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 filter offers the industry’s lowest power solution (760 μA total quiescent current using ±15 V supplies) that yields not only precision and low noise but high gain. Using two duals rather than a single quad op amp also provides flexibility and ease in the PCB layout. The ADA4062-2 is also available in a 1.3 mm × 1.6 mm LFCSP package (smaller than an SC-70), making it the smallest JFET-input op amp in the industry. The smaller size is ideal if designers are concerned about board space when using two duals instead of a quad.

Low-pass filters are often used as antialiasing filters in data acquisition systems or as noise filters to limit high frequency noise. The Butterworth filter is a maximally flat magnitude response filter with no ripple in both the pass band and the stop band. However, this is achieved at the expense of a wider transition band in the frequency response, as compared to other responses that have ripple. Therefore, higher order Butterworth filters are often required. An active filter design is more complex than a passive filter because both the topology and the proper op amps must be selected. However, the active design allows signal gain, and also eliminates the need for large inductors and capacitors required to implement a low frequency passive filter.

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)

Возможности

  • Optimized low pass active filter for 10 Hz
  • Low noise precision Butterworth sallen key topology
  • Very small PCB

Используемые компоненты

Производитель: Analog Devices Inc.
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