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!elliott From: elliott@optilink.COM (Paul Elliott) Newsgroups: sci.electronics Subject: Re: Analog switches/Balanced Demodulators Message-ID: <15485@optilink.COM> Date: 21 Apr 93 00:37:22 GMT References: Distribution: na Organization: DSC/Optilink Access Products Lines: 53 In article 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 analog multiplexer, with positive and negative power supplies (can be run from a single-ended supply as well). With dual supplies, the logic inputs still range from ground (0 Volts) to VDD. This is a neat (well, I think so) design for a switchable-polarity amplifier: +-----/\/\/\-------+ | | | /--------\ | INPUT -+-/\/\/\--+----| - | | | | opamp |----+------- OUTPUT +-/\/\/\--+----| + | | \--------/ | CONTROL ---------X (analog switch) | | --- GND All resistors are equal-value. When the analog switch is closed, the amp is inverting-gain-of-one. With the switch open, it is non-inverting-gain-of-one. You can clean up the circuit to trim out input offset current if this hurts the balance (this would show up 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."