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YOu can use the XF simulation function or just find the gains from the power supply to the output for PSRR and the common mode gain from the input to the output for the CMRR.
XF is the transfer function simulation. It is there in HSPICE and Cadence. So you can set up the simulation for XF from the input to the output ports (for CMRR) and from supply port to output port for PSRR.
Otherwise just connect a simple AC source at the input terminals (after shorting them) and then just do an AC analysis and find the frequency response. Now calculate the gain from this output response at the desired frequency and you will have the common mode gain. Simulate the differential mode gain and find CMRR. Do the same for PSRR.
XF is a transfer function analysis. For simulating PSRR, you can put an ac signal of 1 Von the power supplies with the DC part while giving a dc bias to the inputs. Take the output. Divide the output by the normal gain curve of the circuit.
For CMRR, you give a differential signal on the input and then measure the gain. Now tie both the inputs to a single ac source and then resimulate and measure the output. Take the ratio of both the cases...
Are you aiming for a particular bandwidth? If you are then you can use the pole and zer information to find out which pole is hurting your bandwidth the most and then try to push it further. If you are testing for stability, you can plot the Bode plot using the pole and zero information and then find the gain and phase margins and then add compensation as needed.
how do you setup xf for CMRR and PSRR in Cadence?
for CMRR do you have to apply a common mode signal and a differential signal. if bothe then after running the analysis in direct plot which source do you have to choose, the differential or the common mode?
for PSRR, again in direct plot form which source do you have to select, AC or the supply?
Hi Vijay,
For xf analysis go through cadence manuals.
In short it works like this.
Connect a small signal voltage source to i/p of Op Amp.
Break any feedback
In spectre select xf analysis, than select the output as positive node and gnd as neg probe.
Run the simulations, you will get the results.
For this go to direct results from there select xf analysis, and select the voltage source.
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