Achieving Precision Absolute Value Output Using a Dual-Channel Difference Amplifier

Traditionally, precision half-wave and full-wave rectifiers use carefully selected components, including high-speed operational amplifiers, fast diodes, and precision resistors. The large number of components makes this solution expensive and is not free from the problems of crossover distortion and temperature drift between components.

This article shows how to configure a dual-channel difference amplifier —without any external components—to provide precision absolute-value outputs. This innovative approach can achieve higher accuracy, lower cost, and lower power consumption than traditional approaches.

As shown in Figure 1, the difference amplifier 1 consists of an operational amplifier and four resistors configured as a subtractor. The low-cost monolithic difference amplifier has internal laser wafer-trimmed resistors and offers very high gain accuracy, low offset, low offset drift, high common-mode rejection, and better overall performance than discrete alternatives.

Figure 1. Difference amplifier.

Traditional absolute value circuit

Figure 2 shows a schematic diagram of a commonly used full-wave rectifier circuit. This design relies on two fast op amps and five precision resistors to achieve high performance. When the input signal is positive, the output of A1 is negative, so D1 is reverse biased. D2 is forward biased, closing the feedback loop around A1 through R2 and forming an inverting amplifier. A2 adds the output of A1 multiplied by a gain of -2 and the input signal multiplied by a gain of -1, resulting in a net gain of +1. When the input signal is negative, D1 is forward biased, closing the feedback loop around A1. D2 is reverse biased and does not conduct. A2 inverts the input signal, producing a positive output. Therefore, the output of A2 is a positive voltage, representing the absolute value of the positive and negative inputs.

Figure 2. Standard full-wave rectifier 2, 3

This design has several inherent performance and system disadvantages, such as cost, crossover distortion, gain error, and noise. The design requires dual supplies and many high-performance components, further increasing cost and complexity. Because the input signal spans 0 V + ΔV and 0 V – ∆V, the output of A1 must swing from –VBE to +VBE, so the response time can be long. High-speed op amps and diodes can help alleviate this problem, but at the expense of higher power consumption. The gain accuracy of the absolute value output depends on the matching of R1, R2, R3, R4, and R5. Even a small mismatch in one resistor can cause a large error between the positive and negative absolute value peaks. The overall noise gain is 6, amplifying the effects of op amp noise, offset, and drift. This article is from Electronics Enthusiasts Network ( http://www.elecfans.com )

Improved absolute value circuit

Figure 3 shows a simpler and more effective absolute value circuit that requires only an AD82774 dual-channel difference amplifier and a positive supply. When the input signal is positive, A1 acts as a voltage follower. The potential of both inputs of A2 is the same as the input signal, so A2 simply passes the positive signal to the output. When the input signal is negative, the output of A1 is at 0 V, and A2 inverts the input signal. The absolute value of the input signal is finally obtained. Signals up to ±10 V can be rectified at frequencies up to 10 kHz. If the signal to be rectified is very small, placing a pull-down resistor at each op amp output can improve the circuit performance near 0 V.

Figure 3. Single-supply absolute value circuit using the AD8277.

This circuit looks simple, but it works, all thanks to the AD8277's excellent input and output characteristics and single-supply operation. Unlike most single-supply applications, the input of this difference amplifier can be driven below 0 V. This allows the input of A1 to accept negative input signals while maintaining a 0 V output. The input integrates ESD diodes for more robust overvoltage protection. Figure 4 shows the input and output waveforms and characteristics of a 1 kHz 20 V pp input signal.

Figure 4. (a) Input and output of a 1 kHz 20-V pp input signal (b) Input and output characteristic curves

This improved absolute value circuit overcomes many of the shortcomings of conventional rectifier designs, and its value is beyond imagination. The most prominent one is the reduction in the number of components required: only one device is needed. The external diode is eliminated, and crossover distortion is also eliminated. Laser wafer trimmed resistors are precisely matched to ensure gain error of less than 0.02%. The circuit has a noise gain of only 2, with lower noise, offset, and drift. Operating from a single 2 V to 36 V supply, the AD8277 consumes less than 400 μA of quiescent current.

in conclusion

A precision full-wave rectifier built with a single dual-channel difference amplifier surpasses traditional designs in several ways. Most notably, the elimination of high-performance external components and dual supplies significantly reduces cost and complexity. This difference amplifier solution does not have crossover recovery issues and is optimized for low drift over a wide temperature range. Using the AD8277, a low power, low cost, high precision absolute value circuit can be implemented with a single IC.