Thanks for a very pertinent article as I am designing an ADC PSU using the TPS62590 right now.
A couple of items;
If the Murata part is a 27uF capacitor, then are the 33uF Tant and 10uF Ceramic doing anything?
In your setup, was there any impedance between the TPS62590 PSU output cap and the Murata cap, or was the Murata cap effectively part of the PSU output capacitance?
Keep in mind that the effective in-circuit self-resonant frequency of a surface mount ceramic capacitor is heavily influenced by the length of copper etch from vias to pads and by via inductance to planes. For best results place 3 vias at each pad with just the bare minimum of etch length needed to keep solder confined to the pad. Also capacitor dielectric should be good RF quality such as NP0 or X7R.
GREAT-Terry, thanks for reading my article. You’ve raised a good point. Our customers typically use the low noise LDO in two ways – either as the main power source for the ADC to generate a ‘clean, noise-free’ power rail or as a ‘clean-up’ device after a dc/dc converter for a more power efficient solution. As you mentioned, the actual PSRR of the LDO in the switching frequency range is not very high, so its benefit is limited. In the cases we’ve investigated, that’s usually sufficient for the ADC, but it results in increased power consumption, and we’ve found dissipating an extra 20-40% of power in the LDO unnecessarily, especially in portable battery powered products. With proper design, the ADC can be powered by a dc/dc converter and achieve similar performance as with the LDO. However attention must be given to the power supply filter, and that’s where we’ve noticed a big difference. I’ve tried many different ferrite beads, and it always came down to impedance around the switching frequency, which is what we’ve tried to optimize. While our experiments in this area have focused on improving the power efficiency of designs with high-speed ADCs, there’s a significant opportunity for further research in the RF arena to drive down power consumption even more.
Thanks -- interesting info.
It's interesting that the Murata part is effective, since it's apparently a 27 uF feedthrough cap, with no series inductance. The insertion loss measurements may be misleading, because without any series inductance the 50 ohm source impedance may play a dominant role.
LDO itself usually won't have good attenuation at switching frequency up at MHz so I guess this comparison is not yet optimized. In fact, having only 20dB attenuation may be good enough for ADC but it is still not good for very noise sensitive sections like PLL in which very low phase noise is desirable. Jim Williams of Linear Technology has a very good article that tells people a ferrite bead is good for high switching noise filtering while LDO is only good for low frequency noise (in which PLL still has to rely on).