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From: email@example.com (Gibbo)
Date: 07 Dec 2002 12:47:22 GMT
Organization: AOL, http://www.aol.co.uk
Subject: Re: CMMR op amp outside voltage rails ?
John Larkin wrote:
>>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
That kinda depends on your definition of "almost".
Our application calls for a single rail supply and negative or positive input
offset. In those circumstances the circuit I posted seems to be the only
Grounding the Rc junction will not permit negative input offset with a single
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