Product & Circuit Design Brainstorming - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: March 05, 2017, 11:11:10 pm
content: Dave and David2 brainstorm new uCurrent and other product design options.Autoranging systems, low current measurement design, noise, overload, USB isolation, ADC's, Gold vs Platinum, marketing, market direction, and a host of other stuff in this often tangential session.And what common component name did Dave pronounce embarrassingly wrong for his entire childhood?A video from the archives.

author: f4eru
author_rank: Super Contributor
timestamp: March 05, 2017, 11:47:18 pm
content: from the "16 bit isn't enough" : nope, I don't think so.You can use two converter inputs, and a take off before your final amplification, for example x10 and x1000Effectively an after the fact software autoranging.

author: f4eru
author_rank: Super Contributor
timestamp: March 06, 2017, 12:06:03 am
content: Concerning current measuring dataloggers, the EFM8SB1-SLSTK2010A has one integrated, it can show the result in a log graph on the PC.Part schematic is available in the toolchain, don't know if the current measurement is included.Concerning the leaky relays, a solution is to use reed relays, to pot the result, and to stagger the matrix, so only to have a few in paralell...

author: EEVblog
author_rank: Unknown
timestamp: March 06, 2017, 12:19:35 am
content: Quote from: f4eru on March 05, 2017, 11:47:18 pmIf you ignore burden voltage and noise floor, sure.Assume 1A full scale and a 100mohm shunt, so 100mV burden voltage, acceptable.10nA resolution = 1nV per bit. Good luck with that.

author: FrankBuss
author_rank: Unknown
timestamp: March 06, 2017, 01:20:28 am
content: Interesting discussion. A ucurrent with a digital output, maybe even with USB or one of these fancy cheap WiFi modules, would be cool.BTW, have you seen the problem I had with the nA range?https://www.eevblog.com/forum/testgear/problem-with-ucurrent-gold/Of course was no problem with the ucurrent, but just me being an analog noob, and maybe I missed a warning or information about this problem in the product description.

author: TheRevva
author_rank: Regular Contributor
timestamp: March 06, 2017, 01:35:10 am
content: I'm not INTENTIONALLY being an a$$hole here but...This video is something I can use as an example explaining the need of at least some rudimentary agenda from the outset.90 mins of time discussing stuff, and the conclusions at the end are...?

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 06, 2017, 03:53:40 am
content: I like this discussion type video, very interesting to see the thought processes in action. But, I also want to hear the discussion that you had before this that involved the toilet seat that you erased.

author: bitwelder
author_rank: Super Contributor
timestamp: March 06, 2017, 06:45:32 am
content: Quote from: EEVblog on March 05, 2017, 11:11:10 pmCan you tell how long ago have those brainstorming sessions been shot?BTW they are very interesting.I wonder if it would be feasible to make some 'interactive YT brainstorming' episodes:- take a commercial product that could use some improvement- publish plenty of photos and/or short intro clip of said product- announce that on a certain date you are going to discuss it live- go live and unchain the engineers

author: EEVblog
author_rank: Unknown
timestamp: March 06, 2017, 07:05:12 am
content: Quote from: TheRevva on March 06, 2017, 01:35:10 amIt was never really meant for release, it was kinda like just a live thing that we recorded a while ago. It is what it is.

author: bktemp
author_rank: Super Contributor
timestamp: March 06, 2017, 07:11:46 am
content: The video was both interesting and disappointing to me, because it looks like you tried to solve a problem that does not exist: You need to have a fast high dynamic range mesurement (>100kS/s, >1000000:1 dynamic range) for measuring both standby and operating current for calculating the total energy consumption.Why not reduce the dynamic range range first before feeding it into the adc? If you add a capacitor across the input, this will low pass filter the current signal and avarage the low standby current and high operating current burst. If the burst is short enough, the peak signal is low enough to stay in the low current range, or if it is long, the slew rate will be rather slow, reducing the error due to a range switching deadtime. That is the easiest and cheapest way for measuring the power consumption of low power devices with high peak operating current.If you want to record the current waveform, that is a completely different story, making the device much more complex.Is there an analogue way of compressing the dynamic range first? I have seen a diy power supply using a dual power diode as a current shunt for driving an analogue meter: It wasn't accurate, but you could see if the current was in the single digit mA range or >1A. The other diode in the package was used for temperature compensation.As you said in the video, 10% accuracy is good enough. If you could make a cheap device capable of measuring current over a large range without the problems associated with traditional range switching meters, it would be a nice and useful product.The main problem is probably offset errors at the lower end of the range, because of thermal effects after high current bursts.

author: jonovid
author_rank: Super Contributor
timestamp: March 06, 2017, 10:30:34 am
content: for your ranging did you think to try a binary switch Pushwheel or Thumbwheel Switches or dip switchs ? Daves good video . but a picture in picture for close-ups may had an improvement.Tip- with more then one character on set you may need 3rd person off-set to help with script & props. also setting up the close-ups as you need them. like what was the website you both was looking at? close-ups of your uCurrent™ ? the circuit on the whiteboard , creative direction Fail. little more professionalism goes a long way Dave & Dave.

author: max_torque
author_rank: Super Contributor
timestamp: March 06, 2017, 01:07:48 pm
content: I agree there are two disparate aims here:1) Accurately MEASURE the average power consumption of a load: Here you want an accurate integrated current measurement, with enough dynamic range to capture all sorts of events. A precision current source with a large capacitor parallel to the load will achieve this. (Of course the system will need to auto cal to negate the capacitors self discharge if you want a really high precision at low power consumptions)2) Accurately PROFILE the instantaneous power consumption of a load: Here you either have to settle for a realtime analogue solution with a few fixed ranges (outputting to a scope etc) OR you have to move to a system that includes digitisation and non-real time recording to get a high bandwidth sample of the DUTs instantaneous power consumption. That Digital system them dumps it's memory to a suitable bit of software for visualisation and further processing (RMS/Harmonic/Spectrum calcs etc)These two approaches are necessarily very different, because the requirements in each case are very different, although if you develop the digital solution, then a software averaging process becomes viable to generate, er, average, power consumption data

author: SydB
author_rank: Contributor
timestamp: March 06, 2017, 05:47:52 pm
content: Deciding the spec, as mentioned, is key here. Otherwise you end up designing a scope!Absolutely agree with previous post of the two different (and useful) uses here - signal profile and power measurement.I think the simplest single step forward for the ucurrrent is to have a fast low value shunt switch in very quickly when the current goes above range. This stops the micro from crashing when high current pulses are drawn and means you don't have to use a big supply cap downstream of the ucurrent and so you get to see more detail. While out of range, you don't need to measure, you just need the application circuit to keep working. Then ucurrent needs to then switch the low value shunt out of the way again quickly when the high current falls. The high current periods are effectively just 'blocks' of out of range data, which is fine, because you can measure them another time using a higher range. The important thing is that you have not throttled the current when the circuit needs it so the micro does not crash due to low volts, and you get to see the scope signal before and after the high current event.

author: f4eru
author_rank: Super Contributor
timestamp: March 06, 2017, 06:44:03 pm
content: Quote from: EEVblog on March 06, 2017, 12:19:35 amI don't see what a bigger resolution on your ADC brings to the table if the limitation is the offset voltage of the OP amp anyway. So 16 bit is plenty, and you absolutely need an autoranging to avoid getting stuck with your offset voltage, which is 1/25000 of your measurement anyway.Now, there's a great way to do a soft autoranging with essentially a null recovery time : try this :Calculating the measurement result is super easy : just add the two output values. That's it. No complicated processing involved.Eventually discard the high range measurement 1-2 LSBs when the low range output is lower than 400mVNow the same with 3 ranges :Exercice for the reader : translate this to shunt style measurement. Yes it's possible, but you have to take a variable shunt in the OP loop

author: twam
author_rank: Unknown
timestamp: March 06, 2017, 06:50:03 pm
content: Why not 2 outputs with different ranges, likeKeysight's N2820A?

author: FrankBuss
author_rank: Unknown
timestamp: March 06, 2017, 08:10:13 pm
content: Quote from: twam on March 06, 2017, 06:50:03 pmThey have some demonstration videos, too, where they use this probe. Doesn't look too complicated: one fixed shunt, 20 mOhm, 100 mOhm and user-defined, and then two voltage outputs, one with a gain of 1.97 (strange factor) and a bandwith of 3 MHz, and one with a gain of 300 and a bandwidth of 500 kHz. Only 1% tolerance, but still very useful for many tasks like measuring how much power your microcontroller circuit needs. You can get it fora bargain of EUR 3,741.00. I would probably buy it, if someone sells such a thing for less than EUR 100.

author: stefanL
author_rank: Newbie
timestamp: March 06, 2017, 08:11:05 pm
content: It was very interesting to follow this discussion, since I've recently worked on solving essentially the same problem. I designed the analog front-end for a high-dynamic range current/power logger as a semester project at my university. We are planning to release the design data in the next couple of weeks, so it might also be interesting for you to look at our design.We wanted to design a portable device, that you can use to log the power consumption of a low power / energy harvesting device. We built a analog front-end cape (shield) for the BeagleBone to sample 2 current and 4 voltage channels at the same time. The BeagleBone then stores the data on a MicroSD card and allows the control of the measurement via a webserver. The maximum current it can measure is 500 mA and the RMS noise floor is about 1.5 nA (172dB SNR @1ksps). Our sample rates / bandwith are somewhat lower than what you're talking about: 1ksps nominal, 64 ksps maximum. On the other hand, we support longer-term measurements (hours, days).We achieved this high dynamic range, by having two different ranges and automatically switching between the two. We have a high range, which covers 2-500mA and is a regular shunt current meter with a 50mOhm shunt, and a high-bandwidth instrumentation amplifier. The low range is a feedback current meter with a feefback resistor of ~1kOhm. We can use the high range for range switching, since it is always valid, using a window comparator. Then we just use two MosFETs two control the path of the current (the gate-source leakage was no problem with our MosFETs, but the gate charge was). We always sample both channels and then re-assemble the current waveform later.There were some struggles with accuracy, drift & stability, but in the end, the result was very good, and we manufactured a small batch of these loggers (but the design is not really optimized for sale).We have some additional ressources, if you're intersted in more details (or you can also contact me):DATE 2017 PublicationProject HomepageDesign dataEdit: Updated links

author: FrankBuss
author_rank: Unknown
timestamp: March 06, 2017, 08:27:01 pm
content: Quote from: stefanL on March 06, 2017, 08:11:05 pmNice project. The feedback ammeter looks interesting. There is a NI low-current measurement device (PXI-4022), which uses this concept and has a burden voltage of 20 uV for the 100 nA range (so that's 200 ohm instead of 1 kOhm) and with a resolution of 0.5 pA or better, in combination with a good voltmeter:http://www.ni.com/tutorial/5448/en/

author: stefanL
author_rank: Newbie
timestamp: March 06, 2017, 08:30:37 pm
content: Quote from: FrankBuss on March 06, 2017, 08:27:01 pmThanks. The effect of the feedback resistor is scaled with the gain of the amplifier. Therefore the burden vultage would usually be determined by the op amp (with our ADA4522 typically 0.7 µV). At a very low current that's not an issue, but it's really handy at 2mA.

author: thm_w
author_rank: Super Contributor
timestamp: March 06, 2017, 08:34:09 pm
content: For some reference, the R&S NGMO power supply uses 3 ranges (7A/0.5A/5mA) and has a specified resolution of 100nA (but will display <10nA). It is designed for measuring sleep current and high transmit current of a cell phone.ADC used is AD7722 (http://www.digikey.ca/products/en?lang=en&site=ca&KeyWords=AD7722AS), 195ksps and 16-bit.An a few DACs, possibly for automatic gain adjustment.Thats not quite as good as Dave is envisioning, but it is an old design.Quote from: twam on March 06, 2017, 06:50:03 pmEven better example, problem is how to get one to reverse engineer.

author: f4eru
author_rank: Super Contributor
timestamp: March 06, 2017, 09:07:59 pm
content: no need to reverse engineer one, just design one. But this one is obviously simply a cdascaded amp, both outputs having a diff amp for allowing floating. Not very difficult to design. But the performance is not so good also.

author: f4eru
author_rank: Super Contributor
timestamp: March 06, 2017, 09:44:15 pm
content: Hello StefanL,Question : why not replacing the mosfet M2 by a simple protection resistor ? ( 100 ohm - 1kohm range)My proposal up there for having a "soft autoranging" with a shunt ammeter would be for example to put a mos in linear mode as a protection as you did. But soft-switching, so regulating the max burden voltage over the transimpedance current measurement in case of overload.Obviously, all these feedback ammeters have a drawback : speed in extremely fast overload condition. Protection diodes will help. How much do your protection diodes and mosfets slow down the small signal measurement by added capacity ?

author: stefanL
author_rank: Newbie
timestamp: March 06, 2017, 10:11:27 pm
content: Quote from: f4eru on March 06, 2017, 09:44:15 pmTbh, I didn't think of that. It should also keep the feedback ammeter output voltage in check. It would, however, add to the burden voltage.Sounds like a good idea, but I don't know how you would implement the details. IIRC Agilent is doing something similar in their SMUs ("seamless autoranging").Yes, that is a drawback of the feedback topology.Ediit: I didn't measure/calculate the impact , since the design has a rather low BW (10 kHz). It might be relevant for a faster design.

author: KNSSoftware
author_rank: Regular Contributor
timestamp: March 06, 2017, 11:04:15 pm
content: Nice to see Dave2 again. Dave said he was potentially looking/in discussions, to bring on a new co-host... Warning us up, for a big reveal? I would say his odds are EEVen.

author: Stephan_T
author_rank: Contributor
timestamp: March 06, 2017, 11:13:19 pm
content: Hi Dave,you may take a look at this Video of the "Keysight Oscilloscopes" Youtube channel:

author: Yotson
author_rank: Newbie
timestamp: March 07, 2017, 07:28:06 am
content: Far from ideal as it gives a somewhat logarithmic result and has a temperature coefficient that has to be handled. Both are talked about in the article. What about something like:fromhttp://www.edn.com/design/test-and-measurement/4443319/Multi-decade-current-monitor-the-epitome-of-simplicity

author: M@rcel
author_rank: Newbie
timestamp: March 07, 2017, 10:36:35 am
content: Perhaps a tip for USB isolation (see video at 29:00): Look into ICE08USBx fromhttps://www.silanna.com/(not affiliated in any way). The cheapest (I'm Dutch) product I could find that uses the ICE08USBC is found athttp://hifimediy.com/high-speed-usb-isolator-480Mbps(not affiliated).They work very well. I use them for embedded design testing.HTH

author: xyzzy
author_rank: Contributor
timestamp: March 09, 2017, 05:26:07 am
content: Dave and David,24 bits of dynamic range will get you from 10 nA to 167 mA. A typical scope A/D is 8 bits. What happens if you have 3 simultaneous analog outputs, each scaled by 256x relative to the previous output (ie. no gain, after one 256x gain stage, and after two 256x gain stages)?For example, let's hook the 1x output to channel 1, 256x to channel 2, and 65,536x to channel 3 of your scope. If your product has a 200 nA sleep current and a 10 mA wake current, then you'd read 78% of full scale on channel 3 during sleep, and 6% of full scale on channel 1 during wake (with channels 2 and 3 showing the full rail voltage of your opamp). The best part is that, for those awkward measurements just beyond* a range threshold, you can adjust the vertical scale of each scope channel independently.*http://www.catb.org/jargon/html/P/pessimal.htmlBTW, I really enjoyed this video. It's very educational to see the entire thought process (dead-ends included) vs just seeing an explanation of the final design.

author: FrankBuss
author_rank: Unknown
timestamp: March 09, 2017, 07:20:48 am
content: Quote from: xyzzy on March 09, 2017, 05:26:07 am24 bits are a myth. If you carefully design the board, shield it, battery power it, use it at the lowest samplerate specified by the datasheet (higher samplerate means less usable bits, always read the fine print in the datasheets) etc., you might get 20 usable bits out of a 24 bit ADC. Once I used an AD7793 (24 bit ADC) on a 4 layer board, but my layout was not that good (e.g. no star-grounding, just one big ground plane, didn't know Dave's videos back then) and with a low samplerate and the internal voltage reference I got like 18 usable bits, and 20 bits with oversampling and averaging, which is pretty much what the datasheet says. With an external reference, 4 Hz samplerate and gain 1 the datasheet says 23 bits with RMS noise (meaning if you don't average, you don't get perfect values with this resolution), but 20 bits with peak to peak noise.

author: xyzzy
author_rank: Contributor
timestamp: March 09, 2017, 05:41:43 pm
content: Quote from: FrankBuss on March 09, 2017, 07:20:48 amYessir, I tend to agree. My suggestion removes the ADC from the product entirely and relies on the ADC inside the user's oscilloscope instead. By using 3 scope channels with 8 bits each and multiplying the signal by 256 and 256^2, respectively, in the analog domain, you can measure currents from nA to mA without autoranging. You're effectively measuring three separate ranges in parallel, and when a lower range starts to clip, the next range up starts to provide useful information. (This works with two-channel scopes as well; you simply omit whichever range isn't appropriate for your particular bi-modal distribution of currents.) It's not a perfect solution, but I think it may be a cheap way to provide sufficient information in a useful form.My only concern is if you can multiply an analog signal by 65536 (albeit in two stages) without introducing significant additional error.

author: bktemp
author_rank: Super Contributor
timestamp: March 09, 2017, 06:19:37 pm
content: I would prefer having an ADC, because it allows a cheap digital isolation.Being able to measure the current on the supply rail instead of the GND wire can be really useful in many situation.If you still want an analogue output, 16bit DACs are quite cheap. If the ADC is good enough, you could add a dynamic range compression features like outputting the current waveform in a logarithmic scale instead of linear for viewing the waveform on a scope.

author: FrankBuss
author_rank: Unknown
timestamp: March 09, 2017, 06:22:46 pm
content: I guess 24 bit dynamic range with analog components is also not possible, or gets really expensive, or slow, and if it is 20 bits, the last stage has only a dynamic range of 4 bits, which might be too noisy for many measurement applications.

author: hli
author_rank: Frequent Contributor
timestamp: March 09, 2017, 09:57:45 pm
content: Linear has ADCs with 18 bits and 5 Msps, e.g. theLTC2385-18. You get 24bits at 2Msps with the LTC2380-24. Drawback: the 2385 comes only with LVDS interface.

author: f4eru
author_rank: Super Contributor
timestamp: March 09, 2017, 11:01:19 pm
content: Quote from: xyzzy on March 09, 2017, 05:41:43 pmThat doesn't work well. You need sme overlap, or you'll lose precision by getting some 2x steps, which is a very bad precision:1,2,3.....,127,256,512,....

author: FrankBuss
author_rank: Unknown
timestamp: March 09, 2017, 11:43:20 pm
content: Quote from: hli on March 09, 2017, 09:57:45 pmNo, you don't get 24 bits at 2 Msps. Always read the fine print: 145 dB (=24 bit) dynamic range at 30.5 sps (still quite impressive), 100 dB SNR (=16.6 bits) at 1.5 Msps.http://www.linear.com/product/LTC2380-24

author: hli
author_rank: Frequent Contributor
timestamp: March 10, 2017, 07:35:32 am
content: Quote from: FrankBuss on March 09, 2017, 11:43:20 pmAh, I was just looking at the overview table and not at the detail page. Dave actually wanted at least 18bits at more than 4Msps, so the LTC2385 was the better fit anyway.

author: stefanL
author_rank: Newbie
timestamp: March 17, 2017, 10:22:44 pm
content: A quick update on the RocketLogger: We have just made our project public, so you can now check out our design on ourupdated homepage.The data includes:SchematicBOMBeagleBone SoftwareEnjoyWe will also present our project at DATE 2017 in two weeks.

author: ksrm
author_rank: Newbie
timestamp: March 27, 2017, 08:59:59 pm
content: That's a really interesting project, Stefan. I've also been thinking about how to do high dynamic range current measurements, but I thought I would need a much faster range switching time than the 1.4 us you have. Does it cause any problems in practice?

author: stefanL
author_rank: Newbie
timestamp: March 27, 2017, 09:34:15 pm
content: Quote from: ksrm on March 27, 2017, 08:59:59 pmThanks, it certainly was a lot of funWe never saw any problems related to the range switching with our own logger, while other we did with other equipment. The fact that the voltage drop is limited by the diodes helps a lot, also 1.4µs is not that long. That beign said, there are certainly situations, where it's still too slow and you just have to disable the low range.I think it would be really hard to get even faster rangeswitching with our design. We ran into many issues around the same delay: Noise bandwidth, propagation delay and the RF filter step response. In that regard it might be interesting to look at a design that controls the input burden voltage (see f4eru's post).