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. . -> [Subject]  Any reason for the first opamp?

Any reason for the first opamp?

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Posted by Lingyueqing   (2 posts)  [Biography] bio
Date Mon 22 Jan 2018 09:25 AM (UTC)
Message
I saw this application example in the LM324 datasheet.

[img]https://i.stack.imgur.com/PXQrs.png[/img]

In a case like this:

https://i.stack.imgur.com/QeWdT.png

where R1/R2 is equal to R4/R3 and R4/R3 is large (330 like above), the first op-amp seems to act like a unity gain buffer; its output isn't much different from its non-inverting input.

Also, the inputs (and thus, the output) to the op-amp are already offset by Vcc/2 and buffered. Is there still any need for op-amp U1A above? Can both inputs just be fed into U2A directly without compromising the "High Input Z" quality (or anything else) of the original circuit? I'd also like to understand what is meant by "CMRR depends on this resistor ratio match" in the first image.
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Posted by Nick Gammon   Australia  (23,003 posts)  [Biography] bio   Forum Administrator
Date Reply #1 on Tue 23 Jan 2018 12:32 AM (UTC)
Message
This section of the forum is really intended for tutorials. Please post general questions about op-amps to Electronics StackExchange.

- Nick Gammon

www.gammon.com.au, www.mushclient.com
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