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.