CMRR and PSRR frequency response

[calculus_cuthbert]

Hi,

I'd like to understand CMRR and PSRR in detail. The question I had was how to make more sense out of CMRR and PSRR frequency response. Why does the frequency response of CMRR and PSRR differ from the open loop gain frequency response?

Could someone please explain CMRR and PSRR frequency response in more detail? And how to improve it.

Thanks

 

[tupi]

What is a freq response? It is the small-signal (ac) response of a particular node (generally the output) with respect to the excitation of another node (generally the input) at various frequencies (.. like a laplace transfer function). For loop gain (which makes sense within a loop of a network), it is the found by injecting a small ac input by breaking the loop at a suitable point (say A) and measure the output at the other end of the broken point (B). This way, you measure how much the signal gains/attenuates while covering the loop of the network.
For differential input circuits like an opamp, two gains are defined: differential gain and common mode gain. CMRR which measures common mode gain is found by exciting both the input of the opamp with a common ac signal and finding its effect on the variation in the output( unlike a diifferential where you put a small ac input at one input node and set the other node at ac gnd). This way, circuit designers want to find whether the opamp can reject common mode variation in the differential input signal. Ideally, you want zero common mode gain and large differential gain in an opamp.
PSRR is found by exciting the supply with an ac signal and finding its effect at the output (gain/attenuate). A bad circuit will amplify the supply ac signal. However, a good PSRR circuit should attenuate the ac variation in the supply. It is like finding how much the circuit can reject power supply noise. For example, a 20 dB attenuation (PSRR) at 1MHz would mean a 100 mV ac variation of power supply at 1 MHz would cause 10 mV variation at the output at 1MHz.
At this point, you may also want to distinguish the various frequency response terminology like open loop gain ( for e.g differential gain of an opamp), closed loop gain ( gain of an opamp with feedback), loop gain ( used to find gain around a loop for bode plot and stability analysis), Comon mode gain, differential gain and power supply rejection ratio. As explained, all these are found by exciting a node and finding its effect in another node. And hence all these are some kind of laplace transfer function after linearizing the circuit (network) around a DC operating point.