PRECISION ABSOLUTE VALUE CIRCUITS:You can build a precision absolute value circuit using two op amps and two precision resistors. If you use an op amp and an IC difference amplifier, no user supplied precision resistors or resistor adjustments are required. Circuits shown are suitable for precision split supply operation and for singlesupply operation. When used with a rail-to-rail op amp, the single supply circuit can approach a 0 to 5V full-wave rectified output from a ±5V input when operating from a single +5V power supply. The circuit shown in Figure 1 is a split supply circuit preferred when high input impedance is desired. To understand how the circuit works, notice that for positive input signals D1 becomes reverse biased resulting in the active circuit fragment shown in Figure 2. A1 drives the noninverting input of A2 through forward biased diode D2. The feedback to the inverting inputs of A1 and A2 is from the output of A2 through resistors R1 and R2. Since no current flows through resistors R1 or R2, in this condition, VOUT is precisely equal to VIN.