LDO PSRR Measurement
How to Measure LDO PSRR
Measure LDO PSRR with Network Analyzer
Power supply rejection ratio (PSRR) or some time called power supply ripple rejection measurements are often difficult to measure, especially when the device under test (DUT) is heavily loaded. Typically regulator PSRR is measured with a network analyzer. The network analyzer output signal is AC-coupled into LDO (low dropout) voltage input (Figure 1). The network analyzer generates a sinewave ridding on top of the DC voltage. The LDO input DC voltage is provided by VCC through a small resistor (1 to 10 ohm). The network analyzer is then measure the sinewave amplitude at the LDO input and output using Input-A and Input-B. The ratio of the sinewave at LDO output and input is PSRR.
This method works well if the LDO output is no load or very light load. At heavy load there will be too much voltage drop across the series resistor. Because most network analyzer source output is 50 ohm, it cannot effectively drive the LDO at heavy load (i.e. 1A load). For that reason, regulator PSRR testing often is done at no load or light load. LDO PSRR tends to decrease at heavier load and generally worst at maximum specified load. Measuring power supply rejection at no load is not representative LDO PSRR.