piclist 2000\05\04\135750a >
Thread: [EE] 24-bit A/D. Are We in the Twilite Zone Here?
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BY : Andrew Kunz email (remove spam text)

We were measuring an RF signal with a high noise component (0-15 was reasonable,
ie, 160% noise on 100% signal!)

The 10-50 was counts; I don't have the formula which converted counts to dBmV.

It worked much better than w/o the noise correction.

Andy

Scott Dattalo <scottDATTALO.COM> on 05/04/2000 01:32:11 PM

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Subject: Re: [OT] [EE] 24-bit A/D. Are We in the Twilite
Zone Here?

On Thu, 4 May 2000, Andrew Kunz wrote:

> James,
>
> What we would do is measure an analog signal using the time-honored cap/res
> delay time with a PIC.  Our time was typically 10-50, where our signal was
> acquired (sample-and-hold setup).
>
> Then we would add a random number to this in the range 0-15.  The random
number
> could come from any of a number of places; ours was the CRC of the last
message
> we received (we were continuously receiving messages).
>
> These results were then continuously averaged (Avg = ((sum of samples) - Avg)
+
> newsample).
>
> The result was surprisingly accurate.

James, now I know why you were confused. I have to admit that after Andy's
explanation I'm confused as well. First a couple of questions which Andy may
wish to answer. When you say 'time was typically 10-50', what does that
mean? 10-50 milliseconds, 10-50 samples per second? Are you sure that the
formula is correct? (There's a scaling factor missing.)

I suspect if you go back and repeat your experiments with the "digital random
number" stuff removed that you'll get the same results - perhaps even improved
results. From the information you've provided, I can confidently assert that
adding this digital noise is of no beneficial value whatsoever. Instead, the
averaging algorithm removes the random variations in the REAL analog signal
along with the digital variations that were artificially added. Theory aside,
let's look at it practically. If your A/D converter is absolutely solid and
your signal contains no noise, then you'll get the same value everytime you read
the A/D (assuming a DC input of course). The information from the A/D is
constant. There's nothing you can do to increase the information content. If you
add a random value that has a mean of zero, this is not going to affect the A/D
result.

Scott

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