# Absolute-Value Meter With Polarity Detector

Posted on Feb 7, 2014

This circuit breaks an input voltage signal down into its components: (1) the absolute value and (2) the polarity or sign` (+ or ). It will handle direct input voltages as well as alternating voltages up to several kHz. With a supply voltage of ±9 V, the input level should remain below ±6V. The circuit consists of two sections, each having it

s own function. Operational amplifiers IC1a and IC1b form a full-wave rectifier, its output terminal supplying the absolute value of the input signal, while operational amplifiers IC1c and IC1d examine the polarity of the input voltage. For negative input voltages, the output of IC1a goes high. Consequently D2 is reverse-biased so that IC1a has no effect on the rest of the circuit. IC1b then acts as an inverter because its amplification is ( R5/R3) or 1. Thus, the output voltage is positive. For positive input voltages, D2 conducts and the amplification of IC1a is -1. The output voltage is then determined by the sum of currents that flow through R3 and R4. Taking into account the polarities and the value of all resistors, the overall amplification becomes This means that the value of the output voltage at the output terminal is the same as the input voltage, but the polarity is always positive. The accuracy of the rectification process is determined by the accuracy of resistors R1-R4; close-tolerance (1%) types are recommended. At low input voltages (smaller than 20 mV), the input offset voltage of the operational amplifiers may introduce significant errors. If this is the case, use individual operational amplifiers instead of an array of four (TL061, TLC271, AD548, . ), because they have pins for offset voltage compensation. Alternatively, use an operational amplifier with a low offset voltage like the OP07. In the...

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