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stabilize a circuit containing transimpedance amplifier, mixer, low pass filter, high pass filter and gain stage

Question asked by niloyeee147@gmail.com on May 30, 2015

Hello,

 

I am trying to simulate a system containing transimpedance amplifier (ADA4817), mixer (AD835), high pass filter, low pass filter and a gain stage. For simulation I am using LTSpice.

 

The target is to modulate a microvolt level signal at input with high frequency, then demodulate, filter and amplify it at output.

For low pass filter I need sharp roll off to minimize stop band noise. That is why I am using cascade of three 3rd order LPF. 2nd order HPF is placed before LPF to get rid of dc baseline. 1000 gain inverting amplifier is placed at output of LPF. I am not getting desired gain at output and also phase goes below -180 before gain drops to 0 dB so system is unstable. My observation is with increase of order of LPF, which introduces more poles, phase drops even sharply. Also HPF deteriorates phase situation. I am attaching the schematic file and the frequency response image.


I also tried with cascade of Sallen key 2nd order LPF and HPF with gain 10 each. Output gain in pass band is -20 dB! I am attaching schematic and frequency response image. If I cascade LPFs then gain and phase get even deteriorated. I am trying to understand what is happening. Any help is highly appreciated.


I previously posted question in this forum and got great feedback. Thanks a lot for that.


Niloy.

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