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?


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

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  • \$\begingroup\$ 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? \$\endgroup\$ – AJN May 11 at 10:47
  • \$\begingroup\$ I operate the system in KHz but the response is almost the same. \$\endgroup\$ – Bam_Khel May 11 at 11:35
  • \$\begingroup\$ Moreover, I also added figures to understand the ultimate output. \$\endgroup\$ – Bam_Khel May 11 at 11:41
  • \$\begingroup\$ There is no bias or signal specs \$\endgroup\$ – Tony Stewart EE75 May 11 at 11:41
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    \$\begingroup\$ circuit shown will not demodulate \$\endgroup\$ – Tony Stewart EE75 May 11 at 11:56

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