UnLogic !! Strange Result from AD623

hiii..

the following circuit is so strange..it was working so fine but after i just added a cap in the input ,the voltage just after the cap became so so strange and sure not logic
notes
1- changing the cap value doesn't change such huge strange offset
2- in the image below Rin , Cin is the input Z of the Instrument Amp.
3- AD623 is Instumention Amp. with very high Zin ( 2G||2p) from datasheet

Please I need your help

also if any one has another way to remove the offset 500mv from the input , plz express it

I'm not sure how the macromodel for the AD623 is constructed, but could there be an input bias current issue here? Adding a DC path for the + input or bridging the input cap with a 10meg resistor might help.

The reason why you are getting an offset voltage on the output of the AD623 is that the PNP transistors on input of the instrumentation amplifier do not have a path for the input bias current to flow, which results in an offset voltage at the output.

lladnar23 is right. You could just bridge the cap with a 10M resistor, which would approximate the leakage current in a realistic capacitor, or you could do the best solution, which is to add an explicit path for the DC input bias current.

This solution is shown on Figure 47c on page 14 on the AD623 DATASHEET. Please read the datasheet.

Is there a particular reason why you are using an instrumentation amp in a single-ended measurement (ct_1.jpg)? It really would be cheaper to do this measurement with an op-amp, and you could use a nice JFET input opamp and blissfully ignore input bias current issues.

By the way, is this some sort of school project? I noticed that there is another post about AD623 issues with a very similar circuit.

-elcap

elcap said:

By the way, is this some sort of school project? I noticed that there is another post about AD623 issues with a very similar circuit.

-elcap

Click to expand...

:arrow: he is a friend to me & we working together in a certain project and it is not at all school project and about using op-amp insted of Inst. AMp ,indeed the cir above was just for testing & not the real project......

Thx all for ur help