Design Goals

Design Description

This single-supply precision absolute value circuit is optimized for low-input voltages. It is designed to function up to 50kHz and has excellent linearity at signal levels as low as 5mVpp. The design uses a negative charge pump (such as LM7705) on the negative op amp supply rails to maintain linearity with signal levels near 0V.

Design Notes

1. Observe common-mode and output swing limitations of op amps.
2. R₃ should be sized small enough that the leakage current from D₁ does not cause errors in positive input cycles while verifying the op amp can drive the load.
3. Use a fast switching diode for D₁.
4. Removing the input buffer allows for input signals with peak-to-peak values twice as large as the supply voltage at the expense of lower input impedance and slight gain error.
5. Use precision resistors to minimize gain error.

Design Steps

1. Circuit analysis for positive input signals.
   
   $\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{i}}}=\left(-\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}\right)+\left(1+\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}\right)=1$
   ${\mathrm{V}}_{\mathrm{o}}={\mathrm{V}}_{\mathrm{i}}$
2. Circuit analysis for negative input signals.
   
   $\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{i}}}=\left(-\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}\right)=-1$
   ${\mathrm{V}}_{\mathrm{o}}=-{\mathrm{V}}_{\mathrm{i}}$
3. Select R₁, R₂, and R₃.
   $\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{i}}}=-\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}$
   $\text{If }{\mathrm{R}}_2={\mathrm{R}}_1 \text{ then }{\mathrm{V}}_{\mathrm{o}}=-{\mathrm{V}}_{\mathrm{i}}$
   $\text{Set }{\mathrm{R}}_1={\mathrm{R}}_2={\mathrm{R}}_3=1 \mathrm{k\Omega }$

Design Simulations

Transient Simulation Results

5mVpp at 1kHz Input

400mVpp at 1kHz Input

Design References

Texas Instruments, Simulation for Single-Supply, Low-Input Voltage Full-Wave Rectifier, circuit SPICE simulation file

Texas Instruments, Single-Supply Low-Input Voltage Optimized Precision Full-Wave Rectifier, reference design

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