Relatively high voltage precision DAC amplification - Page 1
Forum section: electronics / projects

author: Laval
author_rank: Regular Contributor
timestamp: September 22, 2024, 07:47:40 pm
content: Hello everyone.I'm working on a DC Voltage Reference/Calibrator project that would be comparable to Data Precision 8200 (hopefully better) in terms of specification i.e. about 0.01% accuracy (preferably better) with 6 digits resolution. The unit will be using a much more modern approach and components of course. I will receive my evaluation board for the AD5791 this week. That should allow me to experiment and prototype the DAC/Reference part.I would like the unit to have a 100mV, 10V, 100V and 1kV or 1.2kV ranges. Since the DAC obviously won't output more then 2.5 to 10 volts (depending on vref), it will need some accurate amplification. This is not a big deal up to about 40V or so as there are many good precision op amps available. For the 100V range, I found this op amp:https://www.analog.com/media/en/technical-documentation/data-sheets/adhv4702-1.pdfThis should be fine for 100V. An application note in the datasheet suggests a voltage range extension (basically bootstrapping) using an n and p MOSFETs on the positive and negative power rails respectively but it can only double the range. 200V is nowhere near 1kV. I'm also a bit concerned about stability since this is a rather fast amplifier for a DC application. It doesn't look like there is much choice anyway.There is a 1kV op amp made by a company called Apex Microtechnology but it is very expansive (over 1k CAD). I guess I could go discrete and make an amplifier stage using an LTP feeding a cascode with negative feedback for linearity much like the VAS stage of classic audio amplifiers. This is what I'm considering at the moment.I'm interested in any suggestion for the 1kV amplification (as well as the rest if you like). What would be a good strategy to get to 1kV while maintaining the precision and low noise.Some complementary information:The device could be used to calibrate instruments that have a low enough precision but it would be used mostly as a precision voltage source.The device will be a benchtop unit with its internal power supply (one of the challenge because of high voltage).The voltage source doesn't have to be able to source much current as it will mostly be used with high impedance circuits. Maybe 10 or 20 ma max at 1Kv.I may eventually add a current source that could go to 100mA or 200mA, not sure though.The complete and definitive specification is not yet determine as a lot will depend on what I can do with available components but accuracy is the main goal.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 22, 2024, 11:21:45 pm
content: Quote from: Laval on September 22, 2024, 07:47:40 pmThat's still 20 watts...One advantage you have is this does not need to be fast, it will be a challenge getting high precision high voltage resistors...You could experiment with photovoltaic drivers as they allow slow and isolated drive into a high voltage MOSFET, and keep things simple ?Your HV supply will need a soft start, as the capacitance of any MOSFET will tend to give a spike at power up.

author: Laval
author_rank: Regular Contributor
timestamp: September 22, 2024, 11:46:29 pm
content: Quote from: PCB.Wiz on September 22, 2024, 11:21:45 pm20ma isn't really an important design goal. 5ma at 1Kv would be fine which is still 5 watts.Quote from: PCB.Wiz on September 22, 2024, 11:21:45 pmThis is my main concern at the moment though, I assume, it could be possible to bodge some less precise resistors to get the desired values as long as they don't drift too much and have good TC. It looks like Caddock does make such resistors but I just don't want to look at the price for nowQuote from: PCB.Wiz on September 22, 2024, 11:21:45 pmI didn't know about these. I assume you are talking about these components:https://www.mouser.ca/ProductDetail/Littelfuse/FDA117GRTR?qs=sGAEpiMZZMug9GoBKXZ75woEA5q4EdH8PW4%2FqvT8hLwjDiSWmxKgRA%3D%3DThat may be interesting. I can't open the datasheet PDF file for some reason. They would give isolation for sure but how do they compare to traditional opto couplers ?Quote from: PCB.Wiz on September 22, 2024, 11:21:45 pmFor sure. I have worked on inrush current limiting before and inrush current measure using zero crossing opto. I don't even know yet if I will use a SMPS or a linear PSU. Getting good transformers for linear PSU is a bit of a pain and they make the unit large and heavy.Thanks for the thoughts.

author: mawyatt
author_rank: Super Contributor
timestamp: September 23, 2024, 12:17:36 am
content: Consider using a Switchmode Boost Regulator operating with the feedback resistor chain (use a few series Precision Rs for the HV) that will regulate to a few volts above the desired DAC set point. Using the same resistor divider chain with a different tap off as feedback with an op-amp and optical driver to drive a HV MOSFET as the output of a "Linear Regulator" to provide the desired output voltage. Since the HV Switchmode Boost output to the "Linear Regulator" is only a few volts above the desired output, power dissipation will be low.Or use a separate low resolution DAC to control the HV Switchmode Boost Regulator, this is likely easier and cheaper as no need for precision DAC, Resistors or anything.Be sure and provide proper overcurrent and short circuit protection, maybe a quick overcurrent/short circuit latching type circuit type protection which requires a user Reset to reactivate for user and component safety.Best,

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 23, 2024, 01:03:54 am
content: Quote from: Laval on September 22, 2024, 11:46:29 pmYes, broadly. They are voltage clamped low current generators, so can provide gate voltage too. Traditional opto couplers are Diodes+transistor so need additional power.The fine print matters for your application.I find Toshiba photovoltaic drivers TLX9905 and TLX9906 and they differ slightly.The TLX9905 sims like a simple clamped current source (needs external discharge resistor), while the TLX9906 has an active turn off helper, which I can sim as a diode+transistor+resistors.For linear control of a FET, parts like the TLX9905 are what you need, as they will behave better in a feedback loop.Spice indicates it can loop-regulate ok, with a slew response ballpark similar to the switching times. Expected from uA and nF.You have stable resistive loads and can tolerate slower response times.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2024, 01:41:57 am
content: Quote from: Laval on September 22, 2024, 11:46:29 pmAt high voltage, it is not only about drift and temperature coefficient; the voltage coefficient of resistance becomes a significant error term.If I could not get a suitable divider network, I would consider using multiples of the same resistor in series to make a divider. This way the same voltage is across each resistor and the voltage coefficient of resistance should at least partially cancel out. With a proper thermally balanced layout, the temperature coefficients should also partially cancel out.

author: jbb
author_rank: Super Contributor
timestamp: September 23, 2024, 02:14:15 am
content: This is a totally sideways post, so feel free to disregard.I saw a note once (I think pertaining to a TI part) thatshunttype references can be series connected to form a higher voltage reference directly. This could perhaps be useable in the vicinity of 100 V. But probably impractical for 1 kV. Also may not be stable enough.

author: Salitronic
author_rank: Contributor
timestamp: September 23, 2024, 06:15:46 am
content: You could consider a cascaded amplifier configuration such as this:https://www.researchgate.net/publication/283661284_A_Cascaded_Linear_High-Voltage_Amplifier_Circuit_for_Dielectric_Measurementand scale it to your required voltage range.I have designed similar cascaded systems for high voltage (kV) signal generation but typically using cascaded H-bridge (switch-mode) converters which is not suitable for your very low noise requirement. It is a very convenient design topology as you have to deal with multiple modules but each one at a low voltage. Power is also split among all modules making thermal management a lot easier. Obviously it is a complex design with multiple modules each requiring independent isolation and synchronized control of all modules.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 23, 2024, 06:49:14 am
content: Quote from: Laval on September 22, 2024, 07:47:40 pmSide note, kilovolt issmallletter "k" followed bycapitalletter "V", so "2kV", not"2Kv"."2Kv" is incorrect, and it means nothing. Kilo prefix is always denoted by the small letter "k", never the capital letter "K". Volt happened to be denoted by capital "V". Other units might be with a small letter, for example meter is with a small "m", so 1.6km should be with small "k" and small "m".For current, it's "mA" for milliampere, not"ma". Capital "A" stands for ampere, small "m" prefix stands for mili (10-3) and capital "M" prefix stands for Mega (106):https://en.wikipedia.org/wiki/Unit_prefix

author: moffy
author_rank: Super Contributor
timestamp: September 23, 2024, 07:19:37 am
content: Quote from: RoGeorge on September 23, 2024, 06:49:14 amWhile I agree that 'mA' is technically correct, using a capital to abbreviate a proper name (Ampere, Volta), I think 'ma' has won by common usage.

author: Hydron
author_rank: Super Contributor
timestamp: September 23, 2024, 07:53:56 am
content: Personally I would not regard "ma" as ok just because some people haven't been educated as to what the proper capitalisation is.On topic, maybe look at the schematics of older instruments which do have similar source ranges. I have a Keithley 237 which comes in handy as a high-ish voltage source (up to 1.1kV), though it's held back a bit by it's age - only 14 bit DACs are used so you only get 4 digit source resolution.

author: SteveThackery
author_rank: Super Contributor
timestamp: September 23, 2024, 08:47:04 am
content: Quote from: moffy on September 23, 2024, 07:19:37 amOh, nooooo! The SI system of units is brilliant and we engineers owe it to ourselves, to learners and students, and to future generations to protect them against abuse.

author: Laval
author_rank: Regular Contributor
timestamp: September 23, 2024, 09:49:47 am
content: Quote from: Salitronic on September 23, 2024, 06:15:46 amThat's an interesting read, thank you.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 23, 2024, 12:05:16 pm
content: Long ago I worked for Burleigh Instruments who made HV amplifiers for piezoelectric devices. We used a fairly standard circuit (and simple) circuit with HV TV transistors. One trick was using series feedback resistors because voltage coefficient was a big factor. Nothing special, just RN70D resistors. We did find that MOX resistors were bad in this application. You'll find a HV amplifier circuit in The Art of Electronics; it's a direct rip-off of the Burleigh circuit and they hint as much in the text, though they did make one small but important improvement. Suggest you look at that. I've also attached the manual/schematic for one of the Burleigh products. It's a terrible scan but all I've got. We had only primitive op-amps back in the day and you'll do far better today as more open loop gain will improve linearity.

author: voltsandjolts
author_rank: Unknown
timestamp: September 23, 2024, 12:38:29 pm
content: AoE chapter 9has some ideas at page 695 onwards.

author: Laval
author_rank: Regular Contributor
timestamp: September 23, 2024, 03:24:36 pm
content: That is a lot of information and material to digest and thinker with. Thank you to everyone. Now, I have something to start with.An interesting (and scary) thing. VCR really is a big problem with HV.https://www.mouser.com/pdfdocs/Welwyn-VCR-Characteristics.pdf

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 23, 2024, 04:31:37 pm
content: Find an AoE2nd edition(there are pdfs online) and look at page 169. That's the basic circuit we used to use for just about everything, both adjustable supplies and amplifiers. Back then we used Delco HV transistors and then Telefunken BU209 and various others.

author: ZGoode
author_rank: Frequent Contributor
timestamp: September 23, 2024, 05:55:44 pm
content: Just want to join in here since I need something similar for my LCR bias fixture (although I only need up to 50V). I was looking at the PA89A chip for this since it seems to be a nice ready to go amplifier for higher voltage. Only downside is it is very expensive. Getting a second hand one on eBay (a used pull that may or may not work) is still going to be at least $100 if I go this route for this.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2024, 06:15:16 pm
content: What about noise requirements?There is something to learn about the design of the Data Precision 8200 from its specifications. Notice that the noise is 10 times higher on the 100 volt range than the 10 volt range. This means that the 100 volt output stage is operating at 10 times the gain of the 10 volt output stage, which makes sense. The gain of the output stage is multiplying the input voltage noise of the output stage.10/100 microvolts RMS over 10 kHz is not particularly demanding if the adjustable reference source is 10 volts. Any of the proposed high voltage output stages should meet this requirement.Given your output current requirements, I would be tempted to use a high voltage class-a instead of class-ab output stage, with an NPN emitter follower driving the output positive and an NPN current sink pulling the output negative with 5 milliamps or whatever. Or high voltage n-channel MOSFETs could be used just as easily. Many oscilloscope high voltage z-axis amplifiers operate somewhat like this with a transimpedance configuration, which is equivalent to an inverting amplifier.

author: Laval
author_rank: Regular Contributor
timestamp: September 23, 2024, 06:16:30 pm
content: Quote from: ZGoode on September 23, 2024, 05:55:44 pmYes, PA89A is the one i was referring to in my original post. I don't know your complete specification but would this amplifier do ?https://www.mouser.com/ProductDetail/Analog-Devices/ADA4522-2ARMZ?qs=UfUFg%2FkmHHHprAoiiQKXwA%3D%3D

author: ZGoode
author_rank: Frequent Contributor
timestamp: September 23, 2024, 06:23:17 pm
content: Oop, don't know how I missed a zero. I meant +/-500V, not 50VQuote from: Laval on September 23, 2024, 06:16:30 pm

author: Laval
author_rank: Regular Contributor
timestamp: September 23, 2024, 06:24:47 pm
content: Quote from: David Hess on September 23, 2024, 06:15:16 pmIndeed, the relative amount of noise is the same in terms of ppm for both ranges. I could probably live with that noise specification unless I can make better.

author: ZGoode
author_rank: Frequent Contributor
timestamp: September 23, 2024, 06:30:55 pm
content: The Systron Donner M107 service manual might be a good starting point for a high voltage reference/amplifier. I was looking at this as well. The service manual has the full schematics for a high voltage system, I just haven't had the time to do a deep dive into the schematic yet.

author: David Hess
author_rank: Super Contributor
timestamp: September 23, 2024, 06:33:53 pm
content: Quote from: Laval on September 23, 2024, 06:24:47 pmLow frequency noise from 0.1 to 10 Hz is usually more relevant.Reference noise almost always overwhelms amplifier noise by at least an order of magnitude. In this case, the noise from the high voltage divider is also a significant source. The feedback resistor of the high voltage divider can be bypassed with a capacitor to reduce high frequency gain, lowering high frequency noise, at the expense of settling time and bandwidth which probably do not matter. If this is done, then the amplifier's feedback point needs to be protected from high voltages, but this is not difficult.Under those conditions with an inverting high voltage amplifier, I think the noise could be improved by 10 times, at which point the reference noise will dominate.In the intended applications, 10 or 100 microvolts of noise over 10 kHz was probably insignificant, so I would not worry too much about achieving lower noise.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 23, 2024, 07:49:40 pm
content: We used to use the Apex parts when we needed a complete amplifier in a small space, or several of them. They make great parts but I'd never use them for a personal project because if I pop it, I can't fix it, only replace it. A discrete circuit can be fixed at low cost.

author: Laval
author_rank: Regular Contributor
timestamp: September 23, 2024, 11:41:37 pm
content: I looked at the schematics of the Systron-Donner M107 as hinted by ZGoode. I attached the schematics of the high voltage amplifier. Maybe it's because it's getting late but I don't understand how the feedback loop works.It requires so many power rails.

author: Someone
author_rank: Super Contributor
timestamp: September 24, 2024, 01:31:56 am
content: Quote from: Laval on September 23, 2024, 11:41:37 pmPretty simple if you draw it out with functional blocks. Note R15-R16 does not have a wire join dot.

author: moffy
author_rank: Super Contributor
timestamp: September 24, 2024, 02:09:57 am
content: For the feedback network, refer to 'A5, Analogue Schematic' which takes the output of the HV amp(25) and feeds it into the resistor networks and then into U6 which controls the input to the HV amp(15).

author: Laval
author_rank: Regular Contributor
timestamp: September 24, 2024, 11:33:43 am
content: Quote from: moffy on September 24, 2024, 02:09:57 amSometimes you just need to step back and look at the big picture ... and get a Coffee.I attached the A5 board schematics for completeness.

author: peter-h
author_rank: Super Contributor
timestamp: September 24, 2024, 02:26:39 pm
content: I've done tons of precision HV stuff years ago (ppm level stability; accuracy as good as an HP3420B which was state of the art then) and Q1 would be whether you need this software-controlled all the way up, or are happy with switchable ranges.The former is a very tricky analog design, but perhaps more importantly would be potentially really dangerous.Building a 1kV level op-amp is basically easy. You just need the supply rails and then some transistors. May have to use 2 or more transistors in series to get around Vce(max) - like they do in the M107 above (you could do it with a lot fewer bits today - see e.g.https://www.mouser.co.uk/c/?marcom=166756581). Optocouplers come in useful for level shifting. How to get the supply rails to start with, you can probably buy some photomultiplier PSU (I used to design those), or just a step-up mains transformer, rectifier and capacitorFeedback should be wirewound for best noise and stability and that is expensive.The switch will be multipole and switching various bits of circuitry in and out.

author: Laval
author_rank: Regular Contributor
timestamp: September 24, 2024, 02:59:14 pm
content: Quote from: peter-h on September 24, 2024, 02:26:39 pmI would really like to have everything controlled by software (digital control panel) but i can live with having to select the range manually preferably using digital control.

author: peter-h
author_rank: Super Contributor
timestamp: September 24, 2024, 04:09:14 pm
content: Do you need microvolt (say 1ppm) stability at 1V, and 1ppm also at 1000V? Imagine the loop gain variation there. But with a very slow control loop it might work.One approach which HP did on the 3314A: use relays (reed relays can go up to kV levels; I used them on PM power supplies to switch a few kV) to switch ranges. Then you can optimise each couple of decades or so.HV switching was solved in the 1970s with beam penetration CRT driving, and one was switching much higher voltages there.

author: Laval
author_rank: Regular Contributor
timestamp: September 24, 2024, 05:11:47 pm
content: Quote from: peter-h on September 24, 2024, 04:09:14 pmI'm not sure it will be possible to reach 1 ppm at 1kV. I can lower the spec a bit for that range.

author: Laval
author_rank: Regular Contributor
timestamp: September 24, 2024, 06:25:31 pm
content: Note on the power supply: I didn't think is was easy to find step up transformer for mains power.https://www.mouser.ca/ProductDetail/Hammond-Manufacturing/1182E110?qs=Uu2yXFAsHaxQsgMjFUxzqg%3D%3D220 V rms would give arounf 310 V peak. A 4x voltage multiplier would give around 1200V. 136 mA is way more then needed and 30 VA would hardly have a significant inrush current when powered on.This would make a quiet power supply too.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2024, 10:41:11 pm
content: Quote from: peter-h on September 24, 2024, 04:09:14 pmThere are a couple of ways around that.If a gain stage is added to the output of the operational amplifier, then the gain stage multiplies the operational amplifier's open loop gain. It also multiplies the operational amplifier's gain-bandwidth product. This is the equivalent to decompensating the operational amplifier, except that the open loop gain is also increased.Another way if a limited output range is acceptable is to add an offset to the output of the operational amplifier. So for instance with an operational amplifier output range of +/-10 volts, adding 1000 volts produces a 9990 to 1010 voltage output, with unity gain, so noise and offset are only multiplied by 1.In practice however with a good design, the precision high voltage divider will limit noise and accuracy. High impedance, voltage coefficient of resistance, and self heating will kill accuracy.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 24, 2024, 11:11:00 pm
content: Back when we were building HV stuff we always used a voltage doubler power supply. I've also made fairly decent 1 kV switchers with a self-wound pot core transformer- hint: double build enamel wire. They ran with a simple 12 VDC input. Also, for calibration use I can't see wanting more then 500 uA because you need to avoid heating whatever your calibrating.

author: peter-h
author_rank: Super Contributor
timestamp: September 25, 2024, 07:59:53 am
content: Yes, HV engineering is a lot of funI started my college/univ holiday jobs hand winding thoseQuoteYou can, but not using thesamecircuit for the low voltages.Stability is also hard at HV because of the heating effect on feedback resistors. My job was at a firm called Wallis Electronics and we used to individually test the resistors for TC (by placing them on a stable 10kV supply and seeing how they drift) and then building them in matched sets. Those were metal film or some such. One 500kV power supply used wirewound feedback though!! But at 1kV it should be doable.Is this a one-off, or production?

author: Laval
author_rank: Regular Contributor
timestamp: September 25, 2024, 10:16:13 am
content: Quote from: peter-h on September 25, 2024, 07:59:53 amYes, I'm thinking about having a different path for low voltage and HV. In fact, I'm thinking about developing the HV part in a modular way so that it can be added as an option. Of course, that requires the module to be prototyped and tested.Quote from: peter-h on September 25, 2024, 07:59:53 amLike Conrad Hoffman said, I will keep current low at least at high voltage (0.5 mA at 1kV is 0.5 W, 1W if I go to 1 mA). Hopefully this will help mitigate TC. I read that VCR (voltage coefficient) particularly effect thick film resistors and that specialized HV film resistors use a different internal geometry to mitigate that effect. Does voltage effect wirewound resistors as well and as much ?HV is something I don't know all that well. I have never worked with anything above 150 VDC and it scares the hell out of me. When I start prototyping with HV, I'll make sure to be very careful. I'm worried about the feedback loop, connecting 1kV output to the input of a precision op amp, even with a divider looks scary to me.Quote from: peter-h on September 25, 2024, 07:59:53 amYes it is a one-off product. I don't intend to market this product. If I get a decent design, I will certainly make it open source but only if the result is good. I'm at a very early stage and perhaps I should have waited to have something to share before posting. I'm supposed to get my DAC today so I can start evaluating figures like noise and accuracy. Unfortunately, it comes with an ADR445 voltage reference which is probably not the best for my application. I'm thinking more about using an ADR1399 but that's a discussion for the Metrology forum (which have already been discussed quite a lot).

author: peter-h
author_rank: Super Contributor
timestamp: September 25, 2024, 01:36:50 pm
content: Well, for a one-off, cost is irrelevant, so buy the best parts you can find, and keep it simple.Does it need to sink current as well as source? IOW, do you need to drive a capacitive load up and down? If no, use an emitter (or source) follower, otherwise use push pull or some such. This is the simplest circuit I can think ofThe constant current cct would get the output closer to the rail, but you don't need that if you can have say a +1200V rail.The reference etc will be trivial in this case. You can fill your boots with precision refs and precision op-amps today. ADR441 is nice, and I use a TLV2333 for op-amps. Dirt cheap and super precise.To drive the output, use some DAC and again, how much do you want to pay? For super resolution you can get 24 bit DACs.Above circuit may need some cap in the feedback, for stability. There are procedures for determining stability margins. You feed in some small signal AC and measure the gain over the frequency and make sure the gain is < 1 before the phase shift reaches 180. But in reality just put in some cap until it is stable and then increase it 10x

author: Laval
author_rank: Regular Contributor
timestamp: September 25, 2024, 05:50:24 pm
content: Quote from: peter-h on September 25, 2024, 01:36:50 pmIt is meant as a precision voltage source for high impedance load so it doesn't really have to source or sink any significant amount of current. I may at some point later on add a small current source (100mA - 250mA) but that would be a different feature.Thank you for the circuit. The small constant current source might probably be good since it would remove one resistor exposed to HV (and probably help mitigate voltage coefficient ... maybe).1.2 kV is about what the power supply should give considering the transformer linked above (rough estimation). That power supply is the first thing I'll have to prototype since I don't have any PSU that can provide anything above 60 VDC.Quote from: peter-h on September 25, 2024, 01:36:50 pmThe TLV2333 is great. 0.02 μV/°C is a great TC and it really is very cheap. The datasheet doesn't mention the offset current but it's fine.Quote from: peter-h on September 25, 2024, 01:36:50 pmThe DAC I'm planning to use is the AD5791. It's a 20 bit DAC with allegedly 1 ppm relative accuracy. Of course, even if it lives up to the expectations, overall accuracy will depend also on circuit design and the voltage reference.AD5791 is the most expensive component of the device so far. I should receive the evaluation board today. It comes with ADR441 vref.I have a couple of ADR1399 at home but they will need to be aged (1500 to 2000 hr) before i can really use them. ADR1399 (like LM399 and LTZ1000A) don't output an exact voltage value so it would need some trimming via an op amp with precise gain. Ref like LTC6655 or AD4XX or AD5XX series have more accurate output but they are less stable.

author: peter-h
author_rank: Super Contributor
timestamp: September 25, 2024, 05:51:44 pm
content: The only precision resistors are the two output ones which form the divider.

author: RandomUser
author_rank: Contributor
timestamp: September 25, 2024, 06:35:40 pm
content: Do you think those could be an option?Those are integrated high voltage dividers implemented on same substrate for good TCR/ratio coupling.https://www.koaspeer.com/products/resistors/networks/hvdp08/https://www.vishay.com/docs/68020/cdhv.pdfWhat do you think?The Vishay part has pretty high-ohmic values only, not sure if that becomes a problem.VCR + TempCo are not super great, but if thermally stable in the layout and used for a fixed voltage, that can be characterized and compensated for in software?

author: Zero999
author_rank: Super Contributor
timestamp: September 25, 2024, 06:53:11 pm
content: It might be easier to stabilise, if you add local feedback to the transistor output stage. This example has a gain of 101. The op-amp IC will need to have a -2V supply for the circuit to output voltages below 70V.

author: Laval
author_rank: Regular Contributor
timestamp: September 25, 2024, 11:49:10 pm
content: Quote from: RandomUser on September 25, 2024, 06:35:40 pmThey look interesting. I wish they had 99:1 ratio for a gain of 100 with a non-inverting amp. Are the VCR and TC given for the resistors or the ratio ? If it is for the resistors, then it would be good. Considering the circuit posted by Zero999, at 1kV, the voltage across R5would be about 9.9V and 990.1V across R4. Assuming 10 ppm/V, that means R5becomes (worst case) 10000.000099 Ohms and R4becomes 1000000.009901 Ohms so the gain becomes 1+(R4/R5) =101.0000000001 well within 0.01%. That's not counting TC though which is 100 ppm.It's time to unpack my DAC

author: BrianHG
author_rank: Super Contributor
timestamp: September 26, 2024, 01:13:07 am
content: Quote from: Zero999 on September 25, 2024, 06:53:11 pmDamn, I like that circuit....Could it be improved with a constant current source sink?I guess it doesn't matter if the OP only needs a DC reference output voltage instead of a higher speed AC waveform.

author: jbb
author_rank: Super Contributor
timestamp: September 26, 2024, 02:16:20 am
content: Any suggestions for main transistor Q1? Seems like a candidate for some series (cascode) devices.The base drive for Q1 might be an issue - trade off between power dissipation and required amplifier headroom. It would seem that a constant current source would do better, but I’ve never built such a high voltage linear circuit; maybe someone who has experience with such designs could comment. There’s nothing intrinsically wrong with just using a resistor and a higher DC input bus.Some other features come to mind…- output enable/disable feature- overcurrent protection- constant current pulldown so output capacitance can be discharged a bit faster than the divider resistance alone

author: peter-h
author_rank: Super Contributor
timestamp: September 26, 2024, 07:16:48 am
content: I posted a link to some 4kV MOSFETs above. Two of those... or three if replacing that pullup with a current source.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: September 26, 2024, 08:08:50 am
content: Quote from: peter-h on September 26, 2024, 07:16:48 amI wonder how a photovoltaic optocoupler + Gate-Source resistor would go here ? (One that is pure opto-current source without optional active turnoff)That would allow just one HV MOSFET. Be interesting to see relative noise numbers ?

author: peter-h
author_rank: Super Contributor
timestamp: September 26, 2024, 01:24:25 pm
content: Many years ago I used cadmium sulphide LDRs with an LED shining on them, as a 5kV optoisolator. The big LDRs are about 3cm diameterVery very slow response time though. Pointless in this case.The noise of these is irrelevant. They are inside the loop.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2024, 08:32:47 pm
content: Quote from: jbb on September 26, 2024, 02:16:20 amA cascode is a good idea.Here's an idea which uses an NPN cascode to drive a PNP cascode. Each transistor only ever sees up to a third of the supply voltage. I recommend more than just three transistors in each string, perhaps eight or ten, that way ordinary jellybean parts can be used.Note that the safe operating area of BJTs means that to get any more than a couple of mA, more than one device, whether it be in series (cascode) or parallel (use current sharing resistors), is required. In this case since it's high voltage, not current, cascode makes more sense.

author: peter-h
author_rank: Super Contributor
timestamp: September 26, 2024, 08:51:26 pm
content: However, you don't need to do this, in the 21st century. In 1970 etc it was the only way hence HP and Tek etc developed all those fancy circuits, but now you can get HV devices.

author: jbb
author_rank: Super Contributor
timestamp: September 27, 2024, 07:01:13 am
content: … that’s a good point. For a low volume (especially a one-off!) design, just buying a premium high V device is likely cheaper than developing & properly testing a cascode arrangement.Same argument applies to precision resistor arrays, I guess.

author: peter-h
author_rank: Super Contributor
timestamp: September 27, 2024, 07:21:02 am
content: Yes. I was doing precision analog, also HV, in the 1970s, and back then you had to develop all kinds of weird techniques. So a job got solved with 10x more parts than would be the case today. A lot of stuff used hand-selected / matched components, especially references, and (for HV) feedback resistors, temperature ovens for references, etc. It was an interesting area because those who developed those skills were able to sell products which nobody else could make. And some of it hasnotgot easier because so few people understand analog.Today you just buy a 1ppm/C ref for a few quid, an op-amp for less, and you are mostly done.Today's precision is amazing. I am building a product with an ADS1118. All of them have their internal Vref within about 0.01% - many times better than the spec. Even the 10 pence TL431 tends to be within 0.01% or so. Well, they are laser trimming, so why not laser trim to 0.01% even if the spec says 1%.In this case, building a 1kV op-amp, just throw in some 4kV devices and you are done. Just don't blow yourself up - this stuff is fatal!

author: Zero999
author_rank: Super Contributor
timestamp: September 29, 2024, 06:22:17 pm
content: Quote from: peter-h on September 26, 2024, 08:51:26 pmI looked at the links in your post. The IXTT02N450HV looks good. Would you advise using two of them, or just one and a cascode for the low current level shifting/driver transistors?https://www.mouser.co.uk/datasheet/2/240/media-3321478.pdf

author: peter-h
author_rank: Super Contributor
timestamp: October 01, 2024, 08:37:38 am
content: But why complicate things.

author: Laval
author_rank: Regular Contributor
timestamp: October 03, 2024, 02:27:28 pm
content: Quote from: peter-h on September 25, 2024, 01:36:50 pmThis circuit cannot output 0v or even near 0v. The transistor on the left will hit saturation before.Quote from: Zero999 on September 25, 2024, 06:53:11 pmThis circuit is puzzling me. It looks almost like Q2 is in a common base configuration but without the base being to ground (and in fact varying with the output). Could you give some explanation as to how it works ?

author: Zero999
author_rank: Super Contributor
timestamp: October 03, 2024, 08:49:24 pm
content: Quote from: Laval on October 03, 2024, 02:27:28 pmIt is common base, of sorts. Just because the base voltage can vary, it doesn't mean it's not common base. The same is true for a common emitter amplifier, without a capacitor, even though the emitter voltage varies, depending on the input. Where it's common base or emitter, depends on which terminal the input is connected to.Another way to look at it is, the BJT is really a type of differential amplifier. The collector current, depends on the voltage difference between the base and emitter. The collector resistor converts this current into a voltage, which is buffered by another transistor to reduce the impedance, so with now have a type of differential voltage amplifier. The emitter of Q2 is the non-inverting input and the base is the inverting input and Q1's emitter is the output. The output is connected to the inverting input via a potential divider (R2 & R3) which acts as a negative feedback network and the input signal is connected to the non-inverting input. This is similar to that of an op-amp in non-inverting configuration, except it has a massive voltage offset of VBE(about 0.6V), due to Q2's base-emitter junction. The output voltage is approximately equal to the following formula:VOUT= (VIN+VBE)*(1+R2/R3).Here's a graph showing the voltage at Q2's base (oa_out), when the input voltage is swept from 0 to 10V. The U1 corrects the voltage offset, hence a negative voltage is required to get to zero. The voltage drop across R2 due to the base current, means U1's output needs to be a little more negative, than the simple formula above would suggest.

author: BrianHG
author_rank: Super Contributor
timestamp: October 03, 2024, 09:00:04 pm
content: Quote from: Laval on October 03, 2024, 02:27:28 pmYou just figured out why I just 'love' that circuit...Quote from: BrianHG on September 26, 2024, 01:13:07 am

author: peter-h
author_rank: Super Contributor
timestamp: October 04, 2024, 11:39:12 am
content: QuoteYes, but this is really basic electronics. You can put a couple of diodes in series with the output transistor's emitter, which will drop 1.2V or so, so the output can swing down to 0.But if you want the ability tocontrola zero voltage then of course you also need a negative rail, which can be quite small, say -0.5V.