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AD5933 Question... Where do these errors come from?

Question asked by Dan-B on Jul 25, 2012
Latest reply on Jul 26, 2012 by musach

Hi guys,

 

Just got a hold of the EVAL-AD5933EBZ development board and I'm getting some weird results. I am not going to be referring to the results I got from the electronics, but from the java interactive design tool at this address:

 

http://designtools.analog.com/dt/ad593x/ad593x.html

 

First question, why does the impedance wobble like it does in the following screenshot at frequencies below ~1K

 

http://snag.gy/Tcmbs.jpg

 

Secondly, in the following screenshot with different values plugged in, as I increase the gain of the transimpedance amplifier (by increasing Rfb), the frequency response of the circuit drops off. All I am asking is confirmation of what I think is happening - the GBWP of the op amp on the evaluation board is limiting the frequency response of the circuit? Another quick question about this screenshot... how come the distortion is so much worse at the lower frequencies than the previous screenshot?

 

http://snag.gy/YqdYp.jpg

 

I realise these are quite basic fundamental questions but I am just getting to grips with the circuit so any help is appreciated.

 

Thanks

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