Help choosing an opamp for precision voltage amplification. "Volt Nut Thread" - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 20, 2017, 09:52:07 pm
content: So I am restoring an HP3455A 6 digit rack meter and when I'm done I would like to calibrate it myself, Im not sure how it costs to send a precision vom to the keysight lab but I am pretty sure I cant afford it. Anyways I have a [ur="http://www.voltagestandard.com/DMMCheck.html"l]DMM Check, which I bought two years ago and has been very handy for handheld meters. I recently took it out and connected it to a linear PSU, I plan to leave it on to burn the reference in until January when I send it in for a recal right after the calibrator has had his HP3458 recaled at keysight.So I would like to make a little piece of gear that I can plug the DMMCheck in to, which uses it's DC reference to generate ranges (preferably dynamically programmable) of DC voltage (i.e 5,10,20,50 etc), and then build an AC transfer and do the same thing. I also would love to do the same thing with it's one mA current source. The problem is, while I do a lot of electronics work it is mostly either digital or RF, so as far as my experience with opamps go it is very basic, current buffers, comparators, single supply amplification without feedback, things like that.Really I don't need help as far as opamp theory goes, I can RTFM. What I need is someone to point me in the right direction on what sort of opamp I should choose and whether I need negative feedback or not. I would prefer the opamp be single rail supply instead of having to create a v ground or something but if that is not possible I understand.Really what I require from the opamp is just gain stability and precision, I know I will need to use .1% or better low tempco resistors to bias it. For example if my DC reference is calibrated and measured to 5.0000 volts when I add a gain of 2 I need 10.0000 volts, it would be nice if I could get the gain to the 100v+(600v would be awesome) range but if I need to use multiple amps for that I understand.Currently I am looking at theLT1001along with a digital pot, but I have a ton of precision opamps around that range all the way up to like 30 dollars an amp, some jfet some bipolar. So exactly what specs does an opamp need to meet to give me a precision DC amplification up to 4 digits past the decimal? Is a digital pot going to cause accuracy issues? Is there maybe an easy programmable gain amplifier out there that meets the specs I need? Lastly if I wanted to amplify the current source from 1.0000mA to 10.0000 amps what kind of parts should I be looking at to do that? Is that what a current feedback opamp is for?

author: danadak
author_rank: Super Contributor
timestamp: December 20, 2017, 11:12:10 pm
content: A starting point -http://www.nutsvolts.com/magazine/article/build_a_01_accurate_voltage_referencehttps://hackaday.com/2015/03/23/build-a-precision-voltage-reference-box/Regards, Dana.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 12:34:02 am
content: Both of those projects are only voltage references/standards just like my DMMCheck. I am looking for a way to amplify the calibrated Voltage Reference output (DMMCheck) to multiple ranges and retain accuracy of the original reference while amplifying. Using multiple references won't work either because they wont all be calibrated on a an HP multi digit meter that itself has been freshly calibrated.As I said in the posting above Im sure this problem is pretty solvable with an opamp, im just not sure how to choose the right opamp, im pretty sure just using an LM386 in single supply no fead back configuration with 1% resistors isn't going to cut it, or maybe it will. Im just not sure about things like do I care about noise figures, I mean my voltages are all high enough to consume them, I don't think offset voltages or rail to rail outputs are really something I would need to worry about either? So is drift the only thing im looking at and what is the minimal drift that I need to keep the accuracy I listed above? Im not sure if I need feed back and if so + or -, Maybe I should just run the DMMCheck + and - terminals to an INA? Are there PGAs that will work in applications like mine?I would love to read a project about someone taking a voltage reference and then using it to make higher voltages with the same accuracy.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2017, 01:13:57 am
content: You must use negative feedback for the precision that you want.Precision operational amplifiers have low offset voltage, low offset voltage drift, low input bias current (except where the lowest noise is required), low input bias current drift, low noise, high open loop gain, high common mode rejection, and high power supply rejection.The LT1001 is a good place to start when source impedances are in the 10s of kilohms. The LT1007 is used at lower impedances where self heating of the feedback network and output stage of the operational amplifier will become a problem and the LT1008/LT1012/LT1097 are used at higher impedances. The trade off is noise with lower input bias current parts having higher voltage noise. For just messing around, the LT1097 would be my first choice because of its flexibility and price but there is nothing wrong with the LT1001.The LT1001 is an "OP-07" type of operational amplifier so there are lots of alternatives including the OP-07, OP-77, OP-177, and whatever the equivalents from Analog Devices and Burr-Brown were; I never memorized their part numbers.For greater precision at the cost of higher noise and some other compromises, a chopper stabilized amplifier would be suitable but I do not think there would be any advantage to using one here.

author: Cerebus
author_rank: Super Contributor
timestamp: December 21, 2017, 01:43:32 am
content: Quote from: rwgast_lowlevellogicdesin on December 20, 2017, 09:52:07 pmIf you don't realize that youwillneed negative feedback just to use an op ampat all(unless you're aiming at producing a rather poor comparator) then youreallydon't yet have a handle on op amp theory at the most basic level. That you asked that about negative feedback betrays your ignorance (I don't mean the word in a pejorative fashion) on the whole topic of op amps. Time to RTFM and once you've done that you'll have a much better idea of what you're looking for and can come back and ask some rather more focused questions.There are a zillion suggestions in the forum on what literature there is available on op amp basics and I don't propose to reiterate those lists here - the forum search tool is your friend.

author: danadak
author_rank: Super Contributor
timestamp: December 21, 2017, 02:06:13 am
content: Key issues for OpAmp choice -1) Drift2) Linearity3) PSRR4) Noise (you do care about this)5) Offsets6) Ratio match fdbk resistorsA fixed gain block might be useful with laser trimmed G -http://www.analog.com/media/en/technical-documentation/data-sheets/AD8273.pdfhttp://www.analog.com/media/en/technical-documentation/tech-articles/Single-IC-Gain-Block-Provides-Accurate-Gains-from-0p25-to-6.pdfRegards, Dana.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 04:57:04 am
content: @David, if you had bothered to read my post before commenting, I clearly stated I don't know much about opamps besides the basics, I haven't had any real reason to use them until now, in RF everything mmic's and transistors. Anytime I have needed a control loop for something I just do with a micro controller and some sort of sensor. Why would you waste your time typing a reply about my ignorance, frankly if you wanted to participate in the thread im sure it would have taken you just as much or less time to recommend an amplifier.I hoped someone could suggest a part or at least minimal specs for what I would like to attempt, seems like a pretty simple project to learn opamp theory with as long as the right one is chosen.. I have absoultely nothing else I want to build that requires an opamp so this is as good as any project to get my feet wet with opamp theory.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 05:05:46 am
content: @danadak, offset is just the smallest difference that can be handled between the inverting and non inverting inputs right? Why do I need to be concerned with that when the 5vdc is going to be the smallest input swing the opamp will ever see?

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: December 21, 2017, 05:11:33 am
content: To go from 5 to 10 volts the best thing would be *2 using an LTC1043 (the schematic is in the datasheet). Its 4 ppm accuracy. You will not get that using an opamp. You may need to buffer the output--that bit is not shown in the datasheet. Use a zero-drift buffer, ADA4522, LT2057, etc.The problem with any opamp is not just the opamp but also the accompanying resistors. Good resistors (low TCR, high precision) will cost $$.Unfortunately the LTC1043 solution runs out of voltage quickly. Its Vdd max is 18volts IIRC.Lots of information about this on the net and this forum.

author: danadak
author_rank: Super Contributor
timestamp: December 21, 2017, 11:18:00 am
content: OpAmp input offset contributes to the output error.http://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdfLook at page 15 for equation defining output including offset error term -http://www.ti.com/lit/an/sloa059/sloa059.pdfDC errors ref material -http://ww1.microchip.com/downloads/en/AppNotes/01177a.pdfhttps://www.maximintegrated.com/en/app-notes/index.mvp/id/5693http://www.analog.com/media/en/training-seminars/tutorials/MT-054.pdfRegards, Dana.

author: Cerebus
author_rank: Super Contributor
timestamp: December 21, 2017, 01:46:58 pm
content: Quote from: rwgast_lowlevellogicdesin on December 21, 2017, 04:57:04 amI suspect that it's me that you're replying to, not David. A precision voltage reference isn't the thing to try designing and building to get your feet wet with op amps. Precision voltage references are some of the hardest things to design and build, there is a thread in the Metrology topic on the design of one reference that currently runs to 95 pages. What you're saying is something like a neophyte machinist deciding that his first project is going to be designing and building a vernier micrometer and his question is "what alloy should I use and how do you turn on the milling machine?" or a neophyte logic designer saying "I'm going to design and build a DSSS decoder, what NAND gate should I use and I don't think I'll need to use registers or a clock?".You don't seem to want to accept that, contrary to what you say, you'd don't even understand the very basics of op amp theory. Statements like:Quote from: rwgast_lowlevellogicdesin on December 21, 2017, 05:05:46 ambetray the fact that you just haven't grasped the basics. Both the statements you make there are wrong.It is clear that you don't understand that this a case of "learning to walk before you run". In fact it's more like "learning to walk before you attempt the 110m hurdles". If you can't accept that then there's little more any of us can offer you.We're all trying to give you what youneedbut you seem to be holding out for getting what youwant. If you won't listen to advice that is intended to be helpful I can at least give you what you're asking for (even though the question is wrong, hence the answer will be too): use an LT1028 op amp, use either Vishay S102 series or VHP series resistors.

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2017, 04:18:52 pm
content: Quote from: rwgast_lowlevellogicdesin on December 21, 2017, 05:05:46 amNo.I'm afraid Cerberus' comments seem perceptive.

author: Cerebus
author_rank: Super Contributor
timestamp: December 21, 2017, 05:28:07 pm
content: Quote from: tggzzz on December 21, 2017, 04:18:52 pmIt's "Cerebus", the misspelling is deliberate (but, possibly apocryphally, wasn't when Dave Sim originally named the comic book character).

author: tggzzz
author_rank: Super Contributor
timestamp: December 21, 2017, 06:16:14 pm
content: Quote from: Cerebus on December 21, 2017, 05:28:07 pmOh..... picky picky pickyBut then without guidance my surname gets misspelled ~80% of the time!

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2017, 07:40:31 pm
content: To get twice the voltage one could consider building something like a second 5 V and maybe 10 V reference that can be tuned to match the existing 5 V two 5 V sources in series gives 10 V, and so on.For the larger voltages, they is the option to have a string of references powered from a single source. For cost reasons this may be different values (e.g. around 7 V) and not the nice even steps of 5 V. In a stabilized (could be passive) case even higher TC references can be acceptable.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 21, 2017, 10:57:20 pm
content: Ok thank you all for chip suggestions.Maybe I am biting off more than I can chew, I guess I looked at this problem like I want to build a headphone amp lets say, but I am new to opamps. There is plenty of documentation on the best opamps to use for this type of thing around the internet. I guess what I am asking is on a different level though.Im not even sure I should be using an opamp... as I said I have never needed to use an opamp for anything besides the basics like a buffer or comparator when attaching things to a micro. In RF everything is done with transistors/mmic gain blocks because op amps don't generally have a good enough NF, and even with RF I usually use a gain block if I am doing anything above HF because it takes a whole lot of equipment I don't own to make sure there is the lowest noise possible and there is no way I can hand build something better for signal amplification that an mmic with a .5nF.Basically I just want to be able to calibrate this HP3455a, and I thought to myself weaw all I have a super accurate trimmed voltage reference which I can even have re trimmed by someone with a calibrated setup if I wanted to translate that accuracy it should be a simple as amplifying it. In RF there is a whole lot of things require to highest accurate drift free gain, including heating/cooling to temperature stabilize the drift. I figured at DC this is exactly the kind of thing people use a precision opamp for.Out of curiosity if I am asking the wrong questions and its not what type of opamp should I use to amplify a precision reference what should be? The best way to solve this problem is my opinion is to somehow get 1000v DC ref have it cal'ed and then use low drift .01% resistors (maybe even warmed a little) to make voltage dividers down all the ranges. One time I just saw a video of someone using a some box from Fluke where you enter in a voltage for each range and it would generate it, I have no idea how it worked but Id assumed it only used one accurate calibrated voltage reference and the just amplified or attenuated it depending on the range.

author: danadak
author_rank: Super Contributor
timestamp: December 21, 2017, 11:09:26 pm
content: Here is a manual of a calibrator, and the method they used to generatethe various voltages -https://www.eevblog.com/forum/blog/eevblog-649-power-designs-2005-psu-teardown/?action=dlattach;attach=104692Regards, Dana.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: December 22, 2017, 12:00:18 am
content: After walking away from this and thinking a bit it does seem like the most practical way to make voltage ranging at higher voltages is just to connect a bunch of 10v references which can be switched in and out. The only problem with that is once again I would need a caled meter to make it work. Then there is using the 5v as a ref for a dac, but that seems like it would produce alot more error than an just an opamp.Finally what kind of precision are we looking at if one was just to pump the 5.0000v in to an lm386? Can the error be modled in spice?

author: hermit
author_rank: Frequent Contributor
timestamp: December 22, 2017, 04:58:38 am
content: Quote from: Cerebus on December 21, 2017, 01:46:58 pmWell, he's seems to be leagues ahead of the guy on the Arduino forum that wanted to build a meter to measure nano volts. When pointed out how hard it was he simply decided, no problem he would amplify the input. Problem solved, right? I'm a hack myself but even I knew what he was up against and suggested he build a 6 digit meter and see if he could get the last numbers to stop flopping around like dead fish. Stupid me. It was an easy thing to do. You just take an average!

author: tggzzz
author_rank: Super Contributor
timestamp: December 22, 2017, 09:42:11 am
content: Quote from: rwgast_lowlevellogicdesin on December 22, 2017, 12:00:18 amThat depends on the details of what is included in the spice model of the opamp - and that varies considerably. Quite frankly, it would do better to do a theoretical analysis.More importantly, at microvolt levels there are many other error sources; have a look at some of the threads in the metrology section of this forum.

author: danadak
author_rank: Super Contributor
timestamp: December 22, 2017, 10:19:31 am
content: QuoteI must be missing something. The LM386 is a terrible Amplifier for precisionwork. So why feed it with a precision source, are you trying to look at its linearity ?Regards, Dana.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 22, 2017, 01:42:48 pm
content: The 3455 is likely more accurate than the 5 V reference. So this would be more like an test for plausibility.One useful idea for the higher voltages is a Hammon type divider made from 6 equal resistors. This is a kind of clever setup to get a rather accurate divider for 1:10 and with an other step also 1:100. The main idea is to have a divider with 3 resistors in series against 3 resistors in parallel thus an rather accurate 1:9 resistor ratio. There is a thread in the metrologie section on this type of divider for a ration transfer.So for the high ranges it would take a resonable stable (but no need for calibration) high voltage reference and than use the Hammon divider to bring the voltage down to the 10 V range and use the 3455 to measure 1/10 or 1/100 the high voltage.

author: Inverted18650
author_rank: Frequent Contributor
timestamp: February 16, 2018, 10:55:15 pm
content: I realize this topic is a bit old, but I recently did something similar and I used the AD586JRZ. I used 2 of them in parallel to get a very precise 10V's.note:I have 4 of them from scrap PCBs that I purchased on eBay very cheaply. The circuit is on the datasheet page RevG. 11 of 16.http://www.analog.com/media/en/technical-documentation/data-sheets/AD586.pdfAlso someone referenced a link to Scullcom Hobby earlier and his latest project is exactly what you are after: