----- Original Message -----
From: "Lindy Mayfield" <Lindy.Mayfield@EUR.SAS.COM>
Subject: [EE]: Op-Amp question: what is "ac-coupled"?


> I cannot seem to find anywhere where it is defined what
> ac-coupled means.  Could someone please clarify this
> for me?

Let me add a little to Richard's comment.  A cap in the input is the
traditional way of AC coupling an input, but there could be others.  The
relevant feature is that the op amp can't see the DC component of the input.
All it sees is the AC component, centered on zero.  Obviously, in this
service, the capacitor needs to be large enough to pass the AC component at
whatever the expected frequency is.  This typically means that the exact
value for the cap in very non-critical.  Often useful values can range over
several orders of magnitude.  When you do this with a capacitor, it is
sometimes called a DC blocking capacitor.

Sometimes in RF circuits you will see transformers used in this service.
Often these transformers are actually part of tuned circuits, so they
provide some frequency sleectivity in addition to DC isolation.

--McD

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