Don't quite get your calculations.

The AD7829 is actually 2MSPS theoretically. However, I havn't looked at the
timing specs to check I can get that. I will be using a Secnix at 50MHz (I
need more than 4MIPs so scenix is about the only way to go). I would assume
with 50MIPS I should get 2MSPS (25 inst per sample isn't unreasonable with a
parallel out ADC) but might want to do less to have a bit of time to do
other things.

With 16 knobs that's 1MSPS per 8 knobs = 1\8th MSPS = 125000 SPS for EACH
knob. 30 samples a sec. isn't acceptable in my book. I figured out with 5
active knobs (hands, feet, mouth :)) I can get one 7-bit message (=20bit at
31250bps) out every 3ms. Hence I would like all knob values updated in a
maximum of 2-3ms. I'd really prefer sub 1 ms delays. There's going to be
enough delays in the MIDI transmission without the box introducing more.

So with 16 knobs I could do 125000Hz sampling.

My noise will be the 50MHz clock, the 50\60Hz hum (like it to be suitable
for both apart from actual PSU) and frequencies generated by knob turning
which should be less than 50Hz.

I gather with a sampling frequency of 125000Hz all I need to filter is the
50MHz clock. In fact as long as I sample at at least 500Hz (60Hz * 2 plus
lots of margin) I should simply need a filter somewhere between 250Hz and
50MHz, with lower being better.

I would like the filtering to be as flexible as possible as I will basically
be allowing the ADC to free run so will not neccesarily have a fixed
sampling rate. I will simply keep sampling the ADC copntinuously in my
foreground loop with interrupts to trigger MIDI sendng. Thus the ADC section
will simply be keeping a running average which will occasionaly be sent by
the other process.

You mention digital noise. This is an area in which I am slightly confused.
ADC's always have seperate analog and digital ground to prevent clock noise
on the analog ground. I have looked at 2 application notes detailing ADC
setup. One said that the AGND and DGND's should be connected as close to the
ground source as possible. This sounded pretty reasonable. The easiest path
from ground to AGND has nothing to do with DGND. Yet another app. note (I'm
pretty sure it was an Analog Devices app note) said AGND and DGND should be
connected at the chip and one wire run to the ground. Which is correct?


Thanks,
Tom.
----- Original Message -----
From: Robert A. LaBudde <ral@LCFLTD.COM>
Subject: Re: A/D Challenge
> >So, in my system with 16 knobs and a 1MSPS ADC that's 62500 samples per
knob
> >per second theroetically. So would I filter at ~125000kHz would I? Well,
I
> >know I can't turn a knob 62500 times a second so
>
> No, the filter would roll-off at a frequency no higher than 1 kHz, since
> you're sampling at 2,000/sec. More likely the knobs won't be changed even
> that quickly. If a human is changing the knobs, you would be better
advised
> to sample more slowly, say <= 30/sec and set a low-pass filter with
> roll-off at < 15 Hz to kill the inevitable 60 Hz hum.
>
> >I ain't gonna introduce that frequency. And there's a bit or breathing
room
> >with the 50-60Hz hum. So I guess the only thing I'd need to filter is
EMI.
> >Is there an inordinately high ammount of EMI >125kHz? If not then it
> >shouldn't need the filter as if enough high end EMI is getting through
> >there'll be too much low freq. EMI anyway.
>
> 1. See previous remark.
> 2. You probably have a spike in your noise frequency at 4 MHz, or 20 MHz,
> or whatever your clock operates at, plus sum submultiples and higher
> multiples of that. So you can expect plenty of digital noise.
> 3. You probably have power supply ripple at about 1 kHz. However, this is
> usually very amplitude limited (e.g., < 1 mV p-p), so it won't be an issue
> except with high resolution ADCs.
>
> ================================================================
> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral@lcfltd.com
> Least Cost Formulations, Ltd.                   URL: http://lcfltd.com/
> 824 Timberlake Drive                            Tel: 757-467-0954
> Virginia Beach, VA 23464-3239                   Fax: 757-467-2947
>
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