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Differerntial demodulation using single-stage opamp and LPF.

Alex_Bam

Sep 28, 2020
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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.
 

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bertus

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Nov 8, 2019
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Hello,

Your very dark schematic is very hard to read.
I have inverted the colors so it is a bit better to look at:
Alex_Bam diif schematic.JPG
Bertus
 

Harald Kapp

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By doing so will make the Schmitt trigger's job easy to retrieve the output signal.
If you have a Schmitt-Trigger anyway, why not simply put the threshold in the center of the 2.8 V - 5 V range?

Otherwise you could reduce the available voltage from 2.8 V -5 V to 0 V - 2.2 V using a zener diode, then amplify by 2.27 to extenbd the range to 0 V - 5 V. In my opinion this will decrease the signal quality, not increase it, as you probably expect since any additional stage adds noise.
 
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