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Analog devices ADA4805 opamp offset voltage

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brumbarchris:
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?

Vovk_Z:
Does R340 really have 1.8k (have you checked it's value, and R341)?

Wolfram:
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.

Vovk_Z:
+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.

schmitt trigger:
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

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