What is Power Supply Rejection Ratio of Op-Amp ?
Power Supply rejection Ratio (PSRR) specifies that if there is any change in the supply voltage, then how it will affect the output of the op-amp. For op-amp, if there is any change in the supply voltage, then it should not affect the output of the op-amp. With the change in the supply voltage, the output voltage of the op-amp will also change by a small margin. And this is specifically important when the input signal level is very small. This PSRR is specified in terms of the input offset voltage. That means with the change in the supply voltage, how the input offset voltage of the op-amp will change. And typically, the unit of this PSRR is μV / V.
Where,
ΔVio= change in the input offset voltage
ΔVs= change in the supply voltage
For example, for some op-amp if the PSRR is equal to 10 μV / V, it means that when the supply voltage will change by 1 volt, then the input offset voltage of that op-amp will change by a 10 μV. The change in the input offset voltage will lead to the change in the output of the op-amp.
For dual supply op-amps typically it is assumed that the change in the supply voltage is symmetrical. But if the change in the supply voltage is not symmetrical, then it will lead to the common mode error.
In the decibel this PSRR is defined as
Effect of Supply Voltage on input offset voltage of the op-amp
Internally, the first stage of the op-amp consist of differential amplifier. For the differential amplifier, if both the transistors are identical then if we apply the same input at both terminals, then output of the op-amp should be zero. But because of some mismatch between the two transistors, there will be some finite differential output. And that differential output will get amplified by the gain of the op-amp. Hence there will be some finite output voltage even if both input terminals are connected to the same input terminal. The input offset voltage is the additional input voltage that we need to apply between the two input terminals, to make the output of the op-amp zero.
With the variation in the supply voltage, the biasing of the differential amplifier will change and hence, the input offset voltage of the op-amp will also change.
The change in the input offset voltage will lead to the change in the output of the op-amp. If the op-amp is configured in the closed loop configuration (inverting or non-inverting), then the input offset voltage will get multiplied by the noise gain of the op-amp.
The Power Supply Rejection Ratio (PSRR) is the ability of the op-amp to withstand the change in the supply voltage. The smaller value of PSRR (μV/ V) is preferable. If PSRR is specified in dB, then larger value of PSRR is preferable.
PSRR with Frequency
PSRR value will also reduce with the frequency. That means if we have a ripple let’s say of 100 Hertz on top of the supply voltage, then that ripple will not be rejected by the op-amp as effectively as the DC voltage. And because of that, some fraction of that ripple will also appear at the output of the op-amp.
The below figure shows the typical PSRR curve with frequency. As the frequency increases, the PSRR (in dB) reduces.
Therefore, to reduce the effect of high frequency ripple on the output of the op-amp, it is advisable to use a well regulated supply voltage with the op-amp. And to avoid any high frequency noise in the output, the proper value of the decoupling capacitor should always be used with the op-amp. The decoupling capacitors provides the low impedance part to the high frequency noise. And in a way it reduces the effect of the high frequency noise at the output of the op-amp.
