Zero error on a diff op-amp stage due to resistor mismatch - Page 1
Forum section: electronics / projects

author: peter-h
author_rank: Super Contributor
timestamp: August 18, 2023, 11:32:47 am
content: There are a lot of equations on google but none apparently addressing this. I have a TLV2333 auto-zeroing op-amp (i.e. negligible zero error in my application which is thermocouple signal amplification - milivolts) and four 0.1% resistors.The Rs are 2x1k and 2x7.5k to produce a 7.5x gain.There is a Vcm of about 20mV.I am doing a factory test calibration where the two inputs are shorted (with the 20mV Vcm applied) and I am storing the output value - the zero error.This value is amazingly small. Admittedly I have not measured the resistors but they are cheap 0805 ones. The zero error I am seeing is about 3 microvolts which is of the same order as the op-amp itself. This is across several boards.Obviously, with Vcm=0, no resistor mismatch can produce azeroerror. But how can I estimate the worst case zero error with a Vcm=20mV and 0.1% resistors?I am crap at maths but looking at it intuitively I would expect the input Vcm to appear at the output at roughly the proportion of the resistor mismatch i.e. 0.1% i.e. 1/1000 of 20mV which is 20uV, multiplied by the 7.5x gain.Thank you for any pointers.

author: 2N3055
author_rank: Super Contributor
timestamp: August 18, 2023, 01:47:28 pm
content: https://www.analog.com/en/analog-dialogue/articles/deeper-look-into-difference-amplifiers.html

author: peter-h
author_rank: Super Contributor
timestamp: August 18, 2023, 01:55:36 pm
content: Thanks.That suggests that with my 0.1% resistors, the CMRR is 500 V/V x 8.5 which means a 20mV Vcm could produce 4.7uV which is about what I am seeing.I would have never guessed that the more gain the better the CMRR.

author: TimFox
author_rank: Super Contributor
timestamp: August 18, 2023, 03:59:08 pm
content: In this elementary diff amp, as you increase the differential gain, the common-mode gain remains roughly constant, so the CMRR improves.

author: MasterT
author_rank: Frequent Contributor
timestamp: August 18, 2023, 05:30:10 pm
content: Probably your test setup is not correct.DS:Quotehttps://www.ti.com/lit/ds/symlink/tlv2333.pdf?ts=1692374470895&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FTLV2333

author: peter-h
author_rank: Super Contributor
timestamp: August 18, 2023, 06:12:53 pm
content: I get your drift but it should be OK on that front because it has a -0.2V negative rail.It's an interesting point though. I need to re-test the CMRR over a range of differential inputs.The DS spec on min Vout is with a 10k load to mid-supply. My load on it is negligible - many megs.

author: MasterT
author_rank: Frequent Contributor
timestamp: August 18, 2023, 09:55:50 pm
content: Quote from: peter-h on August 18, 2023, 06:12:53 pmDoesn't matter, R-load could go to infinity but minimum output voltage doesn't drops to zero. Class AB has standby current, it's not like logic inverter where other side switched-off.Good idea to verify V-offset accros full common mode range. If my memory is not laying me I remmember 10 pieces of this IC I get from digikey, they all misbiheave in 0-0.2V input range. Circuits is the simplest, follower, I was measuring diff voltage in between non-inverting input and output. All o'k before pot drops voltage to 0.2, than a few mV (!!!) offset. I ditched a bunch, and switched to microchip mcp6v9x

author: Terry Bites
author_rank: Super Contributor
timestamp: August 19, 2023, 12:18:56 am
content: I found this recently, the full monty:www.planetanalog.com/seemingly-simple-circuits-part-3-effect-of-resistor-tolerances-on-diff-amp-gains/Its very tedious but ultimately simple maths. Just accept the results as gospel. Its a proof of what your intuition is saying.Resistor drift over temperature will dominate gain errors and degrade CMRR. Even resistors of the same type can't be trusted to all drift by the same proportion in the same direction.Source impedance imbalance (never seen that mentioned in a text book) will squish the CMRR. Maybe get a Monte Carlo Sim going on LTspice to see the full spread of anoyances.CMRR is the ratio of differential gain to common mode gain. More dif gain more CMRR.Since the dawn of instrumentation amps I have vowed to ignore the opamp diffamp on the grounds that its very poor VFM.By the time you've tweaked the trufflle priced precision resistors and suddenly realised the opamp just wont cut it, your out of pocket.Based on cost and performance, I would be very uncomforable about designing one in to a product I cared about. OK, analog synth maybe...

author: peter-h
author_rank: Super Contributor
timestamp: August 19, 2023, 08:23:38 am
content: There are two vendors of the TLV chip: TI and NXP.I found the NXP version much worse in Vos. I discovered this because I built an elaborate calibrator for the analog part of my product, all to < 0.01% specs, but for ease of soldering I bought DIP, not SMT, versions of the chip, and they were NXP, and they were pretty crap.But anyway as I said I have a -200mV supply on it so swing to GND is fine.0.1% resistors, 0805, are fairly cheap. The 0.01% ones aren'tOn the topic of Vcm I am feeding the output of this into an ADS1118 ADC. This is an interesting device. It is "obvious" that the ADC part will work all the way to GND (those switched-cap designs always do) and slightly below, but the PGA which fronts it obviously cannot swing below GND. The DS is silent on this. The ADS1118 does not have the benefit of the -200mV GND and even though doing so would not interfere with the logic side, I can't do it because I am measuring inputs relative to the GND pin, and the -200mV isn't accurate. That stupid chip has the same pin for AGND and DGND. There is only one aspect of my product functionality which needs operation down to GND and I can document this. Specifically it means that some rare thermocouples (there are 8 types of which 4 - BRST - are very low output) will not be able to measure temps below the CJ temp but this would not matter in most applications. Only Type T might be used in cryogenics.

author: MasterT
author_rank: Frequent Contributor
timestamp: August 19, 2023, 12:14:30 pm
content: Can't locate tlv2333 on nxp web-site. Anyway, I had TI.Now, when you mention interfacing AZ opa to SD adc, I become uncertain if I didn't make mistake on my part rejection tlv2333 based on evaluation test results. I also used SD adc, and was much less experinced at that time that it is a bad idea. Beating in frequency between chopper of the AZ and sampling SD may produce unexpected artifacts, Vos is just one of them that easy to observe.I'd not do it again, there are excelent opa2365 that beats up AZ amp on all aspects except drift, which is not guaranteed when SD adc interfaced

author: peter-h
author_rank: Super Contributor
timestamp: August 19, 2023, 04:18:49 pm
content: The TLV2333 claims to not have these problems, and anyway I can use the ADS1118 with a sampling interval up to 1/8 sec which pretty well attenuates noise. The general accuracy I am getting is really good.A strange thing: I have experimentally verified that the ADS1118 can read values below its GND. The PGA inside it is some really weird design...https://www.eevblog.com/forum/projects/ads1118-common-mode-range/