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More Value from Your Absolute Value CircuitDifference
Amplifier Enables Low-Power,
Precision half- and full-wave rectifiers are traditionally built using carefully selected components, including high speed op amps, fast diodes, and precision resistors. The high component count makes this solution expensive, and it suffers from crossover distortion and temperature drift variation between components.
This article shows how a dual-channel difference amplifier can be configuredwith no external componentsto provide a precision absolute value output. This innovative approach can achieve better precision, lower cost, and lower power consumption than conventional approaches.
A difference amplifier1 comprises an op amp and four resistors configured as a subtractor, as shown in Figure 1. Featuring laser-wafer-trimmed resistors, low-cost monolithic difference amplifiers offer very high gain accuracy, low offsets, low offset drift, high common-mode rejection, and better overall performance than discrete alternatives.
Figure 1. Difference amplifier.
Absolute Value Circuit
Figure 2.Textbook full-wave rectifier.2, 3
This design has several inherent performance and system shortcomings, including cost, crossover distortion, gain error, and noise. The design requires dual power supplies and many high performance components, further increasing the cost and complexity. The response time can be long because A1's output has to swing from VBE to +VBE as the input signal crosses from 0 V + ΔV to 0 V ∆V. High speed op amps and diodes can help mitigate this problem, but only at the cost of increased power dissipation. The gain accuracy of the absolute value output is determined by the matching of R1, R2, R3, R4, and R5. A substantial error between the positive and negative absolute value peaks will occur if even a single resistor is mismatched by a small amount. The overall noise gain is 6, amplifying the effects of op-amp noise, offset, and drift.
Figure 3. Single-supply absolute value circuit using AD8277.
The function of this circuit, while deceptively simple, is possible only because of the AD8277's exceptional input and output characteristics and its ability to operate on a single supply. Unlike most op amps running on a single supply, the inputs of the difference amplifier can be driven below 0 V. This allows the inputs of A1 to accept a negative input signal while maintaining a 0 V output. Integrated ESD diodes at the inputs provide additional robust overvoltage protection. Figure 4 shows the input and output waveform and characteristic with a 20-V p-p input signal at 1 kHz.
Figure 4. (a) Output and input for 20-V p-p input signal at 1 kHz. (b) Output vs. input characteristic.
This improved absolute value circuit overcomes many of the limitations of the conventional rectifier design and offers unexpected value. Most remarkable is the reduced number of required components: only a single device is needed. Eliminating the external diodes also eliminates the crossover distortion. The laser-wafer-trimmed resistors are precisely matched, guaranteeing gain error of less than 0.02%. The circuit's noise gain is only 2, resulting in lower noise and lower offset and drift. Operating on a single 2-V to 36-V supply, the AD8277 consumes less than 400 μA of quiescent current.
3Sedra, A.S. and K.C. Smith. Microelectronics Circuits. 4th ed. New York: Oxford University Press. 1998.
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