Offset nulling an op-amp

[Tomble]

Hi again guys! Been off the forum for quite a while now, I've been busy working on my new website (such as it is) and mostly not doing any electronics due to lack of time or free cash for further parts beyond what I have or a new multimeter ho hum.

Anyway, past week or so I got working on a couple useful little projects, and I found that one of the things works about 8 times better after I do a relatively shoddy job of offset nulling its op-amp, compared to having no offset nulling at all, so I'd like to do the job properly. Unfortunately I don't fully understand nulling and the topic seems mostly glossed over where I've seen it, presumably as most applications can get by without it.

My op-amp (TL061) datasheet specifies a 100K trimmer for the job. Presumably part of that is because it's sold as a low-power one so you wouldn't want to waste much current. As it is, I only have a bunch of 1K trimmers and a couple of 200K ones. So far I tried the nulling with the 1K and got as far as I did, but it was pretty hair-trigger. Things seem to be working but it occurred to me after that I may have been quietly toasting sensitive parts of the chip that way?? Even for 741s etc the usual suggested trimmers seem to be 10K.

So first question, is it a problem to use other sizes of trimmer, besides wasted power if it's low R? Is there conversely a point where it's too high?

Secondly, as I don't really get how the nulling network works: If I acheived nulling with eg, a 61% setting on a 1K trimmer, would I achieve it with a 61% setting (ignoring imperfections in the components) on say a 15K network too? I'm thinking I'd like to constrain the range of the trimmer to roughly where the right point is with a bunch of precision resistors either side of it, but obviously that's no good if that point shifts when I change the total size of the divider...

Third question (sorry!): I did my adjusting so far by shorting the inputs together to a moderately stiff 1/2Vcc level (which is the reference point for the real circuit) and then measuring the voltage between that and the output. Is that adequate? It's a FET-input chip if it makes a difference there.

Thanks in advance for any help you can give!

 

[eng1]

Anyway, past week or so I got working on a couple useful little projects, and I found that one of the things works about 8 times better after I do a relatively shoddy job of offset nulling its op-amp, compared to having no offset nulling at all, so I'd like to do the job properly. Unfortunately I don't fully understand nulling and the topic seems mostly glossed over where I've seen it, presumably as most applications can get by without it.

Hi Tomble. Application note SLOA059 by Texas Instruments is a great reading on those topics.
I think that choosing a suitable op amp for you particular application might be more effective, without including trimming reisistors with their drawbacks.
If you require low input offset, a bipolar op amp might be a better option; generally, JFET op amps can be much worse as far as the input offset voltage is concerned.

Secondly, as I don't really get how the nulling network works: If I acheived nulling with eg, a 61% setting on a 1K trimmer, would I achieve it with a 61% setting (ignoring imperfections in the components) on say a 15K network too? I'm thinking I'd like to constrain the range of the trimmer to roughly where the right point is with a bunch of precision resistors either side of it, but obviously that's no good if that point shifts when I change the total size of the divider...

The potentiometer is connected in parallel with the tail reisistors of the differential pair (input stage of the op amp) and they're supposed to 'balance' the currents flowing through each legs, in order to cancel the input offset voltage.
If you want to use the external potentiometer, use the circuit suggested in the datasheet. A multi-turn trim pot would be a good idea.
Also consider that the adjustment might produce a bad result if the operative temperature changes.

 

[Tomble]

Hi, thanks for the reply

Hi Tomble. Application note SLOA059 by Texas Instruments is a great reading on those topics.

Cool, thanks! I've just downloaded it and am having a bit of a look through, although I'm not sure how well I'm following it. I'll probably come back to it a few times and see if it sinks in better.

I think that choosing a suitable op amp for you particular application might be more effective, without including trimming reisistors with their drawbacks.
If you require low input offset, a bipolar op amp might be a better option; generally, JFET op amps can be much worse as far as the input offset voltage is concerned.

*nod* Unfortunately the main requirement for my circuit (which isn't remotely important enough for me to order more parts for) is that it should have as low leakage current as possible. The offset voltage only actually comes into play (and it hadn't occurred to me that it would until I really thought about it) because I made a T-network(?) in the input circuit, referenced to the voltage on the + input. If both inputs are near enough the same voltage, that should be fine but otherwise there's a bit of leakage through that T-network.

I had actually tried one of my non-JFET op-amps (crappy old LM358) earlier on, and the leakage was much too much for it. I don't intend to spend money on other things for this project, it's not worth it. I've still a few other options for improving the circuit, and I'm confident I've already elminated most of the error term from the offset, but I wanted to know what I was actually doing with the trimmer- fill in the gaps in my knowledge and that.

The potentiometer is connected in parallel with the tail reisistors of the differential pair (input stage of the op amp) and they're supposed to 'balance' the currents flowing through each legs, in order to cancel the input offset voltage.
If you want to use the external potentiometer, use the circuit suggested in the datasheet. A multi-turn trim pot would be a good idea.

Yeah... funny thing, oftentimes it seems like posting a question helps me think about it better, I was vaguely coming to the conclusion that was how they worked. The detailed internal schematics in the datasheets generally give me a headache

This would explain why my use of a 1K trimmer (1) was horrendously difficult, the barest nudge with my screwdriver sent the output halfway across the scale, and (2) wound up with the setting extremely close to the 50% mark: smaller pots give a larger range of adjustment (too large) by minimising the contribution of those internal resistors, and larger pots give the higher resolution but might be out of range if higher than the datasheet. Well at least now I know the score I can probably sort something out...

Also consider that the adjustment might produce a bad result if the operative temperature changes.

Yup. That's part of why I'd wondered about padding out most of the range with precision resistors to reduce the error from the trimmer's tempco. Re the tempco of the Op-amp's offset itself, well I guess there's not a lot I can do about that short of buying some other op-amps or ditching the t-network (which I might yet do).