Design Goals

Design Description

This low-pass filter topology offers a significant improvement in settling time over the conventional single-pole RC filter. This is achieved through the use of diodes D₁ and D₂, that allow the filter capacitor to charge and discharge much faster when there is a large enough difference between the input and output voltages.

Design Notes

1. Observe the common-mode input limitations of the op amp.
2. Keeping C₁ small will ensure the op amp does not struggle to drive the capacitive load.
3. For the fastest settling time, use fast switching diodes.
4. The selected op amp should have sufficient output drive capability to charge C₁. R₃ limits the maximum charge current.

Design Steps

1. Select standard values for R₁ and C₁ based on f_C = 10kHz.
   ${\mathrm{R}}_1=\text{10}\mathrm{k\Omega }$
   ${\mathrm{C}}_1=\frac{1}{2\mathrm{\pi }\times {\mathrm{f}}_{\mathrm{C}}\times {\mathrm{R}}_1}=\frac{1}{2\mathrm{\pi }\times 10\text{kHz}\times 10\text{kΩ}}=\text{1.6nF}$
2. Set the diode threshold voltage (V_t). This threshold is the minimum difference in voltage between the input and output that will result in diode conduction (fast capacitor charging and discharging).
   ${\mathrm{V}}_{\mathrm{t}}=\frac{{\mathrm{V}}_{\mathrm{f}}}{1+{\displaystyle \frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}}}\approx \frac{\text{0.6}\mathrm{V}}{1+{\displaystyle \frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}}}=\text{20}\mathrm{mV}$
   ${\mathrm{R}}_2=\left(\frac{\text{0.6}\mathrm{V}}{\text{20}\mathrm{mV}}-1\right)\times {\mathrm{R}}_1=\text{290}\mathrm{k\Omega } \approx \text{300}\mathrm{k\Omega } \text{(standard 5\% value)}$
3. Select components for noise pre-filtering.
   ${\mathrm{f}}_{\mathrm{c}2}=10\times {\mathrm{f}}_{\mathrm{c}}=\text{100}\mathrm{kHz}$
   ${\mathrm{f}}_{\mathrm{c}2}=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_4\times {\mathrm{C}}_3}$
   $\text{Select} {\mathrm{R}}_4={\mathrm{R}}_1=\text{10}\mathrm{k\Omega }$
   ${\mathrm{C}}_3=\frac{{\mathrm{C}}_1}{10}=\text{160}\mathrm{pF}$
4. Add compensation components to stabilize U₁. R₃ limits the charge current into C₁ and also serves to isolate the capacitance from the op amp output when the diodes are conducting. Larger values will improve stability but increase C₁ charge time.
   $\text{Select} {\mathrm{R}}_3=\text{100}\mathrm{\Omega }$
5. C₂ provides local high frequency feedback to counteract the interaction between the input capacitance with the parallel combination of R₁ and R₂. To prevent interaction with C₁, select C₂ as the following shows:
   $\text{Select} {\mathrm{C}}_2=\frac{{\mathrm{C}}_1}{50}=\text{32}\mathrm{pF}\approx \text{33}\mathrm{pF}\text{ (standard value)}$

Design Simulations

AC Simulation Results

Transient Simulation Results

Design References

Texas Instruments, SBOMAU1 TINA-TI™ circuit simulation, file download

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