# LPF based Differential demodulator design

I am designing LPF based differential demodulator(LPF_DDM) to retrieve differentially modulated signals, which are on-chip capacitive isolated [shown]. LPF_DDM includes a differential amplifier, LPF, and Schmitt trigger. I designed a two-stage opamp [specification below], as a differential amplifier. Simulation results of the 2-stage opamp matched with desired specifications (GBW, Phase margin, gain, etc ). However when the differential modulated signals are given as input, then opamp does not work. Is my 2-stage opamp design and/or design parameters adequate for differential modulated input signal?

Thanks.

Design Parameters: Input common mode range: ICMRmax = 4V; ICMRmin = 1.5V ; Load capacitance =2pF ; Gain = 1000/60db ; Slew-rate (SR) = 20v/usec. GBW=30MHz Phase margin (PM)>= 60-degree

• Encoded data seems to be order of 10^6 Hz (us in time axis). LPF cut off seems to be 10^4 Hz. What is the rationale behind these choices ? What happens if the encoded data was in kHz range? Does it work then? – AJN May 11 at 10:47
• I operate the system in KHz but the response is almost the same. – Bam_Khel May 11 at 11:35
• Moreover, I also added figures to understand the ultimate output. – Bam_Khel May 11 at 11:41
• There is no bias or signal specs – Tony Stewart EE75 May 11 at 11:41
• circuit shown will not demodulate – Tony Stewart EE75 May 11 at 11:56