3.4 Measuring Output Ripple Voltage

The ripple appearing on the output of the switching regulator can be important to the circuits under power. Getting an accurate measurement of the output ripple voltage is not always simple.

If the output voltage waveform is measured using an oscilloscope, an accurate result can only be obtained using a differential measurement method (see Figure 21).

Figure 21. Differential Output Ripple Measurement

The differential measurement shown uses the second channel of the oscilloscope to cancel out the signal that is common to both channels (by inverting the B channel signal and adding it to the A channel).

The reason this method must be used is because the fast-switching components in a switching regulator generate voltage spikes that have significant energy at very high frequencies. These signals can be picked up very easily by antennas as small as the 3-inch ground lead on the scope probe.

Assuming the probes are reasonably well matched, the B channel probe will pick up the same radiated signal as the A channel probe, which allows this common-mode signal to be eliminated by adding the inverted channel B signal to channel A.

It is often necessary to measure the RMS output ripple voltage, and this is usually done with some type of digital voltmeter. If the reading obtained is to be meaningful, the following must be considered:

1. The meter must be true-RMS reading, because the waveforms to be measured are very non-sinusoidal.
2. The 3-dB bandwidth of the meter should be at least 3X the bandwidth of the measured signal (the output voltage ripple frequency will typically be > 100 kHz).
3. Subtract the noise floor from the measurement. Connect both meter leads to the negative regulator output and record this value. Move the positive meter lead to positive regulator output and record this value. The actual RMS ripple voltage is the difference between these two readings.