Design Goals

Design Description

This absolute value circuit can turn alternating current (AC) signals to single polarity signals. This circuit functions with limited distortion for ±10V input signals at frequencies up to 50kHz and for signals as small as ±25mV at frequencies up to 1kHz.

Design Notes

1. Be sure to select an op amp with sufficient bandwidth and a high slew rate.
2. For greater precision look for an op amp with low offset voltage, low noise, and low total harmonic distortion (THD).
3. The resistors were selected to be 0.1% tolerance to reduce gain error.
4. Selecting too large of a capacitor C₁ causes large distortion on the transition edges when the input signal changes polarity. C₁ may not be required for all op amps.
5. Use a fast switching diode.

Design Steps

1. Select gain resistors.
   1. Gain for positive input signals.
      $\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{i}}}=1\frac{\mathrm{V}}{\mathrm{V}}$
   2. Gain for negative input signals.
      $\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{i}}}=-\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}=-1\frac{\mathrm{V}}{\mathrm{V}}$
2. Select R₁ and R₂ to reduce thermal noise and to minimize voltage drops due to the reverse leakage current of the diode. These resistors will appear as loads to U₁ and U₂ during negative input signals.
   ${\mathrm{R}}_1={\mathrm{R}}_2=1 \mathrm{k\Omega }$
3. R₃ biases the non-inverting node of U₂ to GND during negative input signals. Select R₃ to be the same value as R₁ and R₂. U₁ must be able to drive the R₃ load during positive input signals.
   ${\mathrm{R}}_3=1 \mathrm{k\Omega }$
4. Select C₁ based on the desired transient response. See the Design Reference section for more information.

Design Simulations

Transient Simulation Results

±10V at 50kHz Input

±25mV at 1kHz Input

Design References

Texas Instruments, Simulation for Photodiode Amplifier, circuit SPICE simulation file

Texas Instruments, Prevision Full-Wave Rectifier, Dual-Supply, reference design

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