At 07:58 PM 2/19/03 +1100, Roman Black wrote:

>Cheapest cleverest way to get some noise into the
>ADC input? Small antenna track on PCB? Tiny cap
>coupled to Vdd pin?? :o)

I use the AC mains.  I adjust the sample rate such that I get an integral
number of samples in 1 AC period - 16, 64, 256, 1024 samples, depending
upon the project.

The bonus is that much (most) of the AC component is rejected should I be
sampling an input that has a lot of AC hum in it.  A couple of systems that
I had to interface with one time ran thermocouple transmitter wiring in the
same pipes as their 110Vac control wiring - I had no problems getting good
a/d results.  Surprised the heck out of me at the time - the AC content was
brutal.

If the a/d input is from something local, I'll inject about 1 LSB worth of
hum into the signal.  That's more than the theory says is necessary but I
seem to get better results.  On battery powered systems, I'll generate a
ramp or sawtooth from an available signal and make sure that my a/d sample
rate is harmonically related to half the signal rate (same number of
samples in each half-cycle).

dwayne

--
Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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