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From: "John Jardine"
Subject: Re: CMMR op amp outside voltage rails ?
Date: Fri, 6 Dec 2002 01:18:59 -0000
NNTP-Posting-Date: 6 Dec 2002 01:06:45 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Gibbo wrote in message
> "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
> >be asked to work at bottom end with (say) a -5V signal and a -4.99V
> >How are you removing the negative common mode offset voltage if the amps
> >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
> inputs. Then it goes onto the differential amp. Then the last one is used
> the gain. I couldn't get consistent operation using the differential amp
> 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
> 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
> lots of products. I have a feeling an ex employee showed it me and that he
> it from AoE. Not certain.
You've just made my day!.
I looked and looked and looked at that circuit for about 10 minutes before I
twigged that the top amp always forced the bottom into common mode range.
Then looked and looked again for another 5 minutes as I thought the op-amps
would be fighting each other. Realised they wouldn't and spiced it. It does
work over very large pos/neg common mode voltages (gain as well)
That circuit is very, very clever. In fact I'd say it's clever^clever.
Love to know who originally dreamt it up. Might now have to go down the
library to have a read of that AoE book.
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