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| Nulling Input Offset Voltage |
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| jbb:
I’m sure this application is interesting, so I understand why you can’t share much. Four things come to mind. I’m no analog expert, so please take these with a grain of salt. Firstly, your environment is super dry, so have you been strict about ESD protection? Sometimes an ESD strike will damage a part and cause poor operation rather than simply destroy it. (During one internship in a low humidity lab I was basically The Death of MOSFETs.) Secondly, have you checked everything is nice and clean? No-clean solder leaves residue which is hard to clean. Maybe you could try washable flux and then wash it the boards clean? Thirdly, how about using a preamp? If you have +-5V available, you could use a precision amp to gain your reference signal up from 1.2V to say 4V (G=-3.3) and then reduce the gain of your high V amp to G=-20. This will dilute the impact of main amp input noise and offset (assuming preamp is better!). It will also turn the main amp into an inverting type with the input common mode at 0V (virtual ground for the win) and remove CMRR as an error term. Also makes a guard ring easier, which may be relevant with those high value feedback resistors. Finally, a composite amplifier might be a good solution. There is an LT application note on them somewhere on www.analog.com. |
| TinyMirrors:
Hey jbb, The environment is a production cleanroom with esd protection in everything we do. Wristbands, chairs, anti-static tiled floors are used for protection. I've cleaned the boards using detergent 8 and dionized water in an ultra sonic bath so there should be virtually no residue. There isn't any that I can see at least. I really like the composite amplifier solution. It's also written by Jim Williams!! I've got a 3 hole binder of his application notes on my desk. I should have known he'd have a solution. Thanks! This is really the direction of a solution I'm looking for. This is much easier to implement than the chopper amp setup I was contemplating with the spare channel. There aren't many if any in person analog people around to gather ideas so I really appreciate the help. |
| Gyro:
--- Quote ---I've cleaned the boards using detergent 8 and dionized water in an ultra sonic bath so there should be virtually no residue. There isn't any that I can see at least. --- End quote --- Hmmm, if you've got non-hermetic epoxy packages involved, not to mention the PCB, it may not be as 'humidity controlled' as you think. My bet is that there's a good deal of drying out still going on. After a wash like that you can probably expect a good deal of settling time before it stabilizes. FR4 PCB substrates in particular take a long time to dry out and you've got high voltages involved. |
| nick_d:
Thanks for posting the schematic! It is an interesting problem. I am not super familiar with low-offset stuff or active offset nulling and so on, so my thoughts were more along the lines of avoiding the issue by applying engineering principles, forgive if I am stating the obvious as I feel it useful to eliminate obvious things first. You said the input stage is powered separately and from a 5V reference. Have you checked the specs carefully? The current drive it can provide and the capacitive load it can drive without oscillation? It seems risky to me to use a reference as a power supply. Can you swap it out for something like a 78L05 or LP2950? Op amps have very good CMRR so I do not see the need for a fancy power supply. Having more current drive will probably improve regulation more than basing it off a super accurate reference. Now about the input offset. In principle this occurs because the input current, while theoretically zero, isn't zero. It may be microamps. So even if you hold the input at PRECISELY some reference voltage, the voltage the op-amp sees at the input will vary slightly due to the input current and the tiny voltages that this develops e.g. if there is input resistance. To a large extent this is unavoidable and it also should not matter overly much since it mostly just behaves like an extra input resistor (or impedance) or an extra feedback resistor or an extra resistor to ground or similar, and this should have fairly repeatable and predictable effects through the output swing and the input operating range. Input current can be affected by temperature, but you say the temperature is not changing significantly. So it does seem mysterious so far. The above analysis assumes you are holding the inputs at precisely the wanted voltages and any offset error is internal to the op-amp. I suspect this is not the case. My concern is that changes in the input offset current is changing the voltages in the resistive divider networks feeding the inputs, because of the high value resistors you have chosen. With those 1M resistors you'll get 1 uA per volt so about 5 uA. An input offset current of just .1uA could change the current in the upper or lower leg of the divider by as much as 2%, you're not seeing such an extreme drift however I think you can see my point that such large resistor values are risky. I could see why to use 1M dividers in battery powered equipment but that's normally in places where some drift or offset is unnoticeable, and I would never use 1M in mains powered equipment at all, I would aim for 100k usually. Theoretically, if you have only negative feedback and all factors such as temperature, supply voltage and RFI are controlled, then it is totally impossible for your output to drift over time. However, it can happen if there is some positive feedback in the system. And I think that is your case. As you use different values at the different inputs (680k, 1M etc) then a tiny fluctuation in offset current can have a larger effect at the positive terminal than negative hence causing the drift to become self reinforcing until another fluctuation. You said the filter can't be changed. So you can't change the 680k to 68k? Then what about buffering it with a high quality op-amp... NE5534? Anyway. I must say that blanket statements about what can and can't be changed are not super helpful. Of course a minimal change solution may be sought, but surely if the problem turns out to lie with a particular part then it would be sensible to change it. Anyway, the things proposed here are easy to test. I would have thought that once you have the test operational it would be a fairly simple matter to swap things until you find the source of the drift. Hopefully I gave some ideas to try. cheers, Nick |
| David Hess:
You have a massive DC resistance imbalance between the inputs so unless that is a low input bias current operational amplifier with JFET or MOSFET inputs, additional drift is being created. Adding another 499k resistor in series with the inverting input will remove this source of error. The resistor should be bypassed with a small capacitor to maintain a low AC impedance at the inverting input. Ignore this if the operational amplifier has low input bias current which is likely. See below. 75 microvolts of drift is certainly possible with a JFET or MOSFET input operational amplifier. Since this is apparently a high voltage part, it is probably a MOSFET design and they are the worst in this respect. If this is the case, the only solution is to use a better design with an auxiliary amplifier to null the offset of the high voltage amplifier. This is actually a pretty common problem when high voltage operational amplifiers are used. There is another possible significant source of error in your circuit which should be considered. The 83.3 volts across R1 is likely to cause the gain to drift and may explain the change in output voltage you are seeing. Several smaller resistors in series will improve this situation or a resistor rated for high voltage operation will help. |
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