Electronics > Projects, Designs, and Technical Stuff
Analog devices ADA4805 opamp offset voltage
brumbarchris:
I've checked with an oscilloscope, it is really DC.
StillTrying:
"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 pm ---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.
--- End quote ---
I 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.
SiliconWizard:
--- Quote from: StillTrying on March 24, 2020, 03:47:18 pm ---So the +IN needs 1.8k in series with it to cancel the 2 input bias current voltage offsets.
--- End quote ---
I 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.)
Vovk_Z:
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).
brumbarchris:
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 the inputs of 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): +0.0073mV
Pin 4 (inverting): +0.0473mV
Output: -50.2mV
Results for R340=180R and R341=10k:
Pin 3 (non-inverting): +0.0076mV
Pin 4 (inverting): +0.045mV
Output: -2.7mV
Results for R340=open and R341=short:
Pin 3 (non-inverting): +0.0085mV
Pin 4 (inverting): +0.05mV
Output: +0.05mV
I 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
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