Path: cantaloupe.srv.cs.cmu.edu!crabapple.srv.cs.cmu.edu!fs7.ece.cmu.edu!europa.eng.gtefsd.com!darwin.sura.net!haven.umd.edu!uunet!optilink!elliottFrom: elliott@optilink.COM (Paul Elliott)Newsgroups: sci.electronicsSubject: Re: Analog switches/Balanced DemodulatorsMessage-ID: <15485@optilink.COM>Date: 21 Apr 93 00:37:22 GMTReferences: <msf.734832494@skaro>Distribution: naOrganization: DSC/Optilink Access ProductsLines: 53In article <msf.734832494@skaro> msf@skaro.as.arizona.edu (Michael Fulbright) writes:>I am trying to build a synchronous demodulator and I've hit a snag.>In my application I want to be able to change the gain of an>op amp amplifier from 1 to -1, controlable via a digital input.>The most obvious way I've come up with is to use analog switches>to adjust the gain of the op amp. The only analog switch I have>experience with it the 4066. Unfortunately I want to switch an>AC signal which goes from about -5V to 5V, and the 4066 is only>for positive signals.>[...]> I have a carrier signal which varies between 0V and +5V. I want>to demodulate an input signal by feeding the input signal thru a>amp with a gain of 1 when the carrier is at +5V, and thru a amp>with a gain of -1 when the carrier is 0V. The output is then>low pass filtered. I believe this is called lock in detection as well>as synchronous demodulation.Look at the 4053.  This is a triple 2-to-1 transmission-gate analogmultiplexer, with positive and negative power supplies (can be runfrom a single-ended supply as well).  With dual supplies, the logicinputs still range from ground (0 Volts) to VDD.This is a neat (well, I think so) design for a switchable-polarityamplifier:                 +-----/\/\/\-------+                 |                  |                 |    /--------\    |INPUT -+-/\/\/\--+----| -      |    |       |              |  opamp |----+------- OUTPUT       +-/\/\/\--+----| +      |                 |    \--------/                 |CONTROL	---------X (analog switch)                 |                 |                ---                GNDAll resistors are equal-value.  When the analog switch is closed,the amp is inverting-gain-of-one.  With the switch open, it isnon-inverting-gain-of-one.  You can clean up the circuit to trimout input offset current if this hurts the balance (this would showup as carrier feed-through).For high frequencies, the slew-rate of the opamp might cause problems,especially if it isn't symmetrical (and it usually isn't).-- --------     Paul Elliott - DSC Optilink - Petaluma, CA USA      ----------    {uunet,pyramid,tekbspa}!optilink!elliott -or- elliott@optilink.com       "I used to think I was indecisive, but now I'm not so sure."