Measuring 2 nV/√Hz noise and 120 dB supply rejection in linear regulators; the Quest for Quiet, part 3

February 09, 2016 // By Todd Owen and Amit Patel
A quiet, well regulated supply is important for optimum performance in a number of circuit applications; linear regulators are required to provide quiet power supply rails, but how does one ensure that the regulator performs as specified?

Part 1 of this article is here, and part 2 here.

Measuring regulator output noise

Once the amplifier is checked and calibrated, actual noise measurements are made. Accurately measuring linear regulator output noise and obtaining faithful results requires careful attention to DUT shielding, component choice, layout, and cable management.

Figure 9. Noise measurement bench setup. Shielded box houses noise amplifier, low output impedance of linear regulator removes necessity for shielding, but magnetic fields can still affect output


Figure 9 shows the configuration used for testing a linear regulator, highlighting the construction and shielding used to avoid magnetic fields from intruding on the measurement. Only one instrument is connected at any given time to preclude ground loops from corrupting the measurement.

Battery power is chosen to supply the linear regulator for the same reason as powering the amplifier; the goal is to measure the noise of the linear regulator, not characterise supply rejection. The regulator does not need to be shielded, as the low output impedance of the regulator makes it much less susceptible to low frequency magnetic fields. Connections from the regulator output to the noise amplifier need to be short barrel connectors since long flexible cables will introduce errors due to triboelectric [4] effects.

The amplifier output is fed directly into an oscilloscope to measure peak-to-peak noise. As shown in Figure 10, the peak-to-peak noise of the LT3042 is 4 µVP-P. A spectrum analyser plot of the same regulator (shown in Figure 11) shows the noise for various amounts of SET pin capacitance. The RMS noise from 10 Hz to 100 kHz as a function of SET pin capacitance is shown in Figure 12.

Figure 10. LT3042 noise in 10 Hz to 100 kHz bandwidth. RMS noise measures 0.8 μVRMS

Figure 11. Noise spectral density plot shows effect of increasing SET pin capacitance on LT3042

Figure 12. Increasing SET pin capacitance decreases RMS noise in 10 Hz to 100 kHz bandwidth