3 Measuring PSRR Using Oscilloscope

If the user does not have a network analyzer then there is a simpler but more cumbersome method which uses a signal generator, DC source and oscilloscope to measure the PSRR. An AC signal from signal generator is applied along with DC signal at the input of the LDO, as shown in either of the afore mentioned methods, and the output of the LDO is measured on an oscilloscope at different VAC frequencies. The PSRR is calculated using the Equation 1 where Ripple_(input) is the amplitude of the input AC signal and Ripple_(output) is the amplitude of output signal. This is then repeated at different frequencies of VAC to generate a piecemeal graph of PSRR.

This method can be used along with the set-ups described in the previous section. But this method is only good for LDOs with lower PSRR values due to resolution and sensitivity of oscilloscopes. Since most oscilloscopes can measure down to the millivolt range, the maximum range of PSRR that could realistically be measured using an oscilloscope is about 40dB–50dB.

Using the TPS78101EVM from Texas Instruments, the PSRR is measured using signal generator and oscilloscope. The input and output waveforms are as shown in Figure 5:

Test conditions are:

1. Vout = 3V
2. I_load = 150mA
3. VAC = 1V (p-p) at 1kHz
4. VDC = 4.3V dc

And the PSRR is calculated from waveforms in Figure 5 is:

Ripple_(input) = 984 mV
Ripple_(output) = 194 mV
PSRR = 20 log10(Ripple_(input)/ Ripple_(output)) = 20 log10(0.984/0.194) = 14.10dB which closely matches the PSRR specified in the datasheet of TPS78101.

This application report shows various methods to measure the PSRR of an LDO and also explains different aspects which need to be considered while measuring PSRR.