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Post LPF Gain Stage

Question asked by groger on Aug 6, 2014
Latest reply on Aug 12, 2014 by groger

Hello -

 

Using the ADI filter wizard, I have generated a Sallen-Key LPF that has 33dB of gain.

 

As per the implementation schematic, I have created a voltage reference and tied all reference points to that Vdd/2 reference.

 

I also require a Vdd/2 reference immediately preceding R1E. For that, I am using a Vdd/2 divider with a bypass capacitor across the lower resistor. Using this reference divider, I am losing AC signal. To compensate for that, I added gain (33dB) to the LPF such that I am now getting at the output of the LPF, exactly what I have input at the sensor's input coupling capacitor.

 

Now I need to get gain at the output of the LPF. I've used the NI input to an op-amp. I have tied the center of the divider's feedback

resistors to the inverting input, and connected the other side to the same reference as the LPF (from the AD8031 circuit)

 

However, 2 bizarre things:

1 - I am unable to get more than 31dB gain from that opamp, any combination of Rf & Rg to get higher results in the signal going back to it's input AC value.

2 - When I do set the values to get the above 31dB gain, the output is centered on 2.6V, whereas the input is close to Vdd/2.

 

I could post a screen capture, but it's from another company's simulation package and I'm not sure it's Ok to do that here. Of course, I'm using ADI parts (as always) in my actual design implementation.

 

Please - any suggestions as to what is happening to cause this? Kinda stumped at this point...

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