Fig 1 shows a high-speed, full-wave rectifier-or absolute-value circuit-that uses a high-speed clamping amplifier, IC2. This circuit is faster and performs better than rectifiers employing diodes in their amplifiers' feedback paths. These diode-based circuits have limitations at lower signal-input levels and higher bandwidths. These limitations show themselves most clearly as crossover distortion. This circuit cleanly corrects a 20-MHz, 600-mV p-p input sine wave (50(ohm) back-terminated).
The circuit employs IC2's clamping action while IC1 and IC3 function as a buffer and a difference amplifier, respectively. In less than 5 nsec, IC2 accurately clamps the input signal to a level you define. (Because the internal clamping circuitry of these amplifiers is in their input stages, these amplifiers can clamp only in the noninverting mode.)
IC2 forms a positive half-wave rectifier. Connecting IC2's pin 8 to +VS sets its positive clamping threshold to +VS-a level outside the amplifier's input-voltage range. Similarly, grounding IC2's pin 5 sets its negative clamping threshold to ground. (To achieve negative full-wave rectifying, you must configure IC2 as a negative half-wave rectifier by connecting the positive threshold to ground and the negative threshold to the negative supply.)
IC1 buffers the input signal, VIN, from the dynamic input impedance IC3 presents. IC1 also matches the signal delay of IC2. IC3 subtracts the output of the half-wave rectifier, IC2, from the buffer IC1's output to produce the desired full-wave rectification.
This circuit operates from a ±5V supply and has a maximum input range of ±3.4V. Optionally, potentiometer R1 and its associated resistors can trim out any clamping-offset error from IC2. Employing precision 1% resistors maintains <2% gain accuracy and resistance values under 250(ohm) and will not degrade the phase margin of these amplifiers. But, you must follow proper decoupling and high-speed-PCB-layout guidelines. Bypass the circuit's power-supply lines with 22-µF tantalum capacitors. (DI#1673)