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  • TI Thinks Resolved

PCM5102: Decoupling -- single 10 uf ceramic versus polarized + .1 uf

Prodigy 160 points

Replies: 1

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Part Number: PCM5102

The example schematic for the pcm510x shows a 10 uf polarized cap and a .1uf non polarized cap on each rail (AVDD, DVDD, and CPVDD).

The evaluation board schematic shows 10 uf polarized caps (presumably aluminum electrolytic) as DNP (do not populate) and the description says those can be eliminated to save board space and cost:

For the analog circuitry surrounding the PCM510x device, certain components can be removed at the cost of higher performance. This configurability may allow for optimal ratio between board space and performance. Components such as C1, C3, C7, and C11 can all be removed for additional board space

(C1, C3, C7, and C11 are the polarized decaps)

But what about a single 10 uf  ceramic  (X7R) instead of a both a .1 uf and 10uf electrolytic?

Typically, two capacitors in parallel are used when a large amount of capacitance (in this case 10 uf) is needed, but a low ESR is also needed.  The small cap provides the low ESR, while the larger cap provides the bulk capacitance needed.  But,  10 uf  X7R ceramic caps have such low ESR (not significantly different than .1 uf caps), that I would think parallel decoupling caps would not provide any benefit.

Still, I see designs in production (e.g. PHatDac) that have both a 10 uf ceramic and .1 uf ceramic on some rails.  Is that just because they have implemented the reference schematic and substituted 10 uf ceramics for the polarized caps indicated in the schematic and did not realize the .1 uf weren't needed if a ceramic 10 uf cap was used?

Second question--

If a single 3.3V supply is used for AVDD, DVDD, and CPVDD, is it necessary to have 10 uf decoupling very close to each of those pins?  Or, is it sufficient to have one (or multiple parallel) 10 uf, X7R, 0805 decaps a little farther away from the pins with .1 uf 0603 X7R as close as possible to each pin?

Third question--

While a "noisy" 3.3 is available on the card, a dedicated  .3V is created for the PCM510x from 5.0 using a LINREG with high PSRR.  Would it be better to use that clean 3.3V supply for all the PCM510x rails, or would it be better to use the "dirty" 3.3V for the PCM510x DVDD?

 

  • Hi Dave,

    1. I think the larger capacitors near the device would make the most sense in larger boards with many subcircuits where the supply source would be far away from PCM. The 10µF cap would act as a charge reservoir near to the chip. In smaller systems it is less necessary. I think having on 10µF on supply and a smaller 0.1-.001µF cap near the pins are a good strategy. In addition, we leave the larger electrolytic capacitor footprints on the board if customers want to experiment with larger caps, audio grade caps.

    2. I think having one 10µF cap with smaller near the device is good.

    3. The AVDD and CPVDD pins supply the analog and charge pump sections, so it is more critical to have a clean supply on those pins. While I cannot say if noise on DVDD definitely will not impact performance, I think you are better off separating the supply from the analog supplies as DVDD can be a source of noise as well (fast switching digital circuits).

    Thanks!
    Paul

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