Electronics > Projects, Designs, and Technical Stuff
Analog devices ADA4805 opamp offset voltage
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Vovk_Z:
It can depends on your ground point.
brumbarchris:
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.
Jay_Diddy_B:
Hi,

The issue here is bias currents. These are included in the LTspice model for AD4805.

Model



I 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 430uV

Results





Try replacing your input short with a 1.8k \$\Omega\$ resistor?

Regards,
Jay+Diddy_B

StillTrying:
<snipped junk idea> :)

It just needs the inputs' DC resistances matching.

Kevin.D:

--- Quote from: brumbarchris on March 26, 2020, 08:49:47 am ---
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

--- End quote ---

When 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.
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