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From: "John Jardine"
Subject: Re: CMMR op amp outside voltage rails ?
Date: Fri, 6 Dec 2002 02:29:38 -0000
NNTP-Posting-Date: 6 Dec 2002 02:17:25 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
John Larkin wrote in message
> On Fri, 6 Dec 2002 01:18:59 -0000, "John Jardine"
> >Gibbo wrote in message
> >> "John Jardine" wrote:
> >> >Sorry-- I've no ideas, but I was just for interest wondering how
> >> >arranged that 1st op-amp?.
> >> >(As I read it). If you've got a 0V and +5V supply for the 3 op-amps
> >> >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
> >> >run from 0-5V?. (I'm assuming all the voltages are not isolated from
> >> >other)
> >> >Thanks
> >> >john.
> >> >
> >> The first op amp is used purely to remove the common mode signal from
> >> inputs. Then it goes onto the differential amp. Then the last one is
> >> the gain. I couldn't get consistent operation using the differential
> >> 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
> >> 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
> >> lots of products. I have a feeling an ex employee showed it me and that
> >> it from AoE. Not certain.
> >> Gibbo
> >You've just made my day!.
> >I looked and looked and looked at that circuit for about 10 minutes
> >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
> >would be fighting each other. Realised they wouldn't and spiced it. It
> >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.
> >Thanks again.
> Yes, this is in AoE, p 423, but it calls for 0.01% resistors, so it's
> still no magic bullet. It works almost as well if you ground the Rc
> junction and throw away the extra amp, at which point is *is* the
> INA117 circuit.
> Spice does furnish nicely matched resistors.
And pairs of sine 'limiter' diodes that need no selecting and will generate
0.0000% of even order harmonic distortions!.
I used some INA's in a design the other week. Love 'em. Like how they just
sit there and do the job without worrying about masses of quality resistors.
None of the IC diff amps are much good at +90decC though. Had to go to
discrete OP177's and 0.1% 15ppm resistors for one particular job.
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