===== Opamp CMRR and PSRR simulation =====

  * For CMRR and PSRR, you can do this with an xf (Transfer Function) analysis available in Spectre. 
  * Set up your testbench sources for the supplies (of course), but also a source representing the common mode voltage. 
  * Then run an xf analysis and tell it where the output of the circuit. 
  * You can then plot the transfer function from every source to the differential output of the circuit.
  * Use calculator or an expression to plot CMRR=20*log(VDM/VCM) in the log-domain