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From: firstname.lastname@example.org (Gibbo)
Date: 05 Dec 2002 20:31:07 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: CMMR op amp outside voltage rails ?
"John Jardine" wrote:
>Sorry-- I've no ideas, but I was just for interest wondering how you've
>arranged that 1st op-amp?.
>(As I read it). If you've got a 0V and +5V supply for the 3 op-amps they can
>be asked to work at bottom end with (say) a -5V signal and a -4.99V signal.
>How are you removing the negative common mode offset voltage if the amps are
>run from 0-5V?. (I'm assuming all the voltages are not isolated from each
The first op amp is used purely to remove the common mode signal from the 2
inputs. Then it goes onto the differential amp. Then the last one is used for
the gain. I couldn't get consistent operation using the differential amp for
gain aswell so this is set at 1 in my application. Thus.......
1/2vcc +ve +-+| \
| / |
- ve+ --| / |
| |/ |
in 1 -----\/\/\------+------+------+---\/\/--gnd
RA | | | | | RB
| \ \ | |
| /RC RD/ | | |\
| \ \ | +--------| \
+--+ | | | \___
| +--+ | / |
\ \ -ve +------+-| / |
/RC RD/ | | |/ |
\ \ | | |
RA | | | +--\/\/--+
in 2 -----\/\/\------+------+------+ RB
I haven't put R values in as they depend entirely on the required offset input
range but RA, RB etc are equal values
Look at how the top amp will operate on a common mode signal.
I can't remember where I first saw this circuit. I've used variants of it in
lots of products. I have a feeling an ex employee showed it me and that he got
it from AoE. Not certain.
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