EEVblog #476 - Opamp Offset Voltage Measurement - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: May 31, 2013, 10:25:21 pm
content: Dave investigates an issue with the input offset voltage on the Analog Devices AD8628 zero-drift chopper opamp.The answer will be presented in the next video.

author: Slothie
author_rank: Regular Contributor
timestamp: May 31, 2013, 11:59:12 pm
content: Since the +rail bypass cap seemed so critical I was surprised you didn't try a bigger one to see if that had any effect.Other than that I cant say - to me the 741 is a mysterious new-fangled device!!

author: mswhin63
author_rank: Frequent Contributor
timestamp: June 01, 2013, 01:33:42 am
content: Still good enough to be a fundamental Friday.What was you output load?

author: EEVblog
author_rank: Unknown
timestamp: June 01, 2013, 01:44:58 am
content: Quote from: mswhin63 on June 01, 2013, 01:33:42 amJust the multimeter. Thought I mentioned that? maybe not.Shooting the real FF now...

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2013, 02:21:29 am
content: In an OP-AMP Application Handbook published by AD they a circuit for measuring offset voltage (attached). Perhaps bias current is the reason for inconsistency here because 1K resistor is only present at inverting input.

author: c4757p
author_rank: Super Contributor
timestamp: June 01, 2013, 02:34:49 am
content: Quote from: Alexei.Polkhanov on June 01, 2013, 02:21:29 amWell, at the maximum 100pA bias current, and about 1k on the inverting input, you get an induced bias of 100nV. I don't think that's it.

author: Rufus
author_rank: Super Contributor
timestamp: June 01, 2013, 02:48:54 am
content: I suspect the issue will have something to do with common mode voltage as well as supply voltage.Would be interesting to put your reference on the wiper of a pot across the rails (with some decoupling) and see what happens.I imagine it is something to do the amount or imbalance of charge injection on all the CMOS switches varying with rail and/or rail to signal voltage. I could see the high frequency impedance of the supply rails having an effect on that. Might be interesting to see if a couple of ferrite beads on the rails (to increase the impedance) makes a difference - might even help.Imbalance of impedance feeding the inputs might make a difference, sticking a 1k in series with the other input might be worth a try.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2013, 04:20:56 am
content: Analog Devices have "Multisim Component Evaluator Analog Devices Edition" free to downloat on their website, which presumably have accurate SPICE models for AD devices. I used it once before and model was pretty accurate.Results for single rail supply seem to match what David has on video. With 5V supply it shows 400pV output, but if I lower supply to 3V it will go up to 35mV. When I tried dual, positive negative supply configuration simulation shows something completely unreal - output goes to +2.5V. It can be because I did something stupid in my simulation or it is really trying to tell me that this OPAMP won't work with dual supply.

author: SeanB
author_rank: Super Contributor
timestamp: June 01, 2013, 05:15:15 am
content: Ground needs to move to other side of R1, otherwise you are measuring the full gain of the opamp amplifying the voltage generated across the 1k resistor by the input bias current.

author: toli
author_rank: Frequent Contributor
timestamp: June 01, 2013, 06:51:19 am
content: Quote from: c4757p on June 01, 2013, 02:34:49 amThis current actually flows through the 100K and not the 1K (it can't go through the 1K since the voltage across this resistor is the offset voltage, and doesn't depend on the bias/offset current). The reason we put a (1K||100K) resistor at the non-inverting input to get rid of the offset caused by Ibias is that this voltage will than be amplified by 101.So the gain for the bias current from the inverting input will be -100K, and from the non-inverting will be 101*(100K||1K) which yields 100K and the two gain cancel the effect of IbiasBTW, as Rufus has suggested, there's a difference in the input CM voltage when you switch between dual and single rail. Adding a simple voltage divider between the two supply lines will keep it in the middle of the supply range and will remove this effect from the measurements.

author: EEVblog
author_rank: Unknown
timestamp: June 01, 2013, 06:58:12 am
content: Next video has already been shot, but I'm going to be an evil bastard and hold it over until next Friday

author: mimmus78
author_rank: Unknown
timestamp: June 01, 2013, 07:22:55 am
content: How much donations you want to publish before next Friday?:-)

author: EEVblog
author_rank: Unknown
timestamp: June 01, 2013, 07:26:06 am
content: Quote from: mimmus78 on June 01, 2013, 07:22:55 amA day off next Friday sounds pretty priceless to me

author: Bored@Work
author_rank: Super Contributor
timestamp: June 01, 2013, 07:45:28 am
content: Ok, you are applying a wrong definition of input offset voltage. Input offset voltage is the voltage you need to apply to the two op-amp inputs to get a zero output.

author: IanB
author_rank: Super Contributor
timestamp: June 01, 2013, 07:46:01 am
content: By the end of the video I was definitely looking to see a 1 k resistor on the +ve input between pin 3 and ground. What difference does it make to the results if you insert such a resistor?

author: TheWelly888
author_rank: Frequent Contributor
timestamp: June 01, 2013, 09:46:22 am
content: Dave - you got me looking at the chopper amp video again and in that, you made the point that there needs to be a load across the output of the chopper amp. The only load I can see across the output is the multimeter which I assume is 10M ohms. Maybe try a 10k or 1k load on the output and see if there is any effect?

author: ddavidebor
author_rank: Super Contributor
timestamp: June 01, 2013, 10:06:32 am
content: Baaad foundamental friday.No, solution, no teory, no whiteboard.I suggest you to publish the new one sooner, this was a thumbs down for me.And please do a mailbag, you've alredy 2 sagan of mail to open.

author: kkp
author_rank: Contributor
timestamp: June 01, 2013, 10:47:33 am
content: ..And it's not thermocouple effect either. If so, it wouldn't change that quickly with supply voltage...And the CMRR is typically better than that.I have seen this before; It can happen when you connect an excessive capacitive load and/or antenna (meter lead) directly to the output of the amplifier.Squinting at the video I can't make out the print on R9 (the output series resistor Dave likely put in the ucurrent for the same reason), it is populated, but the print looks a bit short to be '101', and it's not in the DaveCad*.A bit of capacitance across the feedback resistor would probably be an idea, too. There should be 500mV p-p noise coming out of it without one.Come back next week to see if our AD-caped hero defeats the evil Doctor Offset.*Generating netlists in this is a pain.

author: firewalker
author_rank: Super Contributor
timestamp: June 01, 2013, 11:03:43 am
content: Could you try lower values to the feedback network?Alexander.

author: EEVblog
author_rank: Unknown
timestamp: June 01, 2013, 11:45:08 am
content: Quote from: kkp on June 01, 2013, 10:47:33 amHow do you figure that?

author: EEVblog
author_rank: Unknown
timestamp: June 01, 2013, 11:53:24 am
content: Quote from: ddavidebor on June 01, 2013, 10:06:32 amand the world didn't end!

author: millerb
author_rank: Regular Contributor
timestamp: June 01, 2013, 11:56:34 am
content: Quote from: ddavidebor on June 01, 2013, 10:06:32 amI second that, it's fun to see all the weird stuff people send Dave.

author: kkp
author_rank: Contributor
timestamp: June 01, 2013, 12:21:09 pm
content: 500mV noise: By trying to multiply in my head, and failing to do a reality check afterwards.Somehow I was thinking of the gain as 100k, with a resulting bandwidth 20Hz, and got it wrong by another factor of 10.With 20kHz bandwidth, the 22nV/sqrtHz is dominating, so that should be sqrt(20kHz)*22nV/sqrtHz*gain=311uV RMS..(checked. sounds about the right ballpark too.)

author: JasperNL
author_rank: Contributor
timestamp: June 01, 2013, 12:53:55 pm
content: My guess would be the output load. I can remember a similar problem in my constant current load project. I also used auto-zero op-amp. Can't remember the actual part. The offset was little bit out of spec. Causes by a small capacitive load at the output. A decoupling resistor fixed the problem but decreased to bandwidth.Can remember it pissed me off and ditched the partUsed a non auto-zero part instead and accepted the higher offset.

author: SeanB
author_rank: Super Contributor
timestamp: June 01, 2013, 01:36:27 pm
content: In the appnotes they show a 470p capacitor across the top gain setting resistor ( funny enough 100k and 1k) to give a low pass filter, so it is likely that what is being picked up is the output noise, it does have noise spurs at close to DC and at 15kHz. The load will not be a worry, it does already have a 100k load, the gain setting resistor. Adding a low pass RC filter to the output may help in any case.

author: valentinc
author_rank: Regular Contributor
timestamp: June 01, 2013, 01:47:55 pm
content: Quote from: Alexei.Polkhanov on June 01, 2013, 02:21:29 amI agree with this... Due to the bias currents (that are not zero) and the fact that the non-inverting input is tied directly to ground and the inverting one is tied to ground via a 1K resistor... The bias current + the current from the output (the amplified bias current that comes through the other 100K resistor makes the voltage at the inverting input be higher than that at the non-inverting input, hence you don't measure the real offset voltage, but the offset voltage plus that voltage drop across the 1K resistor...To measure the proper offset voltage you must bias the non-inverting and inverting input at the same potential (as in the pictures Alexei posted) ....And the current through the output stage of the op-amp probably varies with the supply voltage, and the higher the current through the feedback resistors, the higher the voltage drop...

author: onlooker
author_rank: Frequent Contributor
timestamp: June 01, 2013, 02:41:40 pm
content: QuoteI thought it was already mentioned, AD8628's bias current is <0.1nA, with Dave's test circuitry, the inference on the output voltage should be <0.01mV (~=0.1nA x 1kR x 100). That is, the bias/leak current can't explain the discrepancy in the order of ~>0.1mV.

author: Unknown
author_rank: Unknown
timestamp: June 01, 2013, 02:52:05 pm
content: I do not think that's the best way to measure the offset.Anyway, it is clear that something is not right in the AD8628 @ 5V.What do you think about figure 7 on page 6?Why not ask to Analog Devices, to see what think?

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2013, 05:58:28 pm
content: Quote from: SeanB on June 01, 2013, 05:15:15 amOps, daaa, Friday night, all out of coffee.Corrected version gives 95uV on output, which is higher than 480pV as in single supply, but not 0.330mV that was on video.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2013, 06:40:38 pm
content: I think I found one of possible solutions. On simulation (see picture below) I have negative supply -5V, while positive is +2.5V. It gives me +223uV on output.I had similar problem when I was playing with data conditioning circuit for photodiodes. I eventually got triple output power supply that can make sure that positive and negative rails always match closely to each other regardless of load.

author: Rufus
author_rank: Super Contributor
timestamp: June 01, 2013, 07:19:53 pm
content: Quote from: Alexei.Polkhanov on June 01, 2013, 06:40:38 pmWith an absolute maximum supply voltage specification of 6v running it on 7.5v isn't a possible solution.Did running the simulation do any permanent damage to the AD8628 model?

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 01, 2013, 07:52:30 pm
content: Quote from: Rufus on June 01, 2013, 07:19:53 pmHahahaha, no, no smoke. If I make it +1.5/-3.5V to keep it within spec results are same.

author: Big_Al
author_rank: Contributor
timestamp: June 02, 2013, 11:30:08 pm
content: The circuit is oscillating man !Bang a cap across the 100k for compensation.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: June 03, 2013, 05:02:36 am
content: Quote from: Big_Al on June 02, 2013, 11:30:08 pmHow do you know that? It certainly does not oscillate in simulation. I don't have any 8628 at hand unfortunately to try it out.

author: envisionelec
author_rank: Frequent Contributor
timestamp: June 03, 2013, 12:02:45 pm
content: Parasitic thermocouples? At such low current, even the tiniest dissimilar metal junction would introduce an offset voltage.Another possibility is that the resistors are self heating and producing flicker noise (pink distribution) whose charge injection interprets as an overall DC offset maybe due to heterodyning with the chopper frequency. Lower tc resistors would likely help for a quick test. One techique is to use an outer chopper with a frequency below the 1/f of the 'inner' chopper.Possibly repair it with Kelvin connection.

author: Bored@Work
author_rank: Super Contributor
timestamp: June 06, 2013, 04:33:58 pm
content: Dave, you can endlessly wank with your buddy Chris on the Amp Hour that it doesn't matter to use the wrong definition. Here is a hint for you. If you compare your measurements to data in the datasheet, it would be really clever to compare the same things, not apples and oranges.And the right definition has nothing to do with academic correctness. Feeding in the offset voltage into the input is what you'd have to do if you care to compensate the offset. A very practical thing, building a voltage divider, adjustable over a small range, to feed into the input.So whatever you will present in your next video, however you will bend your results until they match the value in the datasheet, you have shown nothing - except that you compare apples and oranges.

author: IanB
author_rank: Super Contributor
timestamp: June 07, 2013, 06:05:36 am
content: Quote from: IanB on June 01, 2013, 07:46:01 amSo apparently it does help a bit, but it's not the whole story...

author: MaiMariarti
author_rank: Newbie
timestamp: December 29, 2019, 08:26:11 pm
content: Did anyone find a solution to this? Is having an unbalanced dual supply the answer?Was having no output load the problem (besides the high Z meter)? No solution from ADI?I guess I won't pick this Op Amp and go with the MAX4239 until this problem is resolved.If it has this problem, what other problems are there kinda thing...