From: John Woodgate
Subject: Re: Amplifying small DC signal with large DC bias...
Date: Mon, 11 Nov 2002 22:26:08 +0000
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Mon, 11 Nov 2002 23:00:07 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that Mike Deblis
wrote (in ) about
'Amplifying small DC signal with large DC bias...', on Mon, 11 Nov 2002:
>I'm after a few ideas - I have a DC signal - about 4 volts, on which
>is superimposed a small (few uV) DC/VLF signal. I need aboput 40dB
>amplification to reliable use the wanted signal, but how to I remove
>the 4V DC bias to prevent the opamp clipping?
>I really need to use a differential amp as there will be common mode
>noise I need to lose, else I'll bury the wanted signal.
There is a very simple technique used in PIR detectors. The signal is
applied to both inputs of a differential amplifier, but to one input
through a very long time-constant RC network. This low-pass filter makes
that input follow the 'running average d.c.' level of the signal, while
the faster variations appear only at the other input and are amplified.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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