Design Ideas: October 26,, 1995
The circuit in Fig 1 lets you use a PC's printer port to set the gain of an inverting amplifier. The circuit uses a 4-to-1 analog multiplexer, IC1, to select a pair of external resistors. The multiplexer's connection scheme ensures that the on-state resistance of the analog switches does not affect the performance of the circuit. The on-state series resistance is negligible in comparison with the high input impedance of the op amp. The op amp's gain is, thus, the ratio of the two network resistances: -R2/R1.
The Turbo C++ program in Listing 1 selects the amplifier gain. In Fig 1, the various resistor pairs draw differing values of current from the input source. The noninverting circuit in Fig 2 overcomes this potential drawback. The circuit selects the feedback network in the same manner, but IC2 acts as a demultiplexer. The voltage gain is 1+R1/R2. With the indicated resistances in both configurations, the gain choices are 1, 5, 10, and 100. The CMOS switches contribute no output-voltage drift, so the only trim you need is for the amplifier's offset voltage. (DI #1773)