Opamp long term offset voltage drift

They'll lie :-(

Can you run an auto-zero loop? ...Jim Thompson

Why not just go for a better amplifier?

My first thought was some kind of chopper amplifier, but the Analog Devices web-site doesn't list them as easily identifiable parts any more.

I eventually found this

Neither data sheet says a word about long term stability, but Analog Devices seem very likely to be able to answer that kind of question - contacting the manufacturer can be a very good idea if it's a good manufacturer.

The OPA is bipolar, and the MCP is cmos. Cmos opamps usually have more longterm drift than bipolar parts, charges moving around in the gate oxide or something.

You should ask. The MCP has a specified typ Vos-vs-temp, but no max, so it's not a precison amp. For lowest drift, use a chopamp or software auto-zero somehow.

We often have mux'd inputs into an ADC, and make one of the mux inputs ground, so the software can check the amp+ADC offset and subtract it out.

We need a continous signal, so autozero is difficult to implement (could be a chopper amp, prices have come down on those)

Cheers

Klaus

That is a nice part. Low offset en quite cheap. Thanks :-)

Cheers

Klaus

We do that too, normally. But in this case, the bandwidth is higher and we cannot use offset compensation

Cheers

Klaus

I just went through 150 datasheet, of which 18 had long term drift data. Interestingly, almost all of them, irrespective of manufactor, had about 1/50 of the typical offset voltage as long term drift per month number (so a 1mV typ offset part had 20uV/mo)

Cheers

Klaus

Depending on the mechanism, at least some of the drift should decrease logarithmically with time.

If you can live with lower supply voltages, some auto-zero op-amps are quite inexpensive, but even ones that can handle high supply voltages are not that expensive.

Eg.

Long term stability +/-4uV.

Best regards, Spehro Pefhany

Did the OpAmp you were using meet its own spec?

Were some of those cmos?

eans we need low long term offset drifts (we do production calibration)

re

ts

d we cannot use offset compensation

Interestingly, almost all of them, irrespective of manufactor, had about 1 /50 of the typical offset voltage as long term drift per month number (so a 1mV typ offset part had 20uV/mo)

Bipolar, BiFET, LinCMOS, bipolar, FET, CMOS. The majority CMOS (FET)

Cheers

Klaus

Cheers

Klaus

Interestingly, almost all of them, irrespective of manufactor, had about 1 /50 of the typical offset voltage as long term drift per month number (so a 1mV typ offset part had 20uV/mo)

The cheapest one that fit my bill was the old TLC2272:

Large BoL offset of 300uV, but extremely low long term drift of 2nV/month

I think thats a typo, sounds strange to have large initial drift and no lon g term drift

Cheers

Klaus

Note the conditions it is tested under. That series uses interspersed arrayed input transistors so you'd expect the drift under those completely symmetric conditions to be very low. However, if your application sees some average voltage at the inputs, you may not have as pleasant a time.

See, for example, the even older CA3140:-

Read starting at the weasel phrase "It is well known that..."

Best regards, Spehro Pefhany

Which types were best?

es web-site doesn't list them as easily identifiable parts any more.

Apparently "chopper" has become a dirty word. The new terminology is Zero D rift:

Zero-Drift op amps or Low TC (temperature coefficient) op amps meet the req uirement for applications demanding constant offset voltage over time and t emperature. Analog Devices is the leader in the low-drift op amp architectu re and offers the broadest portfolio of products in this category. Select p arts from the parametric search table below to download parts datasheets an d order samples.

and

Zero-drift, also known as "chopper stabilized" or "auto-zero" operational a mplifiers (op amps) provide the lowest possible input offset voltage and dr ift. Linear Technology's zero drift amplifiers offer initial offset voltage s of less than 3uV and almost immeasurable offset voltage drift over time a nd temperature. Chopper op amps are tested and fully specified with guarant eed limits over the full operating temperature range for offset voltage, of fset voltage drift, common mode rejection ratio (CMRR), open loop gain and more.

- and these people:

TI analog Zero Drift Precision Amplifier Amplifiers and Linear products are a subset of analog Analog & Mixed-Signal solutions. This page is your reso urce to download datasheets, application notes, order samples and use param etric search to research other Amplifiers and Linear-related analog solutio ns.

-and

ST's precision op amp solutions include zero drift amplifiers and amplifier s with a low offset drift over temperature. Thus the precision op amps from ST are a perfect fit for use with any sensor, including gas, temperature, pressure, and position sensors. Moreover, the rail-to-rail input and output feature allows easy and accurate high-side and low-side sensing.

-and

Maxim's autozero technology achieves high precision over time and temperatu re and eliminates low-frequency noise. The latest families of amplifiers th at feature autozero technology work at much higher switching frequencies, m aking it easier to filter out noise while offering a wider, usable signal b andwidth.

etc...

Looks like everyone who makes OAs has them even though they may not call th em out as a separate product line like OnSemi.

pdf

ces seem very likely to be able to answer that kind of question - contactin g the manufacturer can be a very good idea if it's a good manufacturer.

Why not Linear, LTC1047

Cheers

That's a nice part. I need minimum 500kHz GBW though

Cheers

Klaus

Didn't get an answer. Did the OpAmp you were using meet its own spec? If so, how close?

Well, I in the design phase, no circuit has been tested yet

Regards

Klaus