Correct me if I am wrong...

How accurate is this Progammable Gain Amp?

Unless it is extremely accurate, you will be introducing a bias or scale
error into the signal that will be quite a bit more than 1 LSB at 18bits,
won't you?

Bob Ammerman
RAm Systems

----- Original Message -----
From: Barry King <barry@NRGSYSTEMS.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, August 09, 2000 1:04 PM
Subject: Re: [EE]:A2d and PGA? Theory?


> Graham,
>
> This makes me think of the way the successive approximation A/Ds
> work.  You make an approximation, then subtract off the approximation
> from the input.  If you can subtract a reference from the signal
> accurately ahead of the PGA, then you might be able to do this:
>
> 1) Make a first cut conversion of the signal at a gain of 1.  The two
> MSBs of the result tell you what quarter of the range you are in.
> Remember that (two bit) result.
>
> 2) Depending of which quarter, subtract either 0, 0.25, 0.5, or 0.75
> of the range from the input.  The result is always in the 1/4 of the
> range.
>
> 3) Now reset the PGA to 4 times the gain.  Convert the the result to
> 16 bits.  Now your two bit quarter plus the scaled up 16 bit result
> are an 18 bit result.
>
> One can imagine variants where the PGA is used to create the voltages
> at the A/D Hi and Low ref.  If the A/D low ref can be floated as high
> at 3/4 the input range, that would let you get away from a seperate
> subtractor amp.
>
> All this said, I wonder if that can possibly be better than
> 1) sell PGA
> 2) sell 16 bit converter
> 3) Buy 18 + bit delta/sigma converter, implement that.
> 4) buy beer with the difference.
>
> I'll be interested to see what other ideas folks come up with.
>
> ------------
> Barry King, KA1NLH
> NRG Systems "Measuring the Wind's Energy"
> http://www.nrgsystems.com
> Check out the accumulated (PIC) wisdom of the ages at:
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