I'm trying to create a low-noise 3.3V power supply to power an ADC. The TI
TPS7A53 LDO seems like a good option for this. It has low noise of 4.4 μVRMS and a worst-case PSRR of 10 dB for an input frequency of 2 MHz and 3.3V output voltage.
I hooked up the circuit as follows:
Turning everything on, I see a voltage drop across the LDO but a significant amount of noise on the output.
In this figure the buck converter output is in yellow and the LDO output is in blue:
The voltage drop across the LDO is 150 mV, which is larger than the worst case dropout of 110 mV (datasheet, Fig. 20).
Switching the oscilloscope probes to AC-coupled, and zooming in on the noise, we see the following:
The noise on the output consists of two components: (i) a large spike about every 17 us, and a significant amount of lower-amplitude high-frequency noise. The LDO (shown in blue) is not filtering the input noise in any way, and actually appears to be making the p-p amplitude of the switch spikes worse. This appears to be 0 dB PSRR.
Why are we not seeing the 10 dB or more of PSRR promised by the datasheet? The figure in question is Figure 6:
This seems to suggest that even high frequency switch transients (10 MHz+) should see 10 dB of attenuation.
Am I misunderstanding something? Why isn't the LDO filtering my switched power supply output? Thanks.