From: elliott@optilink.COM (Paul Elliott)
Subject: Re: Analog switches/Balanced Demodulators
Distribution: na
Organization: DSC/Optilink Access Products
Lines: 53

In 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 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."

