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From: davidc@vlsisj.VLSI.COM (David Chapman)
Newsgroups: rec.ham-radio,sci.electronics
Subject: Re: Anomaly in A&A AM WEFAX Demodulator's ADC
Summary: sample/hold is almost always a necessity
Keywords: ADC0804
Message-ID: <15378@vlsisj.VLSI.COM>
Date: 31 Oct 89 22:50:53 GMT
References: <1989Oct24.135458.28372@IDA.ORG> <1989Oct25.170818.1007@IDA.ORG>
Reply-To: davidc@vlsisj.UUCP (David Chapman)
Organization: VLSI Technology Inc., San Jose, CA
Lines: 35

In article <1989Oct25.170818.1007@IDA.ORG> roskos@IDA.ORG (Eric Roskos) writes:
>...It appears simply to
>be a problem with the demodulator board itself... possibly that the
>A/D converter that is used is too slow to convert a particular value of the
>demodulated signal before it changes to a new value, so the ADC gets partway
>through the conversion, finds the value has changed significantly, and
>gets 0's for the low-order bits as a result.
>
>...
>
>It looks like some artifact of how the ADC works...  I have an ADC0820
>which I am thinking of trying to see if it works better (the A/D
>converter on the A&A board is an ADC0804), since the ADC0820 has a
>built-in sample and hold, and can convert at around 20,000
>samples/second. 

Your symptoms are definitely those of a missing sample/hold amplifier.  I
was building a data logger using a 12-bit, 70 kHZ ADC (back in '82 when 12-bit
A/D was hard to do cheaply) and I was having severe noise problems.  I was 
getting only 8 of the 12 bits reliably.  I sent the ADC back with a complaint 
(it was a $135 hybrid module).  They couldn't find anything wrong but sent me
a new one anyway.  It failed too.

That caused me to look at my design more closely, and I found that I had
omitted the sample/hold capacitor (the app note made it look optional).  I
put the cap in, and I had 12 bits of A/D again.  How embarrassing.  :-(

If the ADC doesn't have sample/hold, and you can put in one that does, by
all means do so.  I wouldn't trust an ADC at anything close to the Nyquist 
limit for your input data without sample/hold.
-- 
		David Chapman

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