Digital Ammeter Amplifier

        Both of these Op-Amps were required to work at +/- 3V, but the OP-07 does not have good output swing at this supply level. However it comes with a very low input offset voltage, which is why it was selected. The TLC2232 output can almost get to the supply voltages, which is why it was selected.

The gain of the OP-07 preamplifier was selected to make best use of its available output swing. The limit is for AC inputs, which were assumed to be sine waves.

The precision rectifier circuit is well-known. It depends on the resistors being matched, and the diodes having low leakage. The resistors are in SIP packages, which ensure good matching.

It is modified by adding D1, which has no effect on the rectification. It is there because it provides bias to detect a negative input. The voltage will be half for AC. This output is measured and used to indicate the sign character on the display.