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I'm trying to get my head around how the offset NULL works for an op amp. I think i'm getting somewhere with it. The op amp i'm using is the TL081 from this datasheet: http://www.futurlec.com/Datasheet/Linear/TL084CN.pdf

I understand that the input offset is the voltage of the output when the two inputs are equal and I also understand that when you use a set of resistors to get a voltage gain this offset is also multiplied by the gain.

So my main question is by using a variable resistor on the offset NULL legs and getting this closer to 0v than the op amps current input offset does this mean I can use the op amp to amplify voltages in the range of mV now?

So if i use a gain of around 1000 would this now be acceptable to use given that i null the input offset?

Thanks

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  • \$\begingroup\$ A gain of 1000 is huge. I don't know what you're trying to amplify, but you should probably be using an instrumentation amplifier for it. \$\endgroup\$ – Matt Young Jan 29 '14 at 1:35
  • \$\begingroup\$ How about a gain of say 100 instead would this be more acceptable? Also would adjusting the offset NULL be useful in this case? And if i did use an instrumentation amplifier with 1000 gain would i still need to use the offset NULL? \$\endgroup\$ – user2126881 Jan 29 '14 at 1:48
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TL081 is not a particularly good choice for low DC voltages (it's better for AC coupled designs). Precision op-amps have offset voltages in the 10's of microvolts or lower, and there are even auto-zero op-amps that have negligible offset voltage, an drifts in the tens of nV/°C (at some significant cost in other characteristics). It's also quite noisy (25nV/\$ \sqrt{Hz}\$), but at least has a typical flicker noise corner frequency in the 1Hz range. Aside from high input Z, it has basically one really nice advantage, it's really cheap, and widely available, which is why I've actually designed such a 37-year-old op-amp into a new product recently.

A good general purpose (low noise, capable of handling +/-15V supplies, low distorion for AC signals) precision op-amp suitable for mV levels might be the OPA209A.

You can certainly null the offset voltage of your TL081 out using a trimpot as shown on the datasheet, but it won't stay that well nulled for long. A 10°C change will typically change the offset voltage by 100uV, and about one out of every two will be worse (there's no guarantee how much worse, but a guess would be most are better than +/-30uV/°C). An OPA209 is going to be roughly an order of magnitude better.

There are probably going to be better (and many worse) choices for any given application, all things considered. It's amazing what performance you can get for a a few dollars, so it's worth looking around rather than trying to make a silk purse of a sow's ear.

Just to give you an idea of the kind of (in) accuracy you could get, consider that the gain of the TL081 is only guaranteed to be >15,000, so a gain of 1000 amplifier could have a gain error in the 6% range even without the input offset error (which would be very temperature dependent, and has a -3dB corner of something like 20Hz. Cascading two \$\sqrt {1000}\$ gain amplifiers would help with that (null only the first one).

If the range is, say +/-5mV input, frequency is 0.001 to 1Hz and required accuracy 1% of FS + 50uV**, it might be typically *** okay in a lab environment with a light output load, if you null it after warm-up.

** Instrumentation type specification- it means the output could be as much as +/- 100mV from the ideal value with any input, so a 1mV input could give you 900mV or 1100mV.

*** "Typically" means that one chip might be okay, and the next might not meet the requirements. Guaranteed value is probably 10 times worse.

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  • \$\begingroup\$ Currently I don't have the time to wait for a better op-amp to be delivered as my local shop only sells these cheaper ones. The way the offset can vary due to temperature and age is of no issue as it will only be needed for a short time under constant temperature. If i use an instrumental amplifier then could it make sense to use one of these for a short time? \$\endgroup\$ – user2126881 Jan 29 '14 at 3:16
  • \$\begingroup\$ Not sure I understand your question. Connecting TL084s in an in-amp configuration won't improve the offset voltage, in fact it will typically be sqrt(2) worse. \$\endgroup\$ – Spehro Pefhany Jan 29 '14 at 4:31
  • \$\begingroup\$ You said the op amp i have is not a good choice but should it at least work in some way? Or is the small voltages too small for this particular op amp to detect? \$\endgroup\$ – user2126881 Jan 29 '14 at 8:59
  • \$\begingroup\$ @user2126881 Okay, you have not given enough information to determine that. I'll edit my answer to include a guess. \$\endgroup\$ – Spehro Pefhany Jan 29 '14 at 12:45

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