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From: John Larkin
Subject: Re: CMMR op amp outside voltage rails ?
Date: Thu, 05 Dec 2002 17:46:46 -0800
Organization: Posted via Supernews, http://www.supernews.com
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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 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.
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
Spice does furnish nicely matched resistors.
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