Differerntial demodulation using single-stage opamp and LPF.

[imrankhanPNU]

Hello,
I am designing a differential isolator via a capacitive coupling technique in which a control signal is differentially modulated and coupled capacitively to the differential demodulator side.
I have designed a demodulator using single-stage Opamp and LPF [Schematics attached]. The problem is the LPF's output signal swings between 2.8V to 5 V. How can I make LPF's output signal swing between 0-5V?
By doing so will make the Schmitt trigger's job easy to retrieve the output signal.

Thanks.

 

[AMS012]

There are multiple ways to do it.

1. If you know the lowest input frequency, you can AC couple the output of this 1-stage demod, and buffer it up to hit the rails you want.
2. You can use a skewed inverter as a level shifter that shifts the levels to your required values.
3. You can try other kind of level shifters like NAND or NOR based ones.
4. You can get rid of this above demod, and do the demod using self biased inverters. You can have a cap across the feedback resistance to do the low pass filtering, and the output will hit rail to rail with a few stages of buffering.