Design Goals

Design Description

This tunable band-pass attenuator reduces signal level by –40dB over the frequency range from 10Hz to 100kHz. It also allows for independent control of the DC output level. For this design, the pole frequencies were selected outside the pass band to minimize attenuation within the specified bandwidth range.

Design Notes

1. If a DC voltage is applied to V_ref be sure to check common mode limitations.
2. Keep R₃ as small as possible to avoid loading issues while maintaining stability.
3. Keep the frequency of the second pole in the low-pass filter (f_p3) at least twice the frequency of the first low-pass filter pole (f_p2).

Design Steps

1. Set the pass-band gain.
   $\mathrm{Gain}=-\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}=-0.01\frac{\mathrm{V}}{\mathrm{V}}(-40\mathrm{dB})$
   ${\mathrm{R}}_1=100\mathrm{k\Omega }$
   ${\mathrm{R}}_2=0.01\times {\mathrm{R}}_1=1 \mathrm{k\Omega }$
2. Set high-pass filter pole frequency (f_p1) below f_l.
   ${\mathrm{f}}_{\mathrm{l}}=10\mathrm{Hz},{\mathrm{f}}_{\mathrm{p1}}=\mathrm{2.5Hz}$
3. Set low-pass filter pole frequency (f_p2 and f_p3) above f_h.
   ${\mathrm{f}}_{\mathrm{h}}=100\mathrm{kHz}$
   ${\mathrm{f}}_{\mathrm{p}2}=150\mathrm{kHz}$
   ${\mathrm{f}}_{\mathrm{p}3}\ge 2\times {\mathrm{f}}_{\mathrm{p}2}=300\mathrm{kHz}$
   ${\mathrm{f}}_{\mathrm{p}3}=300\mathrm{kHz}$
4. Calculate C₁ to set the location of f_p1.
   ${\mathrm{C}}_1=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_1\times {\mathrm{f}}_{\mathrm{p}1}}=\frac{1}{2\mathrm{\pi }\times 100\mathrm{k\Omega }\times 2.5\mathrm{Hz}}=0.636\mathrm{\mu F}\approx 1 \mathrm{\mu F}\text{ (Standard Value)}$
5. Select components to set f_p2 and f_p3.
   ${\mathrm{R}}_3=8.2\mathrm{\Omega }\text{ (provides stability for cap loads up to 100nF)}$
   $$\begin{gathered} {\mathrm{C}}_2=\frac{1}{2\mathrm{\pi }\times ({\mathrm{R}}_2+{\mathrm{R}}_3)\times {\mathrm{f}}_{\mathrm{p}2}}=\frac{1}{2\mathrm{\pi }\times 1008.2\mathrm{\Omega }\times 150\mathrm{kHz}} \\ =1052\mathrm{pF}\approx 1200\mathrm{pF}\text{ (Standard Value)} \end{gathered}$$
   ${\mathrm{C}}_3=\frac{1}{2\mathrm{\pi }\times {\mathrm{R}}_3\times {\mathrm{f}}_{\mathrm{p}3}}=\frac{1}{2\mathrm{\pi }\times 8.2\mathrm{\Omega }\times 300\mathrm{kHz}}=64.7\mathrm{nF}\approx 68\mathrm{nF}\text{ (Standard Value)}$

Design Simulations

DC Simulation Results

The amplifier passes DC voltages applied to the noninverting pin up to the common mode limitations of the op amp (±13V in this design)

AC Simulation Results

Transient Simulation Results

Design References

Texas Instruments, Simulation for Band Pass Filtered Inverting Attenuator Circuit, circuit SPICE simulation file

Texas Instruments, Bandpass-Filtered -40-DB Attenuator, Less than 0.1-DB Error, reference design

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