Op Amp input offset and drift questions - Page 1
Forum section: electronics / projects

author: jmdejoanelli
author_rank: Contributor
timestamp: May 08, 2013, 10:16:13 pm
content: Hi all,So I have an application where I need to measure very low voltages (around the low uV range) and I had a couple of questions about input offset voltage.Firstly, from what I understand, the input offset voltage of an op amp is a manufacturing inaccuracy (mismatching of input transistors) so should be of fixed value, i.e. doesn't change (except with offset drift obviously), is this correct?Secondly, since the input offset error is constant and the drift is extremely small (by selection), can't I just calibrate the offset out of the system in software after the ADC? And so when I choose an op amp, should I focus entirely on a very low input offset drift instead of a low offset voltage? In my application, I am buffering and amplifying a small dc voltage and then measuring it with an ADC (which I'm also considering input offset and drift for).Lastly, how do people measure nano Volts? I haven't seen many op amps with less than a uV of input offset, but I have seen many with drift well into the low nV range.Cheers,Josef

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 10:39:19 pm
content: Quote from: jmdejoanelli on May 08, 2013, 10:16:13 pmHellno. Set up a little measurement jig, throw some heat on the op amp, and see what happens to your input offset.

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 10:41:25 pm
content: Quote from: jmdejoanelli on May 08, 2013, 10:16:13 pmHave a look through thisKeithley 181 Nanovoltmeter service manual- the Theory of Operation and the schematics.

author: jmdejoanelli
author_rank: Contributor
timestamp: May 08, 2013, 10:47:21 pm
content: Quote from: c4757p on May 08, 2013, 10:39:19 pmGood thinking.Is this due to input offset drift though? In which case, with an input offset drift well below 1uV, will I see much change at all (based on an op amp configured for unity gain to G=10 say)?

author: c4757p
author_rank: Super Contributor
timestamp: May 08, 2013, 11:16:39 pm
content: The datasheet should tell you. (If it doesn't, that op amp is crap for this application.) Example:OP07C/Dspecifies a temperature coefficient for the input offset voltage, up to 1.8 µV/°C for OP07C and 2.5 µV/°C for OP07D.MCP6V06specifies ±50 nV/°C plus 0.15 nV/°C².

author: jmdejoanelli
author_rank: Contributor
timestamp: May 08, 2013, 11:45:49 pm
content: Okay, so, the op amp I'm looking at using is the OPA2335 from TI.It has a 50nV/°C maximum offset drift, and a maximum offset of 5uV.So my thinking was that I can calibrate my system in software to cancel out the offset of 5uV (or whatever it may be) at room temperature. I haven't thought about how I'm going to account for the offset drift yet, perhaps I will use a heater to control the temperature (say fixed 50°C), so that the drift is fixed.

author: AlfBaz
author_rank: Super Contributor
timestamp: May 09, 2013, 12:11:03 am
content: Does your application require continuous sensor monitoring or can it handle loss of signal for a millisecond or so periodically. If the it's the latter you could try auto zeroing technique which is even easier in this situation as you zero it in software. The added bonus is you are cancelling any offset in the adc to.Just short the inputs, measure the output and there's your new software offset

author: jmdejoanelli
author_rank: Contributor
timestamp: May 09, 2013, 12:53:42 am
content: Quote from: AlfBaz on May 09, 2013, 12:11:03 amI like the idea of the auto zeroing. I think I can afford to interrupt the monitoring.How often would you perform the auto zeroing? Once per second? Once every 10 seconds?Also, what kind of device would you use to short the inputs? Just a transistor? A digitally controlled analogue switch?Cheers,Josef

author: AlfBaz
author_rank: Super Contributor
timestamp: May 09, 2013, 01:36:29 am
content: Quote from: jmdejoanelli on May 09, 2013, 12:53:42 amI would imagine it depends on how much it drifts over time and also if the drift exceeds low frequency or external incoming noise. It would also depends on the sensor, can it handle being loaded/unloaded without affecting its performance?QuoteI don't know, but it would need to be something configured like a double pole, double throw switch. The 2 commons connected to the opamp, normally closed to the sensor and the normally open shorted together.If you went for a quality relay you would have to take into account its max operations when considering the "how often..." question

author: SeanB
author_rank: Super Contributor
timestamp: May 09, 2013, 05:43:08 pm
content: Reed relay, which has both low offset voltage and low thermal EMF ( depending on type, gold flashed are around the best) and which has a life into the hundreds of millions of cycles, so every 30 seconds will be essentially forever. You will just need 2 SPCO reeds and a coil that can accomodate both to both disconnect and short the inputs of the opamp, and they will be thermally equivalent so will not have much offset. You will just have to have a copper inner shield on the coil to keep switching noise from the coil to a minimum, as well as driving the coil from a low noise supply ( otherwise noise there is coupled into the inputs).