The peak detector is what I described, without knowing what it was 
called.  It is sufficient for what I need.  (This is not precision, just 
an indication of relative volume level.)  My problem is before that.  I 
need to be able to translate an audio signal that is referenced to audio 
ground to a signal with its reference to digital ground without 
connecting the grounds together.

Phil

Phil Keller wrote:

> I am trying to take an analog signal (out of an MP3 player) and 
> determine its signal amplitude with the PIC's A2D input.  I would like 
> to keep the sampling frequency down (to reduce the clock speed and 
> save power) so I thought of adding a large cap outside the PIC in 
> parallel with a resistor to ground to hold the level.
>
> The problem is that the manufacture spec says that I must never 
> connect Audio ground to Digital Ground.
>
> I thought of simply having the analog input across a resistor to 
> analog ground and using a blocking cap to couple the signal to the 
> base of a PNP (The base is DC biased to Vcc/2 and the Collector to 
> digital ground) with a diode on the emitter to the PIC and large Cap & 
> resistor to digital ground.
>
> I tried this with Linear Technology's LTspice and the results were 
> very ugly.
>
> I am sure that this has been done before but I have been unable to 
> find anything via Google or in the PIClist archives.  The right 
> keywords have escaped me so far.
>
> All suggestion, pointers, ideas or answers are gratefully appreciated.
>
> Phil
>
>
>

-- 
===========================================
PK Consulting
Phil Keller
Fremont, Ca

Phil@PKConsulting.net



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