AD823 schematics review

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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I have a problem with ad823 amplifier in my project. The impedance to inverting
input is high. The Output voltage of the amplifier in the pcb board is not
acting as a simulation . In the simulation I get 2 volts output  Nation profile
in the pcb I get 1. 94-volt output I enclose a circuit diagram.
Do you have an idea?

 

Op-amps when operating in closed loop will be far more accurate than the 2-3%
error you are observing. So, the root cause of the error is not the AD823.

Two things to keep in mind for this setup :

1. The resistor values are very large. A 1% tolerance of either of the
resistors will give you a 20 mV error. The tolerance on these large value
resistors can go out to the specifications limits, so you can easily expect
errors in the 2-3% range. The simulation does not give you the tolerance
effects unless you "sweep" a parameter.
2. Although leakages on the PCB are usually considered insignificant, they will
make significant contributions to errors when such high value resistors are
used. So, it's not surprising if the circuit has a 60mV error on a 2V expected
output.

What is the load resistance past TP6? If it is 10k or less R2 should be removed.

You can attempt to find what causes the error by :
a. Removing D2,D3.
b. Measuring the actual value of R3 and R4 accurately (this may be more
difficult than it sounds)
c. Use conformal coating to minimize leakage.

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