Moshe Gerstanhaber and Chau Tran, Analog Devices
EDN
Many modern A/D converters offer only a 5 V input range, and using these converters with a ±5 V or larger input signal gives the designer a problem: how to discard half of a good analog signal without introducing errors and distortion. To solve the problem, you can use an attenuator comprising two operational amplifiers and two resistors (Figure 1). However, this approach can reduce a system's performance by introducing gain errors due to amplifier offset and drift and resistor mismatch.
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| Figure 1. | A conventional active voltage divider requires two op amps, IC1 and IC2, and two resistors, R1 and R2, that form a 2-to- 1 voltage divider. |
Figure 2 shows an alternative circuit that provides a precision gain of one-half with low offset, low drift, and low input-bias currents and that uses an AD8221 instrumentation amplifier.
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| Figure 2. | You can use an instrumentation amplifier to halve an analog signal’s amplitude. All resistors are internal to the IC. |
The amplifier's output, VO, equals the difference between the two inputs, VIN+ and VIN–:
Connecting the amplifier's output to its inverting input and substituting VO for VIN– yields:
or
Thus, the circuit provides a precision gain of one-half with no external components and, in this configuration, is unconditionally stable. The performance plots of Figure 3 and Figure 4, respectively, show a gain error of less than 300 µV and a maximum nonlinearity error of about 1 ppm over a 26 V input-voltage range.
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| Figure 3. | The circuit in Figure 2 introduces a full-scale gain error of less than 300 µV over a 26 V input-voltage range. |
To introduce an offset voltage, VOS, that equals half of a reference voltage (VOS=VR/2), connect the AD8221's reference input (Pin 6) to voltage VR. To bias the attenuator's output at half of the positive- or negative-power-supply voltage, connect the reference pin to the appropriate power supply.
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| Figure 4. | The circuit in Figure 2 introduces a maximum nonlinearity error of about 1 ppm over a 26 V input-voltage range. |
