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SIGNAL CHAIN BASICS #38: Single-Supply Precision Rectifiers

Precision rectifier circuits are commonly used where the absolute value of a signal is needed, as part of a circuit measuring signal magnitude in metering applications. Countless designs exist for these types of circuits, but realizing this function in a single-supply system can be challenging.

Many recent designs rely on the saturation behavior of a single-supply operational amplifier (op amp) to realize the rectification. In many cases, this is acceptable, but if you want to avoid saturating an op amp and the inherent concerns with that (slow recovery time, potential undesired phase inversion), the circuit in Figure 1 is a good solution.


Figure 1: Single-supply precision rectifier.
(Click on image to enlarge)

The circuit of Figure 1 accepts negative signals (up to the supply rail of the device; 5 V in this example). With a +5 V supply, this circuit will accept up to a 10 Vp-p signal centered on zero volts (i.e ±5 V).

For positive signals (Vin > 0 V), U1 acts as a summer amplifier and U2 and D1 are out of the picture. For negative signals (Vin <0 V) D1 and U2 form a ground clamp and hold the non-inverting input of U1 at 0 V. U1 now acts only as an inverting amplifier. The result is a full wave rectified sine wave at Vout, as shown in Figure 2 .


Figure 2: SPICE simulation of circuit of Figure 1.
(Click on image to enlarge)

For this circuit to operate properly, the op amp outputs must swing to the negative power supply rail on input and output without phase inversion.

The same circuit can be used for input signals that don't swing below ground, but are referenced to Vcc /2 by simply changing the reference point (the non-inverting input) of U2 to the mid-supply reference, as shown in Figure 3 .


Figure 3: Changing the reference voltage on U2 changes the rectifier's input range.
(Click on image to enlarge)

Figure 4 shows the operation of the Figure 3 circuit. In Figure 4, the top trace (Vin ) is shown referenced to the mid-supply voltage, so it appears to be bipolar–the trace labeled Vin_ref is the actual input voltage referred to ground, which clearly remains above ground at all times.


Figure 4: SPICE simulation of circuit of Figure 3.
(Click on image to enlarge)

The circuits of Figures 1 and 3 both work up to the voltage swing limitations of the op amps selected. Rail-to-rail input and output op amps, which allow operation typically within a few tens of millivolts of the supply rails, will give the best results in this application.

Please join us next month when we will discuss Class G audio amplifier architecture.

References
Jones, D. and Stitt, M., “Precision Absolute Value Circuits,” Texas Instruments, SBOA68, December 1997.

About the Author

Rick Downs is applications engineering manager for Texas Instruments' Precision Analog group, Tucson, Arizona. Over the past 23 years, Rick has held various positions in applications and marketing of analog semiconductors focused on audio, data acquisition, digital temperature sensors and battery management products. Rick received his BSEE from the University of Arizona, and holds four patents. He has authored several articles and application notes on analog topics, and prepared and delivered several seminars on data acquisition. You can send your questions to Rick at scb@list.ti.com.

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