Analog devices ADA4805 opamp offset voltage - Page 1
Forum section: electronics / projects

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 24, 2020, 12:50:28 pm
content: Hello everyone,Please consider the schematic and layout portion I represent in the pictures below. It is essentially a non-inverting amplifier, with a gain of some 55, built around the precision Analog Devices ADA4805 opamp . "AMP_OUT" output is disconnected from load. "VC_OUT" input is pulled to ground by making a short circuit across D312 (nothing drives it, anyway). Yet, for some reason, the "AMP_OUT" output measures -50mV, while I would have expected something in the -6.8...+6.8mV (max opamp offset voltage is 125uV at room temperature multiplied by the circuit amplification factor, which is 55). I have checked that pins 3 and 4 of the opamp measure really 0.0000V. I measured with several instruments. I replaced the opamp with a new one. The power supplies are clean, with no unwanted artifacts on them. I suppose layout does matter, but to what extent? There is a solid GND plane (not illustrated in the attachment) on the very next layer under the surface layer represented in the picture.So why this large offset outside of the expected range?

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 24, 2020, 01:15:01 pm
content: Does R340 really have 1.8k (have you checked it's value, and R341)?

author: Wolfram
author_rank: Frequent Contributor
timestamp: March 24, 2020, 01:29:32 pm
content: Beware that this amplifier has a maximum input bias current of 800 nA, which would lead to an offset of 1.4 mV across 1k8||100k. If this was the cause of your problem, it would also be observable when measuring the voltage on the input pins.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 24, 2020, 01:51:20 pm
content: +1. I just wanted to say the same.Both input pins have to have some voltage. For example, pin 3 = 0,0 mV (because it's grouded), but pin 4 = 0,9 mV.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 24, 2020, 02:01:34 pm
content: Could you momentarily short R341 and measure again?Sorry, did not read the whole datasheet. This only works if the ADA4805 is unity gain stable

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 24, 2020, 02:09:00 pm
content: It is unity-gain stable. So TS has to have about 0.9 mV on pin1 (out) with shorted R341.It seems that we have a pretty close to typical current here.470 nA is "Low" dependably on with what opamp we compare. There are a lot of opamps with pico- and fetto-amps bias current.

author: StillTrying
author_rank: Super Contributor
timestamp: March 24, 2020, 02:25:25 pm
content: R340 being open circuit would just about explain it, -0.50uA input bias current through the 100k = -50mV.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 24, 2020, 02:51:17 pm
content: Quote from: StillTrying on March 24, 2020, 02:25:25 pmYep.

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 24, 2020, 03:18:42 pm
content: R340 is not open, I just triple-checked it.I took a multimeter with more digits and better precision and I measured the input pins again (in the original configuration):Pin 3 (non-inverting): +0.0073mVPin 4 (inverting): +0.0473mVOutput: -50.2mVHaving reconsidered the effect of the bias current, as some of you guys pointed out, I actually believe the limits for the output should now be +/-84mV. So the output is indeed within limits, but I would have expected to see the effect of the input bias current at the input pins themselves. At such a large output voltage, I would have expected to see some 0.77mV actual difference between the two input pins, which I do not see.Changed R340 to 180R and R341 to 10k, so as to keep their ratio constant, and measured again:Pin 3 (non-inverting): +0.0076mVPin 4 (inverting): +0.045mVOutput: -2.7mVIn this case the limits are +/-14mV. So well within limits in this case.If I just reconfigure it to a voltage repeater (R340 removed and R341 short-circuit), I get:Pin 3 (non-inverting): +0.0085mVPin 4 (inverting): +0.05mVOutput: +0.05mVIn this case the limits are +/-0.125mV. So well within limits in this case.Why is the effect of the input bias current not visible on the input pins, in the case of the original configuration?Best regards,Cristian

author: voltsandjolts
author_rank: Unknown
timestamp: March 24, 2020, 03:20:23 pm
content: Also might be worth checking the op-amp output is indeed steady-state DC.i.e. measuring with a DMM might show some offset that is actually an AC signal caused by e.g. coupled noise from DCDC converter.

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 24, 2020, 03:39:26 pm
content: I've checked with an oscilloscope, it is really DC.

author: StillTrying
author_rank: Super Contributor
timestamp: March 24, 2020, 03:47:18 pm
content: "R340 is not open, I just triple-checked it."Have you checked its 0V end is connected to a good 0V (on resistance).You do have VC_OUT and AMP_OUT following a long parallel path, but if you've checked the -50mV is not oscillation there's not much left. The +/- 5V decoupling doesn't look tooooo bad.Quote from: Vovk_Z on March 24, 2020, 01:51:20 pmI think this could be it, 0.55uA of bias current flowing though the grounded +IN is 0V, but 0.55uA flowing through the 1.8k|100k = 0.98mV, times 55 = 54mV.So the +IN needs 1.8k in series with it to cancel the 2 input bias current voltage offsets.

author: SiliconWizard
author_rank: Super Contributor
timestamp: March 24, 2020, 05:16:03 pm
content: Quote from: StillTrying on March 24, 2020, 03:47:18 pmI would also try that. (Unless I'm mistaken, I think that would be more like 1.8k // 100k, but for all practical purposes, it won't make a big difference.)

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 24, 2020, 06:31:55 pm
content: That opamp definitely doesn't like large resistor nominals, smaller R have to be about 20-200 Ohms (look at the datasheet typical circuits). It can be even better to use 18 R at that place, I mean 1000R/18R (and 18 R at another input).

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 26, 2020, 08:49:47 am
content: Thank you all for your great help. Indeed I can mitigate the effect of the input bias current by decreasing the resistor values. However, what still puzzles me, is that I cannot see the effect of this offset current when measuring the voltage at theinputsof the opamp. Just to reiterate, the measurement results with various resistor configuration are as follows:Results for R340=1800R and R341=100k:Pin 3 (non-inverting): +0.0073mVPin 4 (inverting): +0.0473mVOutput: -50.2mVResults for R340=180R and R341=10k:Pin 3 (non-inverting): +0.0076mVPin 4 (inverting): +0.045mVOutput: -2.7mVResults for R340=open and R341=short:Pin 3 (non-inverting): +0.0085mVPin 4 (inverting): +0.05mVOutput: +0.05mVI would have expected to see a significantly larger voltage difference between the inverting and non-inverting inputs of the opmap in the first case (R340=1800R and R341=100k), something around 0.77mV. However, that is not the case. Any idea why?Best regards,Cristian

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 26, 2020, 09:26:16 am
content: It can depends on your ground point.

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 26, 2020, 09:48:32 am
content: I have tried several: GND point of D312, GND point of R340, GND point of the decoupling caps, they all show similar readings, no notable difference.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 26, 2020, 10:52:28 am
content: Hi,The issue here is bias currents. These are included in the LTspice model for AD4805.ModelI have two models, one is the OP original circuit. It gives an output voltage of 47.5mV, V(out)In the second model I have included R3 so the impedance at both of the input terminals is approximately equal. The output, V(out_2) is 430uVResultsTry replacing your input short with a 1.8kresistor?Regards,Jay+Diddy_B

author: StillTrying
author_rank: Super Contributor
timestamp: March 26, 2020, 11:46:33 am
content: <snipped junk idea>It just needs the inputs' DC resistances matching.

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 26, 2020, 12:16:10 pm
content: Quote from: brumbarchris on March 26, 2020, 08:49:47 amWhen neg feedback is applied the differential input voltage measured at the input feedback node will be reduced approx by factor of the loop gain (Vin=Vin/(loop gain+1). Whats left is what I call the 'loop gain error voltage' referred to the input which for an ideal amplifier with infinite voltage gain would be zero.The 'Loop gain' is the difference between amplifier open loop gain and the signal gain, e.g At DC Opamp Aol= 100dB signal voltage gain = 35dB so loop gain= 100-35 =65dB your diff input voltage will be reduced by factor of ~ 65dB.

author: brumbarchris
author_rank: Regular Contributor
timestamp: March 26, 2020, 01:55:20 pm
content: Everything is now crystal clear regarding the quations I had. Thank you very much to everybody who contributed.Best regards,Cristian