ADS1299 AVDD and AVSS Supply

Hi Darren,

The LM27761 may work just fine as it seems to have more than enough output current. Also, the internal LDO has about 35dB of PSRR at the 2MHz switching frequency, which is similar to the PSRR given by the TPS72325 at the 250kHz switching frequency of the TPS60403 (under a ~1mA load). The supply pins on the ADS1299 offer around an additional 80dB to 110dB of PSRR, depending on the PGA gain.

I see the advantages of going to a one-chip solution, but my only concern is with the switching frequency of the LM27761, which can vary from 1.7MHz to 2.3MHz. This is very close to (but not exactly) an integer multiple of the modulator frequency in the ADS1299 (1.024MHz typical).

The ADS1299 digital filter response returns to 0dB at n*fMOD. Any unwanted signal content near n*fMOD will be left unfiltered and may show up directly as a low-frequency alias or increase the average noise floor of the system in the bandwidth of interest. If you are using a switching supply, my recommendation would be to synchronize the switching frequency to an integer multiple of the ADC modulator frequency. This way, the frequency content of any remaining ripple will fold back to DC. If that's not possible, the next best thing to do is choose a switching frequency that falls within the digital filter stopband (preferably in one of the sinc filter "nulls" at n*fDR), which is why I believe the TPS72325 was chosen for our EVM.

All in all, I think the part would work. Some of my above concerns can be alleviated with good layout practices too (i.e. keep the switching supply distant from the sensitive analog circuitry and use a solid ground plane).

Best Regards,