8-Pole Active Low-Pass Filter Optimized for Precision, Low Noise, and High Gain Using the AD8622 and the ADA4062-2 Op Amps (cn0127)
This circuit is a precision, low noise, low power, 8-pole active low-pass filter with a gain of 40 dB. It uses the Sallen-Key topology with the Butterworth response.
Rather than simply constructing the filter with a quad op amp, the selected combination of dual op amps offers a more optimized solution. The AD8622 low noise (0.2 μV p-p, 0.1 Hz to 10 Hz), low offset voltage (10 μV typical) op amp is used for the input and gain stages. The AD8622 offers the lowest offset voltage and noise in the industry for its power (215 μA/amplifier @ ±15 V). The ADA4062-2 JFET-input op amp is used for the last two stages and also offers low power (165 μA/amplifier). The low input bias current of the ADA4062-2 allows the use of larger resistors and smaller capacitors in the last two stages, thereby reducing the passive component board area.
The Sallen-Key topology, also known as the voltage controlled voltage source (VCVS), has been widely used in the industry due to its simple design with few circuit elements. The filter shown has a cutoff frequency, fC, of 10 Hz and overall gain of 100 V/V, or 40 dB. The design can be easily scaled to other frequencies using standard filter design techniques.
Figure 1. 8-Pole Sallen Key Butterworth Low-Pass Filter Using the AD8622 and ADA4062-2, VSY = ±15 V (Decoupling Not Shown)
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