Other methods for measurement
Other methods and equipment are available to make supply rejection measurements. A lock-in amplifier uses the reference signal to provide synchronous detection at the desired frequency to help measure small signal levels. A network analyser also provides an oscillator to sweep across frequency while providing the bandpass function to measure both input and output amplitudes and calculate the rejection of the circuit. These methods provide valid results, but one still needs to be fastidious with circuit connections and verify results. Checking both input and output signals on an oscilloscope is essential; signal amplitudes and wave shapes will indicate if the regulator under test is being driven into dropout or if small signal response has given way to large signal behaviour.
Similar to measuring noise, there are pitfalls that can lead one astray when measuring supply rejection. Careful attention to circuit grounding using star grounds is important. Some effects that are seen while measuring supply rejection actually seem counter-intuitive.
Up until now, a solid design would always include some capacitance at the input of the linear regulator to keep the supply impedance as low as possible across frequency. With high enough supply rejection from a device, this can actually increase the amount of ripple seen at the output.
Figure 19. Using the LT3042 to post-regulate the LT8614 Silent Switcher
Consider a circuit as shown in Figure 19 where the LT3042 post-regulates the LT8614 Silent Switcher regulator. The LT8614 delivers approximately 20 mVP-P of ripple at its 500 kHz switching frequency to the input of the LT3042 through a few cm of copper board traces. With only the 22 µF output capacitor of the LT8614, output ripple of the linear regulator is only a few µVP-P. When a 4.7 µF capacitor is added at the input of the LT3042, output