Absolute-Value Meter With Polarity Detector

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 its 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

–R5/R3 + (–R5/R4) ↔ (–R2/R1) = –1 + 2 = 1

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 polarity detector IC1c acts as a comparator, with a certain amount of positive feedback due to R7 and R8.

This feedback causes a hysteresis of 20mV that prevents oscillation when the input voltage changes slowly. IC1d is an ordinary inverter. For input voltages above 10mV, the SIGN output terminal will swing to almost the positive supply. When the input voltage is below –10mV, the SIGN terminal drops low, almost to the negative supply voltage. For input voltages between these two thresholds, the output voltage is well defined, too, because it stays at its previous level. This circuit is the perfect complement to the ‘+/– voltage on bargraph display’ circuit discussed elsewhere in this blog.

The |Uin| and SIGN outputs of the present circuit may be directly connected to Uin and CONTROL IN inputs of the bi-directional bar display. The ±6 V sign indicator signal may be used as the control voltage for the +/– voltage display as long as the reference voltage remains smaller than 3 V. Although presented as a pair, both circuits may of course be used individually for other purposes.