Measuring Vos of 5 different op-amps - Page 1
Forum section: electronics / beginners

author: renzoms
author_rank: Regular Contributor
timestamp: December 13, 2021, 04:06:43 pm
content: Hey guys, thanks for helping me with this difficult material. Today, I'm just gonna move onto new stuff.I measured the offset voltage VOS of 5 different op-ampsand all but 1 value were far from the expected results.Here's (everything)the circuit I used, the measured resistances/expected gain, expected outputs given VOS on each datasheet, and what I measured.I tried to troubleshoot1. Did I set the circuits up right? I think I did.I noticed one thing1. When I set up one of the OP wrong, I read a small negative output exactly like my last measurement. But, I triple checked the AD820 OP config. and I swear it's hooked up right.I don't know what else to do. I disconnected the feedback resistor and watched the OP saturate, and that's when I came here.Hope I can learn something from thisSidebar: I want to mention I learned a bit from reading these data-sheets, and thanks to the help of rstofer I recognized how I can use an op-amp to amplify the offset voltage to get a ballpark estimate of what it is.Here's a nice picture to show you the circuits I used and where I made connections to the real circuit.I also learned I should be using my DMM for these measurements and not my oscilloscope.

author: rstofer
author_rank: Super Contributor
timestamp: December 13, 2021, 05:03:04 pm
content: Vos is not a guaranteed single value parameter. For the TL084, for example, it can vary between +-1mV (typical) and +-4mV (maximum) and +-5mV over temperature range.See section 6.11:https://www.ti.com/lit/ds/symlink/tl084.pdfI didn't check the others but, as long as the value is within the published specs (or better), I would just thank the stars I have a genuine part and not some floor sweeping.Think about what +-4mV actually means: the value can be anywhere between positive 4mV and negative 4mV and that includes 0mV if you're lucky. Manufacturers want to be certain their parts are always within specs so you are likely to find more devices with values closer to 'typical' than 'maximum'.The value of Vos changes with time, of course. That's why some Op Amps have a trim input.ETA: Another implication of a spec like +-4mV is that, yes, the value can be negative.

author: renzoms
author_rank: Regular Contributor
timestamp: December 13, 2021, 05:23:20 pm
content: Ok thanks. I think I've learned what I had to to move on, thanks.

author: Buriedcode
author_rank: Super Contributor
timestamp: December 13, 2021, 06:02:50 pm
content: What are +Vs and -Vs in your setup? And what is the accuracy of the resistors used?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 13, 2021, 06:57:50 pm
content: The maximum specs for the offset are upper and lower test limits, so the offse should not be ouside that +- range.The typical specs are more like the typical scattering range, like expect 70% of the samples to be better. Over time the production and if used the trim process may improve and the fraction of parts better than typical may go up.A better values is not a problem if at all the first OP may be rather close to the limits - there are still different gardes and one has to carefully check which grade one actually has.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2021, 01:10:04 pm
content: Watch out for Ib offset which becomes a voltage offset when that curent flows in your resistors. It often has ugly temperature coefficients.https://training.ti.com/ti-precision-labs-op-amps-vos-and-ib-specifications