PCM2900

John,

Thank you - this answers the question.  I have couple of more from my customer:

1) On the eval board for the PCM2900C, there is an anti aliasing filter.  What kind is it on the schematic? There are different modes such as Sallen key etc.  How does it work?

2) On the eval board you are using a crystal with 18 pF and the 2 caps are also 18 pF each. Normally, it would be higher as the two caps are in series and they'd become 9 pF in your case + 2 pF for trace caps and it comes to 11 pF.  I would think you'd add a cap such as 30 pF.  Can you ask why?

Thanks,

Dan

Hi Dan,

Glad to hear it!

1) Well, there is an anti-aliasing filter built-in to the stereo ADC in the PCM2900C and for the typical filter frequency response, look at page 14 of the d/s.

2) The caps were chosen based on the data sheet for the crystal as well as the requirements of the PCM2900C. On the top of page 21, under Clocks and Reset, choosing caps for the crystal is explained.

Also on figure 36, the notes it say "C5, C6: 10 pF to 33 pF (depending on crystal resonator)"

Regards,

~John

1. On the evaluation schematic, you have op amp based amplifiers driving the line in and ou0,t and also to the line inputs to the CODEC. The way you have it set up looks to me like an active filter. Is it based of multiple feedback topology?
2. The crystal used in evaluation board needs 18 pF. The crystal sees the two external caps in parallel with parasitic impedances or (18/2 + 2 pF(assuming)) = 10 pF. This does not violate the spec but the crystal does not see its rated impedance. I would expect something close to 30 pF so that (30/2 + 2 ) = 17 pF
3. On the eval schematic, you are powering the VCCCI pin of the CODEC with an LDO. I see a diode at the input. Is that an ORRING diode? Do I need the diode? I have a few other LDO parts available in stock and may consider using that and I wanted to know if I can drive the pin direct or it must go via the diode as shown the schematic

Hi Dan,

No problem.

1. Yes, this is an active double order filter.

2. This is an interesting point. For your design, I would follow that same logic (as recommended by the d/s and the crystal).

3. The diode is necessary for the design, as you may also notice it in the recommended design in figure 36 mentioned previously.

Regards,

~John

John,

Thanks for the quick turn on answers in this thread.

Dan

Hello John,

A couple more questions:

1. On PCM2900C, if I am connecting to a stereo headset, do I connect the microphone analog signal to the left or right channel?
2. For mono application, which signal do I drive the ear piece, left or right?

Hi Dan,

1.) This depends on the application, but in most cases, attaching the microphone to both could avoid future issues (microphone output only being on left or right channel). 

2.) Also, application specific. In this application, is there a way to make the USB output have the same for the left and right channels? Then both output channels would be the same and either channel could work.

Regards,

~John