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Volume 44 –
April 2010

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More Value from Your Absolute Value
Circuit—Difference Amplifier Enables Low-Power,

High-Performance Absolute Value Circuit

By Moshe
Gerstenhaber and Reem
Malik

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 configured—with no external components—to 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.

Traditional Absolute Value
Circuit
Figure 2 shows a schematic of a commonly used
full-wave rectifier circuit. To achieve high performance, this
design relies on two fast op amps and five precision resistors.
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 sums the output of A1 times a gain of −2 with the input signal
times a gain of −1, leaving 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, resulting in a positive output. Thus, the output of
A2 is a positive voltage that represents the absolute value of the
input, whether positive or negative.

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.

Improved Absolute Value Circuit
Figure
3 shows a simpler, more effective absolute value circuit that
requires only one AD82774
dual-channel difference amplifier and a single positive
supply. When the input signal is positive, A1 acts as a
voltage follower. Both inputs of A2 are at the same potential 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 overall result is the
absolute value of the input signal. Signals as large as ±10 V can
be rectified at frequencies as high as 10 kHz. If the signal to be
rectified is very small, a pull-down resistor at each op amp
output can improve the circuit performance around 0 V.

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.

Conclusion
A precision full-wave
rectifier built with a single dual-channel difference amplifier
offers several advantages over traditional designs. Notably, the need
for high performance external components and dual power supplies is
eliminated, drastically lowering cost and reducing complexity. The
difference amplifier solution has no crossover recovery problem
and is optimized for low drift over a wide temperature range.
Using the AD8277, a high precision absolute value circuit can be
realized at low power consumption and low cost using a single
IC.

References

1www.analog.com/en/amplifiers-and-comparators/difference-amplifiers/products/index.html.

2http://sound.westhost.com/appnotes/an001.htm.

3Sedra, A.S. and K.C. Smith.
Microelectronics Circuits. 4th ed. New York: Oxford
University Press. 1998.

4www.analog.com/en/amplifiers-and-comparators/difference-amplifiers/ad8277/products/product.html.

Authors

Moshe Gerstenhaber [[email-masked]]
is a Division Fellow at Analog Devices. He began his
career at ADI in 1978 and has held various senior
positions over the years in manufacturing, product
engineering, and product design. Moshe is currently the design
manager of the Integrated Amplifier Products (IAP) Group. He
has made significant contributions in the field of
amplifier design, especially very-high-precision
specialty amplifiers, such as instrumentation and
difference amplifiers.
(return
to top)

Reem Malik [[email-masked]]
is an applications engineer in the Integrated Amplifier
Products (IAP) Group in Wilmington, MA. She supports
customers in the instrumentation, industrial, and
medical areas and is responsible for products such as
difference amplifiers and variable gain amplifiers. Reem holds
BSEE and MSEE degrees, both from Worcester Polytechnic
Institute, earned in 2003 and 2008, respectively. She
joined Analog Devices in June 2008.
(return
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