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| how to route codec (ADC/DAC), analog pins not all on one side |
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| loki42:
Yeah, I thought there might be interaction with the higher internal frequencies of the delta sigma codec. Do you think it's worth adding an LC filter? I've been looking and some 1uH 8.5mOhm inductors that are pretty cheap and would be minimal extra cost. That's similar power loss compared to the ferrite bead and I assume I'd be able to replace it with the LC filter. The annoying bit is the low value resistor for dampening. |
| capt bullshot:
Regarding LC filtering, I always preferred RC filtering with maybe an additional L. LC alone or even a ferrite bead can cause ugly ringing, making things worse. I'd go to include significant resistive damping in the filter. If your circuit allows for the voltage drop, start with 10R (in series to the inductor or instead the inductor), a some 10uF ceramic can achieve a lot then. And check the switching regulator doesn't go into "low power" modes, causing heavy lower frequency ripple. |
| loki42:
The circuit can't cope with that level of voltage drop. I've just been looking at circuits like : https://www.analog.com/en/technical-articles/switching-regulator-noise-reduction-with-an-LC-Filter.html A Pi style filter looks pretty straight forward and the component count is low. Though I'm not sure how to select what the cut off frequency should be. I assume for filtering DC you just want it as low as possible for a given component cost. 1uH and 2 10uF seem to give me a 25khz cutoff frequency unless I'm calculating that wrong. I've been reading through : http://www.ti.com/lit/an/snva538/snva538.pdf as well. |
| capt bullshot:
Analog (and former LT) has some quite useful application notes. To me, the hint "place a filter at the input of your buck regulator too" looks more useful than all the formulas from the TI paper. Corner frequency of the filters doesn't matter too much, as long as it low enough to attenuate the lowest frequency ripple (beware of "pulse-skipping eco-mode" and the like) that occurs in practice - for this reason I like high frequency switchers like the one you used. Anyway, if you do LC filtering, add damping (e.g. ESR of non-ceramic capacitors, high-loss inductance or real resistors in series or parallel with the reactive components). |
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