Full-wave rectifier has programmable gain

Chuck and Chris Wojslaw, Xicor Inc, Milpitas, CA -- EDN, 11/9/2000

The traditional approach to the design of a full-wave rectifier (Figure 1) is to set the gains of IC₁ and IC₂ to 1 and use the steering diodes D₁ and D₂ to sum the sinusoidal half-cycles of the input to form the rectified output. For the positive half-cycle of the input signal, IC₁ is a noninverting amplifier with a gain of 1. For the negative half-cycle of the input signal, IC₂ is an inverting amplifier with a gain of –1. This full-wave-rectifier circuit often combines with a lowpass filter to form a low-cost ac/dc converter. If you need a full-wave rectifier with amplification, the combination of these two functions in one circuit can provide savings in cost, component count, and board space. The circuit in Figure 2 combines rectification and programmable amplification. The two 256-tap Xicor digitally controlled potentiometers, DCP₁ and DCP₂, control the gains of the noninverting amplifer, IC₁, and the inverting amplifier, IC₂, respectively. The gain of IC₁ is ₁G₁=255/P1, where P1 (0P1255) is the programmed decimal value of DCP₁. Similarly, the gain of IC₂ is G=–(255–P2)/P2. The performance of this circuit takes advantage of the resistor matching inherent in the digital potentiometers. The measured data falls within 2% of calculated values.