piclist 2001\02\18\122203a >
Thread: ETS on a PIC ??? Yes - it can be done.
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face BY : Dan Michaels email (remove spam text)



Andrew Vasallo wrote:
>>In addition, the A/D gain internally falls off at about -60 dB/decade,
>>which makes a really nice anti-aliasing filter when you are doing ETS
>>at 1 mhz, but I was hoping for a little more BW.
>
>Maybe I'm barking up the wrong tree here, but my electret microphone input
>on a project (single-stage amplification of 50 nominal) into the A/D results
>in absolutely no amplification at all, really.  Not being very familiar with
>all this audio stuff, does the above statement have any relation to this
>problem?
>

Nothing whatsoever. The A/D doesn't fall off until about 120 khz,
audio is only 20 khz tops.
=========

>I have the mic biased to 2.5V, but I can't get any AC swing.  What I ended
>up doing, only haphazardly, was to crank the gain up to something
>outrageous, in the 1300 range.  Oddly, the noise is only about 2 bits on an
>8-bit A/D.  The overall range still only goes from ~20d to 225d on the A/D
>result, and only saturates if I speak DIRECTLY into the mic.  If I'm about
>one foot away (actually, several feet is my eventual goal), it requires me
>to really shout.  Any idea why the output needs such high gain?  Is this
>even possible?
>

Nothing unusal, you just need a lot of amplification when using
an electret mike. The key, I believe, is to DC bias the mike, and
use AC-coupling to all successive stages: mike --> amp --> amp --> A/D.

Probably best to use at least 2 amplification stages, as a single
stage at gain = 1300 is not gonna have much BW.

LM324 has GBW = 1 Mhz. If setup with gain = 1300, the BW of the
stage is only 1mhz/1300 = 770 hz. This is not enough BW for a
typical audio signal.

- danM

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