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Differerntial demodulation using single-stage opamp and LPF.

Alex_bam

New Member
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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rjenkinsgb

Well-Known Member
Most Helpful Member
With that circuit, I think the only way would be by using a negative supply rather than 0V for the "opamp" negative.
The source of the output devices is at 1.23V so it can never possibly pull lower than that, even if the output device was switched hard.

"Rail to rail" output opamps have a rather different topology than simple / dual supply types, eg. such as these examples:

Simplified-schematic-of-Class-AB-rail-to-rail-op-amp-the-load-and-the-switch-stage-are.png


figure6.jpg
 

ChrisP58

Well-Known Member
Most Helpful Member
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.
I would use a voltage comparator. Set the reference at about 3.5 to 4 Volts. Add ~ 100 millivolts of hysteresis. Use a pullup resistor tied to 5 Volts on the comparator output.
 

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