference on excess noise in bulk metal foil resistors?The Vishay datasheets claim <-40 dB, here they claim -42 dB:https://www.ieee.li/pdf/viewgraphs/ultra_high_precision_resistors.pdfFrank Seifert's LIGO report is a great reference, but it doesn't have much on this type. There's a lot of speculation and hand waving around this whole thing. Not to mention audiophile statements like "this type of resistor sounds better than that type", as if we're comparing tomatoes on the market.I can try to measure it myself, but at the moment I just can't free up enough time and the needed equipment for it.On the other hand, I came up with a test that I can do with the HPM7177. By feeding a common-mode signal from the internal multiplexer I can exclude the ADC as a source of additional 1/f noise -it would still measure near-zero differential voltage. At the same time, the four 20 K elements will be biased with with up to 6.4 V (for CM input of +10V). Three measurements at CM = 5V, 7.1 V and 10 V will be enough to fit the model. The problem is that the HPM7177 setup is also busy, running another long-term stability test

author: MegaVolt
author_rank: Frequent Contributor
timestamp: March 11, 2020, 10:35:17 am
content: Quote from: Castorp on March 11, 2020, 10:24:21 am

author: Castorp
author_rank: Regular Contributor
timestamp: March 11, 2020, 10:51:35 am
content: Thanks, I'm well aware of this reference. There's just one measurement of a 100 Ohm S102K resistor that shows well below -60 dB, but I'm not sure how representative that is for higher values (10 K and higher).

author: Castorp
author_rank: Regular Contributor
timestamp: March 12, 2020, 12:16:54 pm
content: OK, now I have the answer.Today I had to stop the long-term test, so I did the measurement at VCM = 10 V. So far, with 10 FFT averages, it looks pretty much the same as with VCM = 0 V. So the extra noise comes from the ADC, and not from the resistors. If they contribute something, it's really negligible.The voltages across the 20 K elements (R1-R3-R5-R7) and 3.5 K ones (R2-R4-R6-R8) in the R array has the following dependence on VCM (input):As you can see, due to VOCM=2.5 V, all 8 elements see zero voltage when VCM (input) = 2.5 V. At VCM=+10V they see 3 times higher voltages than at 0V.And here's what the noise spectra look like. For completeness, the spectrum with internal ADC short is added.(Edit: replaced the 10V CM plot line (red) with one averaged over longer time)There's no need to take further measurements to confirm it. Those Vishay resistors definitely have noise index of -50 dB or lower.

author: TexasRanger
author_rank: Contributor
timestamp: June 12, 2020, 07:07:51 pm
content: Quote from: Gerhard_dk4xp on February 13, 2020, 10:07:06 amHere you go, the dB scale is however a bit offGetting this Plot with an actual differential fullscale input signal seems quite impossible to me.Compared to the 2380-24 the 2500-32 has better DNL.(Sorry for Gravedigging)EDIT: There is a typo in the plot, Ref Level is 3.5Vrms/10Vpp

author: oPossum
author_rank: Super Contributor
timestamp: February 25, 2021, 10:28:14 am
content: Marco Reps built one...

author: quarks
author_rank: Frequent Contributor
timestamp: February 25, 2021, 11:52:53 am
content: Quote from: oPossum on February 25, 2021, 10:28:14 amvery nice

author: branadic
author_rank: Super Contributor
timestamp: February 26, 2021, 06:25:22 pm
content: Really nice to see Marcos attempts to this Cern project. Fortunately he shared his measurements on github, so I couldn't resist to use his INL data to check my linearization approach.Would be nice to see, if this improves his measurements.-branadic-

author: guenthert
author_rank: Frequent Contributor
timestamp: February 26, 2021, 07:09:15 pm
content: Quote from: branadic on February 26, 2021, 06:25:22 pmWell, can the INL error assumed to be constant in time? Isn't it rather the result of all the little errors which do shift a bit over time?Further, Marcos cheerfully neglected the INL of the source and assumed it to be perfectly linear. The Fluke 5700 certainly has impressive specifications, but how well does that specific unit still perform? Not sure, how strict one should or realistically can be here (what's the objective?), but J. Williams et. al. in AN86 went through great length in determining INL using a (self-calibrating) KVD and multiple 3458A. I'd think pen-ultimately one would use hamon dividers for a few test points, if one doesn't have access to a Josephson junction ...

author: branadic
author_rank: Super Contributor
timestamp: February 26, 2021, 07:21:12 pm
content: Hopefully everyone understood, that this approach is linearization to the source only, not to absolute linearity as most of us don't have access to PJVS or at least 3458A with linearity verified to PJVS. So I used what data are available. Please take the results with a grain of salt.Once Marco showed up at PTB and got results on their PJVS and thus proper INL measurements we can adjust the coefficients or the error correction function I presented-branadic-

author: View[+]Finder
author_rank: Unknown
timestamp: February 26, 2021, 07:36:58 pm
content: "Marco Reps built one" is a bit of an understatement: he did it with "a sugar-powered pick and place machine," in his kitchen, with a chemical-assisted reflow crafted from food-service line parts. Freaking amazing! And it works.branadic, thanks for posting your paper on error correction in INL measurement using fitted regression models. I need to conduct similar measurements on my KS34465 DMM with a transplanted reference from the 7.5 digit version. Also, I would like to experiment with the AD7177 Evaluation Kit as a less expensive way to 8.5 digits. Did you use the actual CERN-design boards in your analysis? Do you have any tips, photos of your 7177 build that you could share?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2021, 07:44:12 pm
content: The fluke 5700 definitely has some INL limitations around zero. The negative side is genrated by switching relays and these can add thermal EMF offsets. Also noise / drift during the offset cal can add to the jump at zero.Another possible contribution to the simpe INL test with the 5700 can be reference drift, At the µV level the low frequency noise of the LTZ1000 reference in the meter and LTFLU in the 5700 can add a bit to the difference. So Ideally one would repeat the sweep of the test points a few times.There are 2 spot tests for the INL, that can be done without special instruments:1) the so called turn over test to check if a votlage is read the same with reversed input therminals. If done at different voltages, this can test the even powers of the INL curve. Due to the internal construnction of the ADC this tends to be quite good.2) with a crude, but stable 1:1 divider (possibly with buffer) one could check the linearit at half the scale, by comparing the sum of the two halves to the direct sum.

author: Castorp
author_rank: Regular Contributor
timestamp: February 26, 2021, 07:52:37 pm
content: @branadic, I'm also very much interested in this approach. Even if it's just for academic reasons (for now).I can tell that the INL curve is stable in time. At least over one year, at least to the uncertainty limit of my measurement (multiple 3458As).There are some newer integrated ADCs that are even more promising in terms of native INL. Check out the plots in the datasheet of AD7768 for example. Well, I already explained why INL is not the most important spec for us, so for now I won't be building any HPM7768s

author: chuckb
author_rank: Frequent Contributor
timestamp: February 26, 2021, 09:20:36 pm
content: Quote from: Castorp on February 26, 2021, 07:52:37 pmDo you know how well the INL tracks between assemblies? Could you make some INL improvement with hard coding a correction curve?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: February 26, 2021, 09:33:44 pm
content: It would be interesting to know how much of this INL is added in analog front end circuit.This would require 3458a measuring -10V ...+10V steps from the calibrator and then measuring the same steps at AFE output.Often in high end devices front end is being bootstrapped to minimize nonlinearity.

author: Castorp
author_rank: Regular Contributor
timestamp: February 27, 2021, 09:01:55 am
content: Only the positive half of the INL is consistent between units. So yes, it's possible to linearize at least 0 to +10 V using some average curve. It would bring it from 0.6 to let's say 0.2 ppm.As for the contribution of the frontend - I have strong reasons to believe it's significantly smaller than the ADC nonlinearity. But I admit I haven't tried to separate it.My indirect evidence comes from many tests of the ADC in different modes - for instance by swapping the internal signals using the built-in mux. Just by swapping IN+ and IN- inside the ADC you get a different curve. Also if you use a quartz oscillator rather than the internal 16 MHz or an external CMOS oscillator. And most importantly - INL seems to be very sensitive to the RC networks on the inputs and the Vref pin. What you see in the latest schematics is the result of empirical optimization. Just to remind you - all internal buffers are disabled. That gives a little advantage in noise and big advantage in INL.

author: branadic
author_rank: Super Contributor
timestamp: February 27, 2021, 10:29:02 am
content: Updated the former document with a less saticfying solution for HPM1, which could still lead to good results. It has to be tested though.QuoteDue to a still pending paper I can't show any details at the moment, but the solution only.-branadic-

author: MK
author_rank: Regular Contributor
timestamp: February 27, 2021, 10:32:55 am
content: Hi Branadic,Which document has been updated?

author: Castorp
author_rank: Regular Contributor
timestamp: February 27, 2021, 01:33:16 pm
content: Branadic, it's really not urgent. But it does look very promising. It would be nice to demonstrate it once I get an absolute INL measurement against 10 V PJAS.I also have some interesting stuff to publish, pending for now. Hopefully later this year.

author: coppercone2
author_rank: Super Contributor
timestamp: February 28, 2021, 08:39:14 pm
content: haha, I got a sample AD7177 (or maybe it was the other 32 bit ADC) back in like 2015 or 2016 and I was imagining making a pickup using it but I stopped because I could not find an application. It's good to see this thing being used for something, the applications were always quite nebulous. I thought something maybe with seismometers but I gave upWhat exactly is this thing going to measure anyway?Specifically I am interested in what transducer its connected to and how the extra resolution benefits research. I really wanted to do something with my old samples, maybe the explanation will inspire me. I just got a nanovolt amplifier and a transformer for it because I got a good deal, but the only thing that came to mind was ELF receiver, depending on the CMRR of the thing, but I am even at a loss for applications despite the fact that it is in a easy to use box. Current amplifiers are a little easier to stomach because you can do light with them.Do you have maybe ideas for me for applications since you have a good feel for what these machines are useful for , is there anything to do with these low voltage levels and st abilities other then making benchtops jiggle less ? Not that stability is not interesting in its own right but I am not ultra fascinated by it like other things so this part remains at a low priority.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2021, 10:30:11 pm
content: A seismometer may be a possible use for a high resolution ADC. However this usually would not need that much of gain stability.AFAIK the main target for the HPM7177 is measuring the current to some beam magnets at the accerator, so the beam can be stable at the same position and also fixed focussing. More stable magents should can allow better beam quality and possibly less loss.The articles show DC current transformers as transducers. In this context I am wondering a little why they need a 10 V range: the shunts (on the secondary) should likely also work with a slightly lower voltage so that one might get away without the divider.Chance are the meter could also be used in some other places, e.g. to replace a HP3458. It should be slightly lower noise and at the full 10 kHz speed could even be more linear. Especially at the higher speeds (using the lower input resistor) the linearity of the 3458 may not be that great.I aggree that there are not that many uses for so much resolution.Chip manufacturers could make use of such a meter to test high resolution DACs - maybe a little faster than the current setup. They may allready use a similar setup.

author: coppercone2
author_rank: Super Contributor
timestamp: February 28, 2021, 10:45:38 pm
content: oh its for the focusing/bending coils, so thats kinda like a really really fine tuning CROseismometer is like the only cool application I ever got from people about it and it was only proposed like 'maybe those big guys do that'. I can maybe fit one magnet from a small accelerator in my garage, maybe.not that I usually care about applications but the nanovolts are so much extra cost and work that its almost unappealing to make/work on.i wish there was a application that can sweeten the deal.

author: mycroft
author_rank: Regular Contributor
timestamp: March 01, 2021, 01:06:21 am
content: Maybe a digital equivalent of "High Precision Scale for Human Subjects" by Jim Williams AN-43https://www.analog.com/media/en/technical-documentation/application-notes/an43f.pdfpage 8.

author: coppercone2
author_rank: Super Contributor
timestamp: March 01, 2021, 01:19:49 am
content: I have a precise scale and the building is more the problem then the measurement circuiti would need to start slashing at the slab.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: March 01, 2021, 06:44:38 am
content: Quote from: Kleinstein on February 28, 2021, 10:30:11 pmShunt for DCCT (also called burden resistor) is typically connected to a nearby differential amplifier which converts it to a standardized -10...+10V voltage.Now this large signal is much easier to send through the cable in noisy environment.

author: branadic
author_rank: Super Contributor
timestamp: March 01, 2021, 11:13:11 am
content: QuoteSure, let's test it once you have measurements available-branadic-

author: branadic
author_rank: Super Contributor
timestamp: March 02, 2021, 02:51:48 pm
content: Marco already published a second run of his linearity measurement:https://github.com/marcoreps/multiinstrumentalist/blob/master/csv/HPM1_formula2.csvand tested the fitting algorithm that was presented for HPM1, with quite good results.https://snapshot.raintank.io/dashboard/snapshot/adrOhhnUwiVpq96kaIl7CJu1QoOdeDcI?orgId=2I will plot both solutions in comparison to the former measurement tonight, so we get a raw idea of the limitations.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: March 02, 2021, 05:54:04 pm
content: So here is the plot. This is just the second run data (from -10V to +10V) fed into the same correction scheme / formula as extracted from the first run. As can be seen the fit appears reproducable, except -10V, -9.5V and -9V are off on HPM1 and looking at -9V it seems like some inrush is happening.The whole second run is based on the same 10 kS averaging resulting in 1Sps, with 100 averaged samples per voltage step. This gives an idea of the spread.With the values and formulas given, everyone can reproduce the results.-branadic-

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 02, 2021, 08:47:12 pm
content: Here is another, much simpler, approach to reduce the INL...This one "learned" from the blue measurement and applied it to the orange one.What I don't understand on branadics improvements is, how the INL noise could also be reduced.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2021, 09:58:56 pm
content: If the nonlinear part is reasonable stable, one can apply a correction function. With the correction applied the ADC would be more linear.This can work reasonable good for the more smooth part, like a low power law. However if the correction includes steeper parts, this may cause trouble if the INL changes by changing the voltage they apply too.It also needs more points to test and correct more local errors. Those tests are not easy - at some point one can no longer guaratee that a 5700 or 3458 is really linear enough. The other problem is noise from the references - it takes quite some time to get a reliable curve. Drift and low frequency noise of the references are usually more limiting than the ADC or DAC noise. So 100 seconds at each point are of limited use. Drift of the reference could effect the curve quite a bit. Better would be some 10 runs with 5 seconds each and the rest of the time spend waiting for settling.One can not test every point, so one needs to use a reasonable interpoation / approximation of the correction curve. A higher oder polynominal may not be the best choice. Chances are something like a spline interpolation would work better.A numercal correction is nothing new. Chances are some of the DMMs already include some nonlinear corrections, like a square part or a different slope for the positive and negative side. AFAIK the HP34401 corrects much of the turn over error this way. The LTC2400 is also sometimes used with correction.

author: e61_phil
author_rank: Frequent Contributor
timestamp: March 02, 2021, 10:04:46 pm
content: The "learning" was also just a lookup table with linear interpolation in between the points (less than a handful lines of code).I heard from a manufacturer of 24-Bit ADCs that some of the more expensive parts are INL tested AND corrected with a table within the ADC. Sounds that something like this isn't uncommon.They did the INL test with a very pure tone.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 02, 2021, 10:44:36 pm
content: A very pure tone can be a way to measure linearity. However this mainly works for fast ADCs with lower resolution, as one needs some statistics and the genrators for the pure tone only work well in a limited frequency range: too fast and the OPs get more tricky, to slow and the capacitors get too large.With some ADCs the linearity may depends on the rate of signal change too. A kHz sine wave may show different numbers than a static signal. Especially some SD ADCs are not that good with a static signal, as idel tones can cause problems. This can cause static INL errors that can look a little like a resonance at some specific points - the not so slow sine wave could average over those ranges. A static test with only a limited number of points may miss those ranges or just hit a peak. There may also be thermal effects that show some delay and may thus be different from DC to some 100 Hz or 1 kHz.

author: View[+]Finder
author_rank: Unknown
timestamp: March 06, 2021, 01:48:01 am
content: Inspired by Marco Reps video and a visit to the HPM7177 ADC documentation website, I ordered the Analog Devices AD7177 EVAL kit from Mouser. Just to see what fun could result. Attached are some photos and other output. Just to be clear: two days ago, all I knew about ADC of voltage was from working with an Arduino and various sensors.The EVAL board can work on its own, but for an extra $100 there is a microprocessor that makes interfacing with the AD software trivial. So my total investment is under $200. The evaluation software is well documented and it will let you set the operational parameters by clicking on the virtual configuration panel. I had to do some research to learn the ADC jargon and the function of all the AVSS, AVDD, etc. (thank you internet). Being a NOOB myself, I will assume that readers will not benefit from my explanation other than to describe the setup as a unipolar input of 3VDC from an HP3245A source and that the AD7177 was configured as shown on two of the photos below.

author: Castorp
author_rank: Regular Contributor
timestamp: March 06, 2021, 09:38:50 am
content: The evaluation board is pretty flexible indeed. And the neat thing is that beside the built-in basic analysis in the software (histogram, rms, effective resolution, etc.) you can also export the raw samples and do your own FFT or ADEV or whatever you like. The buffer is long enough, so that when you operate at low data rate like 20 Sa/s you can record a few hours' worth of data.If you want more flexibility, it's possible to operate the evaluation board without the MCU. The digital interface lines are accessible on a header. You can hook it up to some FPGA board like this. If someone wants to go this way (and can't wait till the HPM7177 HDL and firmware get released), I can provide the VHDL core for communication with the AD7177-2.

author: View[+]Finder
author_rank: Unknown
timestamp: March 06, 2021, 03:03:27 pm
content: Quote from: Castorp on March 06, 2021, 09:38:50 amYes, having the VHDL core for communication with the AD7177-2 would be very nice, thank you! It is fun being a CERN fan-boy these days, having been a space-kid during the early days of NASA. The very idea of sensing fluctuations in kA current for the magnets at nano-volt sensitivity is mind-boggling science of the best kind. I can only imagine the personal energy associated with being part of the CERN team.The Analog Devices software is a very good place to start and the ability to save the raw data after a run is way better than expected from 'evaluation' products. My last run generated 100,000 observations at 20 Sa/s for a 1.81MB CSV file overnight. I use DataGraph for analysis and plotting and it is very good at managing data of that scale.Thanks for your participation in the EEVBLOG forum, having an insider's point of view on the science supporting discovery is most welcome.

author: Castorp
author_rank: Regular Contributor
timestamp: March 08, 2021, 09:01:08 am
content: @ViewFinder, I sent you the code.I think it was a good call to release this project as open hardware and to participate in discussions such as the one in this forum. It's good for me, as I get a lot of feedback and plenty of clever questions. Now I understand that many aspects of our work are not so visible from the outside, and the background information is somehow scattered. Plus, the application is unusual and exotic, so it differs a bit from standard voltage or current metrology.I hope it's also useful for everyone else. Some people have already borrowed bits and pieces for their own use, which is totally fine. Others are building their own complete HPM7177 units.It's certainly an exciting time to be at CERN. There are lots of developments around the High Luminosity upgrade of the LHC. For me this ADC is just one of several sub-projects. There are upgrades of other digitizers and DCCTs, test infrastructure, etc.

author: dietert1
author_rank: Super Contributor
timestamp: March 09, 2021, 10:50:47 am
content: From my own time as a high energy physicist at DESY almost 40 years ago i remember that new and better electronics could pave the way for new physics, like search for SUSY particles. Being able to raise the clock of a real-time processing system by a factor two made a difference. And sometimes those physicists, who spent a lot of time on technology work, got trapped and lost their career as scientist.Regards, Dieter

author: View[+]Finder
author_rank: Unknown
timestamp: March 15, 2021, 11:51:40 pm
content: I've been running the AD7177 EVAL under a variety of settings. It is now in a metal box and that cut the noise by a couple of micro-volts. What I'm seeing is like 2ppm for 4VDC at 5sps, 10 samples for 10 repetitions. Longer runs (20,000 repetitions) are more like 15ppm. I don't have a 1ppm voltage standard under 5VDC, and there is enough variation in ambient temperature to account for the variation in the longer runs.

author: amiq
author_rank: Regular Contributor
timestamp: March 16, 2021, 04:32:27 am
content: Castorp, any time you update the files (or make them public) on the CERN repository could you post a wee message in this thread to let everyone know.

author: View[+]Finder
author_rank: Unknown
timestamp: March 16, 2021, 04:56:44 pm
content: Quote from: branadic on March 02, 2021, 02:51:48 pm@branadicThe data on GitHub from MarcoReps show a vref voltage from -10DC to +10DC with precision to the 15th decimal place in many cases. What sort of equipment would have been used to to achieve that level of precision? A Josephson voltage standard perhaps? How does that compare to the expected level of precision from a voltage reference (and its ambient environment) used by participants in the Metrology section of the eevblog forum? {not intended to be a snarky remark} What is the limit of precision available with current of-the-shelf instruments?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 16, 2021, 06:04:09 pm
content: For a Josephson junction reference the 15 th digit may be stable at cryogenic temperature so that there is no thermal EMF. However going to room temperature would add uncertainty from thermal EMF.For more normal instruments, at higher voltages it is the noise of the references that is likely to a large part the limiting factor. So the LTZ1000 in the HPM7177 and to a slightly lesser degree the dual LTFLU in a Fluke 5700 or similar. Chances are the first collumn would be nominal numbers - so no error there as nominal can be exact. It is just the question how accurate the actual source reflects the nominal. The total deviation is allways the combination of errors of the source and the meter plus the reference(s) noise.Ideally one would have the ADC and DAC (PWM DAC in case of the Fluke 5700) to use the same reference, which is not an easy task, though not totally impossible. The way one could eliminate the reference noise and drift.

author: Castorp
author_rank: Regular Contributor
timestamp: March 16, 2021, 09:59:32 pm
content: Quote from: amiq on March 16, 2021, 04:32:27 amYes, of course.Regarding the question on how many digits (or bits) are meaningful - Kleinstein's answer is spot-on as usualQuantitatively speaking, the short-term (minutes/hours) stability of a measurement with a good LTZ1000 against another one (or something similar) should be on the order of 15-20 ppb (parts per billion), limited by 1/f noise. In terms of voltage noise that's 150-200 nV RMS at 10 V.With the HPM7177 you can't easily arrange a ratiometric setup - that was never the goal. But if you could, you'd be limited by the AD7177-2 noise to something like 10 ppb (short-term) at full scale and 4-5 ppb at zero (mid-scale). Of course, you can do better if you implement auto-zero or more sophisticated self-cal routines. This path may become interesting in the future even for non-ratiometric measurements (stay tuned!) but at the moment for our purposes it's just an overkill.

author: PixelVampyr
author_rank: Newbie
timestamp: March 18, 2021, 05:53:48 pm
content: Hello Everyone, MarcoReps paved me the way :-)Just a very big thank you for opensourcing this great project. I'm also very interested in the PSU and what nice design decisions you did there..So i would be very happy if the schematics for the PSU will also be available for the public.Thanks again and best regardsDaniel

author: Castorp
author_rank: Regular Contributor
timestamp: March 23, 2021, 03:00:11 pm
content: Hi Daniel, and welcome!There's really nothing special about the PSU. I'm working on an improved version now. It will follow the same principle (AC/DC + DC/DC + capacitance multipliers + LDOs), but I'll take some measures to reduce the CM noise (GND to EARTH). Also, the +-12V rails are too high for the Peltier. I'll reduce them to cut down the dissipation in the OPA548. The schematics and (hopefully) final test results should be available by Q3 this year.

author: SigurdR
author_rank: Contributor
timestamp: March 25, 2021, 09:48:55 am
content: Hi,thanks for the information about the ADC!I am considering buying the eval-kit, too.Looking at the performance diagrams, it seems to be rather much noise.What sampling frequency did you use?Is it possible to set the sampling frequency in the eval-Software?Best regards,Sigurd

author: Kleinstein
author_rank: Super Contributor
timestamp: March 25, 2021, 10:20:36 am
content: The noise from the bare eval kit without a lower noise reference can be quite high because of the reference noise.Chances are the sampling rate is still at 10 kSPS. This also means there can be some mains hum included. To get comparable with a DMM one would need to average some 200-5000 readings, so expect the noise to go down by a factor of 10-50 just from averaging - though this does not fully apply to the reference noise, that can have quite some 1/f part.By itself the AD7177 is rather low noise.

author: Castorp
author_rank: Regular Contributor
timestamp: March 25, 2021, 12:41:00 pm
content: The evaluation board comes with an ADR445. It's not too bad, but it's not LTZ1000. And it's well above the ADC noise (even LTZ1000 is).For long-term measurements you probably can't use 10 kSPS with the evaluation board software. And you can't go slower than 20 Sa/s either. If power line interference is a problem, you can use the built-in 50/60 Hz notch filters.

author: SigurdR
author_rank: Contributor
timestamp: March 25, 2021, 05:41:25 pm
content: Indeed, the 7177-2 is rather low noise.Table 6 and 7 in the DS show that noise at 5 SPS is about 0,05uVrms (BW not stated)."The numbers given are for the bipolar input range withan external 5 V reference"...what ref used is not stated. LTC6655-5 maybe.Quote from: Kleinstein on March 25, 2021, 10:20:36 am

author: SigurdR
author_rank: Contributor
timestamp: March 25, 2021, 06:33:03 pm
content: Thanks for the reply!Filtered LTZ1000A is what I will use.I have only need for 1 SPS or even lower.My plan is for a DVM for DC.For AC an audio ADC from ESS is my choice.Quote from: Castorp on March 25, 2021, 12:41:00 pm

author: Castorp
author_rank: Regular Contributor
timestamp: March 26, 2021, 02:10:58 pm
content: I had remembered incorrectly - indeed, the lowest data rate is 5 Sa/s, not 20.You can calculate the effective bandwidth of the sinc3 or sinc5 filter. They are fairly steep, as the first notch is at the output data rate. The equivalent bandwidth is roughly 1/3 of that, so something like 1.7 Hz at 5 Sa/s.Filtering the LTZ1000 won't give you much. You can't filter its low-frequency noise efficiently, and the broadband noise wouldn't matter in this case.

author: SigurdR
author_rank: Contributor
timestamp: March 26, 2021, 09:18:43 pm
content: I have been looking at AD converters trying to find the best alternative for a 8.5 - 9.5 digit DVM. Seems like there is not much development the last 10 years regarding slow, precision ADC:s. I guess there is not much sales for the companies designing them...Fast ADC:s are plenty.The AD7177-2 is about 10 years old already.LT has the LTC2500-32 SAR which might be a candidate.Is the Ad7177-2 still the best ADC for a DVM ?

author: Kleinstein
author_rank: Super Contributor
timestamp: March 26, 2021, 11:12:33 pm
content: Nearly all higher end DVM use seprately build ADCs, not ready made ADC chips. The HPM7177 is the exception in this direction. The next best DMM build around an ADC chip may be the Sigilent SDM3068.There are mainly 2 difficulties using a SD ADC chip in a high resolution meter:1) the reference and range of the ADC is usually 5 V or less. This does not combine easy with a 7 V reference. The extra divider for the reference is a point for additional drift.2) The linearity of most ADC chips is not that great. Low noise does not help that much when the linearity is limited. The LTC2500 and related ones are a relatively new exception in this respect and also the AD7177 with reduced range seems to be relatively good. The linearity specs are usually for a differential drive with suppresed common mode signal. This needs extra effort for the input stage.The input stage is at least different from the usual single ended design used in classical DMMs.Because of the limited linearity and use of a usually not as stable 2.5-5 V reference the ADC chips are normally considered more suitable for the 5.5 to maybe lower end 6 digit meter range.Noise is only one parameter for a DMM and with modern part it is more like the easiest. The other parametes are linearity and stability against drift (with time and temperature).The SD ADC are pretty good in the 5.5 digit range. The 6-7 digit range is still mainly multislope or similar ADC build around a FPGA and multiple analog chips. These can work well from a 7 V reference and the native range usually includes 7 V.In my view the best candidates for high performance from a ADC chip are AD7177, ADS1281 and LTC2378 and related (inlcuding LTC2500-32). New chips come up quite often.Even getting the nominal INL may not be easy, as the layout and reference filtering / buffer can effect the linearity. INL may be the more important parameter, not noise.Some of the ADC chips are pretty good when it comes to high speed. This is an area that is difficult with a classical multi-slope ADC.There is very limited need for even 8 digit resolution. Not many reference are good for this level and for a voltage measurement one has the noise of the reference and the intrinsic noise if the signal source. Unless one is measuring a high end reference the DUT may very well swamp the noise.

author: Castorp
author_rank: Regular Contributor
timestamp: March 27, 2021, 10:37:28 am
content: Well, HPM7177 (and the other digitizers we build and maintain) was never meant to be a general-purpose DVM. It has features specifically tailored to our needs, while completely lacking others. I thought that was clear from the start, but I keep getting questions that suggest otherwiseHigh-resolution ADCs and high-end voltage references don't automatically click together. It's true that high-resolution Sigma-Delta and SAR ADCs progress slower than other ADC types, but they progress nevertheless. Check out AD7768. I think in the coming years we may see more parts with lower INL and equally good noise performance. I doubt there would ever be one that takes 7 V reference though.

author: SigurdR
author_rank: Contributor
timestamp: March 27, 2021, 11:30:15 am
content: I hope I did not say that the HPM7177 would be a general purpose DVM. If so, that was not my intention. It is a grand piece of work for your specific needs!CERN has always been a remarkable place just to be interested in what goes on there. I am a physisist originally but ended up in electronics and management. I still follow what goes on in particle physics.Thanks for the tip about the AD7768 - i will study it.Quote from: Castorp on March 27, 2021, 10:37:28 am

author: Castorp
author_rank: Regular Contributor
timestamp: March 27, 2021, 01:36:51 pm
content: @SigurdR, I wasn't referring to your questions or the discussion going on in this thread. There are plenty of people who do their homework and ask clever questions, either here or through other channels. And then there are others who ask "Why do you make a 8.5-digit DVM and you don't even put a display on it?"

author: SigurdR
author_rank: Contributor
timestamp: March 28, 2021, 08:26:36 am
content: Thank you, Kleinstein, for your informative reply!This makes me wonder how many people, or man/woman hours, are put into a project like designing for manufacturing, an 8.5 digit DVM...Quote from: Kleinstein on March 26, 2021, 11:12:33 pm

author: SigurdR
author_rank: Contributor
timestamp: March 28, 2021, 09:23:40 am
content: After having read about lowering noise from the LTC6655 and LTC6655LNhttps://www.analog.com/en/analog-dialogue/articles/why-does-voltage-reference-noise-matter.htmland herehttps://www.analog.com/en/technical-articles/reference-filter-increases-32-bit-adc-snr-by-6db.htmlfor use with the LTC2508-32 to increase its SNR,I wonder if that method would also work for the LTZ1000, or what is the main issue to why it is difficult to lower its noise efficiently?(my being a 20-20kHz low noise audio person)Kind regards,SigurdQuote from: Castorp on March 26, 2021, 02:10:58 pm

author: Kleinstein
author_rank: Super Contributor
timestamp: March 28, 2021, 09:54:10 am
content: For most electronic designs one would not start from zero. A moderm higher resolution DVM is kind of modular with parts that are relatively independent: display and interface part, power supply, ADC, reference, input protection and input switching and amplification.A first design on paper can be rather fast, the tricky parts comes with refining the actual performance, so that it works like planed or at least close too. How much is needed here is hard to predict: one can be lucky and tricky parts work from the start. However there is also a chance to find nasty surprises, like unexpected ringing, resonances or interactions. A difficulty is testing when at the cutting edge.Finding and fixing such quirks is what can take a lot of time, but it is hard to predict. If lucky, it may be a few weeks and it works, but there is also a chance for failure, so that after 2 years one may have no useful product and just knowlege on how it does not work. One still gains knowledge and the experience of the team can make a big difference. Pushing the envelope at the cutting edge is a difficult thing and not very predictable: it sometimes works nice and than new road blocks come up. A the high end failure is always an option.I spend quite some time for my vesion of a high resolution (now 8 digit range - though the original target was more like cheap 6 digits) MS ADC - the initial version was ready surprisingly fast (~ 20 hours for a version on a bread board). Then there were months (and many hours spend) with no real succes in identifying quirks and improving things that are irrelevant. The closer it gets to perfection the longer it takes to solve the small remaining weaknesses. The ADC part is likely the most tricky one - others like the supply and switching is quite a bit easier. It is still the less intersting part for a hobby project. For a hobby it is not so much about getting the result, but also have the challange - so there is no strict target performance to be met, but also hard to find a good enough.AFAIK it took several years to just redesign the boards for the HP3458 to replace obsolete parts. This was a simpler thing with a known working solution.The software part can also take quite some time. This is more predictable - though a similar problem with bug fixing applies there too.For the reference filtering, there can be some improvement be gained from filtering the higher frequencies where it is easy. There can be kind of an analog to aliasing also at the reference side. This would also apply to the LTZ1000. The SNR tests with the LTC2508 were more at higher frequencies and the LTC6655 has a relatively low 1/f cross over. For a DVM the more nasty noise is the really low frequency part and there filtering is not practical.In my ADC design I also have some similar reference filtering for a LM399 ref. It helps for the more white noise - the main effect is from the 25 Hz and 5-100 kHz range. It still does not help with low frequency noise that is the limiting part of the reference.

author: View[+]Finder
author_rank: Unknown
timestamp: March 31, 2021, 06:31:13 pm
content: Time for a little update on the "fan-boy, making-do" version of the CERN precision current fluctuation measurement device.The attached photo shows the AD7177 EVAL board with a piggy-back AD8475 EVALUATION BOARD attenuator. It functions to buffer and reduce the input voltage to a level acceptable to the AD7177, i.e. from ~10VDC to less than 4VDC. The AD8475 is powered by 3.3V from the black micro-controller board.Having spent most of the last month testing the AD7177 in a variety of configurations, I am happy to report that my investment in the experiment was well worth the time and money. The software (FREE) from AD is both easy to use and sufficiently sophisticated to explore most of the issues raised in this thread. More data will be posted as it becomes available . . .

author: View[+]Finder
author_rank: Unknown
timestamp: March 31, 2021, 07:02:54 pm
content: Another test of AD7177 setup . . .

author: View[+]Finder
author_rank: Unknown
timestamp: March 31, 2021, 07:07:18 pm
content: Some more with different settings . . .

author: View[+]Finder
author_rank: Unknown
timestamp: March 31, 2021, 07:28:36 pm
content: One problem I have encountered is that my AD7177 setup (using what I have determined to be a 'precision' voltage source) places a burden on the source on the order of several millivolts. This might not be a problem for CERN in their application, however it poses a problem for anyone who might want to use it as a precision meter on a reference voltage like the HP3459 A9 board for example. As @Kleinstein noted, there is no easy path to precision measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 31, 2021, 08:08:52 pm
content: Discussing the AD7177 evalutation board goes a bit off topic for this thread. If there is more to come in this dirction it may be better to start an own thread for this.The higher speed very low noise SD ADCs seem to be quite demanding on the drivers, both for the inputs and also for the reference. It is not only the simple DC load current, but the more tricky part seems to be the higher frequency (e.g. 10 MHz) part with fast transients. So the ADC chip is not that easy to use. A 7 V ref adds a little (but not not that much) to this.If one does not have a very low noise reference, a first point to look is at the noise with a zero input voltage. Also the external reference used to generate the 3.x V test voltage would add to the overall noise.

author: Castorp
author_rank: Regular Contributor
timestamp: March 31, 2021, 08:27:20 pm
content: Quote from: View[+]Finder on March 31, 2021, 07:28:36 pmI suspect in this case the culprit is the AD8475. It's a reasonably good FDA with integrated resistors but it doesn't have buffered inputs.Unless you want to squeeze the lowest LF noise and INL from the AD7177-2, it makes perfect sense to use its built-in buffers. They make life much easier when it comes to driving the inputs and Vref pins.

author: View[+]Finder
author_rank: Unknown
timestamp: April 01, 2021, 01:52:07 am
content: Quote from: Kleinstein on March 31, 2021, 08:08:52 pmThe title of the thread is "HPM7177 ADC from CERN" so, considering the EEVBLOG forum as a place for interested amateurs of various levels of skill to exchange information, and noting MarcoReps success to DYI the HPM7177, I offered up a lower-cost alternative for other readers to try out. From my experience thus far, it has been an excellent way to learn about ADC and the difficulties of precision measurement.My objective is to participate (as a mere observer) in the profound effort and accomplishment of CERN scientists and engineers and with the luminance project in particular in mind. I believe my postings are consistent with the public engagement mission evident in the CERN videos on YouTube that I have seen and the cost of my bit of kit is within the budget of, well anyone.What I envisage as my future posts remain within the scope of the CERN HPM7177: the use of an FPGA (or initially a Raspberry Pi) to control the ADC--in principle the same as the HPM7177--and a low-budget alternative to the LabVIEW PXI rack that CERN will use for data collection. This could be a science project for kids . . . of any age.

author: Castorp
author_rank: Regular Contributor
timestamp: April 01, 2021, 08:15:38 am
content: On a similar note, people have been asking me whether it's possible to build a cheaper, lower-grade version of HPM7177. I believe it's possible to reduce the BOM cost by a few hundred euros and keep a similar level of performance. It wouldn't be the same of course, but I think it's possible to have the same noise, linearity and perhaps 2-3 times worse temperature drift (which still means <=0.1 ppm/degree C) with certain thin-film resistor arrays. This approach would open up another possible path - to use a higher temperature set point and replace the Peltier element with a heater.I think replacing the LTZ1000 with another voltage reference wouldn't make sense. Then you may as well replace the ADC, and also get rid of the complex frontend and some other parts. At CERN we already have a solution like that with LTC2378-20, LT1236, THS4531-based driver and SMN resistor networks. The price is an order of magnitude lower, and so is the performance. It's a tried and proven "Swiss army knife" solution, with hundreds of units used throughout the entire accelerator complex (not just LHC).

author: niner_007
author_rank: Frequent Contributor
timestamp: April 04, 2021, 07:57:55 pm
content: Quote from: SigurdR on March 28, 2021, 09:23:40 amIf you want the reduce the noise of an LTZ1000, you can parallel multiple LTZ1000 as described in the data sheet. That’s the only way you can possibly reduce 1/f noise that I’m aware of. Diminished returns though. You might have better luck with a carefully selected zener diode in a heated oven. If low noise and not long term stability is what you need, an LTC6655 in a thermally stabilized chamber could solve your temperature stability and noise.

author: branadic
author_rank: Super Contributor
timestamp: April 04, 2021, 08:35:02 pm
content: QuoteThat is a misconception and here is why: Datasheet states "0.25ppmP-P (0.1Hz to 10Hz) 625nVP-P for the LTC6655-2.5"If you multiply that noise figure by 2.88 (7.2V/2.5V) you end up with 1.8µVpp @ 7.2V, which is larger than the 1.2μVpp noise for the LTZ @5mA and typ. 7.2V.-branadic-

author: branadic
author_rank: Super Contributor
timestamp: April 04, 2021, 08:57:32 pm
content: If you refer to the typ. 0.12ppmpp for the LN version, this value is also misleading if you read it out of context, as the LN version comes in a MSOP plastic package. So you might have lower pink or 1/f noise, but have to deal with mechanical strain and humidity influence and suddenly all your low noise advantage is gone forever.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: April 04, 2021, 09:34:16 pm
content: A single LTC6655 is higher noise than a LTZ1000. However with the LTC6655 it is very feasable to combine 2 references (e.g. 2 x 2.5 V in series) and than the 1/f noise gets comparable to the LTZ1000. The LTC6655 may even get away without an extra reference buffer, as it has quite some capacitive drive capability. The LTZ1000+buffer circuit can include some filtering for the higher frequency range. Not sure how much the AD7177 reacts to this higher frequency noise - it may or may not be a factor.0.1-10 Hz is the standard frequency range used in datasheets, because this is about the practical limit for a measurement with capacitive coupling. Real world may need performance at even lower frequencies, that may be worse with the LTC6655 because of thermal effects.

author: SigurdR
author_rank: Contributor
timestamp: April 08, 2021, 05:19:07 pm
content: CERN must be a gold mine for us electronic engineers! I am realy glad that CERN is making the 8,5 digit DVM open to the public!Is the LTC2378-20, LT1236, THS4531-based driver and SMN resistor networks also open to the public?Quote from: Castorp on April 01, 2021, 08:15:38 am

author: Castorp
author_rank: Regular Contributor
timestamp: April 12, 2021, 08:45:39 am
content: Quote from: SigurdR on April 08, 2021, 05:19:07 pmUnfortunately it isn't. But you can see a block diagram here:https://kt.cern/sites/knowledgetransfer.web.cern.ch/files/technology/magnet-power-supplies/tech-brief/magnet-power-supplies-posterpdfpdf.pdfAnd a photo of the newer version on slide 4 here:https://indico.cern.ch/event/976008/contributions/4117627/attachments/2160132/3644408/HL-LHC_ADC_beev.pdfThe SMNs are on the bottom side of the board under IC4, IC8, etc. The voltage reference is on the bottom right corner, on the island with L-cutout, surrounded by resistor networks used as heaters. There's a small plastic enclosure that goes over this island.

author: View[+]Finder
author_rank: Unknown
timestamp: May 24, 2021, 02:11:17 am
content: The AD5791 demo board, that will be the reference for my AD7177 imitation of CERN's HPM7177 (see earlier posts), now has the LTZ1000 add-on board. I've been running it 24/7 for over a month and will make the results available here. As is the case with the HPM7177, my expected result will be a DC voltmeter that is very good at one voltage range, not a DMM by any measure. What I hope to get is a combination of a better precision reference and a means of verifying other meters in a limited way.

author: Echo88
author_rank: Frequent Contributor
timestamp: July 31, 2021, 10:28:27 pm
content: @Castorp:I have a question regarding the power supply section.Why was the decision made to use a medical grade AC/DC-brick (TPP 30-112-D) instead of a power supply as used in the PCB/Fluke 7001, when CM-noise is a concern?Too much hassle since the big transformer (since it also powers the TEC) and its conductive shell would be a custom part?https://edms.cern.ch/ui/file/2606855/1/EDA-04259-V2-0_sch.pdfhttps://eprintspublications.npl.co.uk/1888/1/bemc99-7.pdf

author: Echo88
author_rank: Frequent Contributor
timestamp: September 12, 2022, 06:43:56 pm
content: I intend to replicate the HPM7177 ADC board, since its a very good and tested basis for contemporary ADC-experiments.For my purposes i`ll modify it a bit:-different pcb size/format-4 Layers instead of 6 if it doesnt degrade the specs, but one gotta say the 6 Layer stackup is very nice-TDP-networks instead PRND, dont want to wait half a year and spent more for them than the LTZsThe attached document shows the usual HPM7177-schematic, modified with TDP-networks and comments.Also the last two pages show ideas on how to modify it for high impedance input by using JFET-OPs or discrete JFET-implementations.Hope for some fruitful discussion. The only HPM7177-copy that i know off is the one from Marco Reps, which he sold.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 12, 2022, 07:23:34 pm
content: The resistors for the reference scaling and gain setting for the ADC input are important for the performance. With lower grade resistors one may have to do a slightly more frequent check of the gain by measureing the raw 7 V reference.The version with the discrete JFETs would normally not need an extra buffer between the input buffer and the differential amplifiers at the ADC inputs.The extra JFET to drive the bootstrapped drain voltage could be a weak point: the JFET is behind the loop and drain-source voltage (and thus the common mode voltage) effects the gate source voltage.This may be good enough for the 34401 but may give INL errors with higher demand. It may help to trim the get currents, to get a low drift for the FET pair.Modern JFET OPs (e.g. OPA140) are quite good and could also be used with a bootstrapped supply. So there is limited need for the discrete JFETs.The input switching part does not provide a path for a precharge phase. So switching at the input can cause quite some current spikes. Nomally one should have an extra high impedance buffered signal as an extra input path to reduce the spike for critical inputs. Depending on the use, there is limited need for the +-10 V ref. levels. The scale factor calibration would use the +7 V reference and ground only. The extra switch chip before the MUX is not really useful. The main mux should already have ground as one input - if more inputs are needed, have an extra MUX for some reference levels like the 5 V or VCM that are less critical. The 7 V ref to the MUX chips should have a series resistor to limit current spikes to the reference.The circuit does not provide protection for the input - this would likely be a part to add.The is usually no 10 Gohm resistor at the DMM input - the specs are usually something like > 10 Gohm and here the > can also allow for a lot larger.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 12, 2022, 09:16:11 pm
content: Input protection was omitted for clarity in the schematic, in the end its needed of course.I was unsure wether the 10G-resistors were needed at all for these low biascurrents of the OPA140, just included them as a biascurrent-path, i guess they could be made bigger.The 34401A JFET-frontend is quite nice and simple, but it most likely will not be good enough for the best contemporary ADCs.One could use the 34420A-frontend, but the selected IF3602 is a difficult point.You raise an interesting point with the precharge phase and questioning the many mux-voltages. I need to read up about it.With 140dB CMRR the OPA140 is already quite good for this task, but bootstrapping is a nice exercise in general.

author: branadic
author_rank: Super Contributor
timestamp: September 13, 2022, 05:50:22 am
content: QuoteAre you sure the IF3602 were selected, as it already is a matched pair JFET, not the single JFET version IF3601.If you don't mind you can select yourself from 2SK147 to create yourself a 2SK146. There are similar JFET devices such as 2SK369 or IFN147/IFN146, LSK389, 2SK117, 2SK170 still available today.-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: September 13, 2022, 07:32:34 am
content: With only a buffer and than a gain off less than 1 towards the ADC there is no need for super low noise FETs or amplifiers. One should get away with relatively normal FETs (e.g. SK209, SK2145, 2N4393).The 34420 uses the special FETs to get super low noise for the cases with high gain, not for very good linearity.The 34401 type circuit uses the FETs as source followers and this is still limited by the linearity of the following OP-amp. Meters like the 34420 or 3458 use the FETs already as a first gain stage and this way add to the loop gain and the CMRR of the OP-amp is not critical.Also a good JFET OP-amp is a very viable option. Bootstrapping the supply can help to improve the linearity. One still has the output cross over distortion, that may need class A biasing.With bootstrapping the circuit is however not that much simpler than the discrete JFET version. Discrete FETs can reach lower noise, but with drift OP-amps are usually easier.Also a zero drift OP-amp can work, though it needs some compromising between noise and bias current. With there usually very good CMRR and loop gain one might get away without bootstrapping the supply, though there are more types available for a 5 V supply.The high impedance DMM inputs usually have no intentional resistor to ground. So the open circuit behaviour is undefined and the bias current may as well make it drift all the ways to saturation. This is not a problem and the DUT will provide the DC path.There is nothing wrong with many possible signals to the ADC. Chances are there will be even more with precharge. One can separate the less critical ones to a 2nd MUX in a side path so leakage there would not effect the sensitive inputs that only go through one MUX chip.For the leakage and bias the actual performance can be quite a bit better than the maximum specs and it may be hard to gurantee low leakage from the spec limits. The actual performance can be quite a bit better. It could be viable to build based on typical specs and still get a good yield (low chance to find excessive leakage) for the input section.

author: Echo88
author_rank: Frequent Contributor
timestamp: September 14, 2022, 02:42:59 pm
content: Apologies, the pdf in post #152 is wrong on the 5. page: its missing the OPA140-section and instead is just a copy of the discrete JFET-section of page 6.You are right branadic: i didnt remember correctly. The IF3602-batches arent measured for matching.According to AoE3 they are most likely measured for their Vgs/Ids characteristics, to ensure the JFET-stage works correctly.Attached is a new pdf with a simple OPA140-stage on page 5. Also page 7 shows a detailed improved OPA140-based input stage with added features. I hope i implemented the suggestions from Kleinstein correctly.A JFET-OP-stage is intended since i dont really wanna deal with the current spikes/bias current of AZ-OPs (gonna miss the very high CMRR/AVOL of them though).The autozero/fullscale-cal done by the Mux together with the temp stabilization by the TEC should be good enough to ensure the necessary stability for this task.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2022, 04:53:10 pm
content: The bootstrapped buffer part still has a few weak points: the voltage range is somewhat limited with just a divider and with only partial (e.g. 3/4) bootstrapping there is only a moderate boost in CMRR.The in loop buffer to get more driving power should not use the bootstrapped supply, but normal supply: the main idea if this amplifier is to provide the current, as it is hard to provide much power from the somewhat soft bootstraped supply.The prechearge part is wrong. As shown the extra path works more like a hold circuit, but does not provide an auxiliary signal with the input potential when the input is not yet connected. Instead the extra buffers should be from the inputs directly (before the mux) and the precharge signals should be more going to the main mux as well. The Photomos switches are rather slow to use them for precharge. The buffer for the raw input can also help with the protection (e.g. support clamps).With a differential input, one would likely need more choices for the input MUX. Currently there is no true differential mode. Chances are one would need independent MUX chips for the positive and negative side. Signals would be something like the pos and neg side input, the pos and neg side precharge signals, maybe ground, the extra MUX for reference signals. One may also want some way to ground (or set to a common mode valze) one of the inputs in case the signal source is not differential, like an isolated input.The protection still needs the clamping and turn off part. With the photomos or similar as protection, there should be a backup with a PTC and/or fusible resistor.

author: macaba
author_rank: Regular Contributor
timestamp: September 15, 2022, 08:50:17 am
content: Some good points from Kleinstein, now adding one of mine:Consider capacitance of switches- MUX36D04 is low capacitance mux and the input signals are mostly DC so that should be fine.- AQW210 in the input mux arrangement might be worthy of simulation, even simulating it as a network of capacitors - the capacitance is much higher (though very low for SSRs in general) so you might have AC signal leakage. Possible way around it - a t-network with 2 SSR ICs, with the middle node connected via high value resistor to the precharge buffer output.

author: Castorp
author_rank: Regular Contributor
timestamp: September 15, 2022, 10:57:18 am
content: Nice work, very interesting. Also a good reminder about the large step from digitizer for a specific fixed application to a more general-purpose instrument (regarding the frontend, power supply, etc.)This year has been extraordinarily busy in many ways, but I finally managed to travel to PTB and do those tests with the PJVS. Now I only have to find time to process all the data

author: Echo88
author_rank: Frequent Contributor
timestamp: September 15, 2022, 06:32:25 pm
content: Attached is the daily modified version, last page is of interest.Only the positive input is complete, the negative one neglected as the schematic is overthrown every day.Also lets view it as a +-10V voltmeter, just this range and just sufficient protection to survive lets say 50V.You use Eagle, right Kleinstein and i assume Macaba uses Altium?Otherwise i could share the kicad-project file to those interested in participating, makes it easier than do the schematic from assuming what you meant and Kleinstein giving up on me.The 34420A and 3458A frontends are interesting for this task naturally, way more JFETs (and their BOOT-bias...) instead of Muxes.Come on hiding greybeards, show us how its done nowadays.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2022, 09:50:36 pm
content: For looking at the schematics a PDF file is OK, though some would prefer a PNG.Kicad files are a bit tricky to share with many references to part libs and different lib versions and paths.For reference it helps to have the parts numberedThe switching still looks overly complicated. For a single input (other side to ground) the critical input should to go to few switches. In the minimal solution there are just 2 switches to contribute to the leakage:1 from the input to the the main amplifier and1 from the amplifier input to a 2nd MUX with all the less critical signals (references, ground, precharge signal,...).In the old times the CMOS switches were limited and JFETs gave better performance. Modern CMOS switches got quite good and are easier to use. If a suitable gate signal is avaible JFETs are still a viable solution. JFET solutions may have quite some charge injection. CMOS switches are often quite good in compensating the charge spikes. However the charge injection is usually voltage dependent and the datasheets often give one of the better operating point.The topic of a good DMM input is quite lengthy. So it may be better so open a new thread for this, to keep this thread more focussed on the HPM7177.