Simply an adjustable low-pass filter

-September 13, 2013

A low-pass filter is the most common filter found in data acquisition systems. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. A signal path requires this type of dedicated filter to match the signal’s requirements. If the circuit has a front-end multiplexer, it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. Consequently, a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors.

An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). The obvious advantage is reduction in chip count, from multiple op amps to one single amplifier. Subsequently, the cost is lower when a single filter serves many analog inputs. You can use a dual digital potentiometer, two capacitors, and a single amplifier to configure a low-pass, second-order Butterworth response with a programmable corner frequency range of 1:100. Table 1 summarizes the digital potentiometer programmed settings.

Second order analog filter
Figure 1 A second-order, analog filter using a dual digital potentiometer, two capacitors, and one operational amplifier reduces chip count.

With this circuit, it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. Additionally, you can realize a combination of Butterworth, Bessel, and Chebyshev filters with the same circuit using a 1:10 corner frequency range.

Figure 1 shows the details of a single-supply, unity gain, second-order programmable low-pass Sallen-key filter. The OPA314 is a single-supply, rail-to-rail op amp. The filter implementation requires two resistors and two capacitors. The dual TPL0102-100, a 100 kΩ 8-bit digital potentiometer, replaces the two resistors in this circuit. The capacitors are hard-wired in. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter’s frequency cut-off and approximation (Butterworth, Bessel, vs. Chebyshev) of this second-order, low-pass filter.

You can calculate the appropriate resistance and capacitance with a little research, a sharp pencil, and a good eraser. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI’s WEBENCH Filter Designer software.

In the input screen, enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V, fc = 100, fs = 1000, and Asb = –35 dB. This will produce a second-order Butterworth filter. In this view, you can also select the supply requirements of Single Supply = +5 V. Then press the green button, “Start Filter Design.”

The next page displays a list of filter response options that meet your requirements. Select a second-order Butterworth from the list and click “Open Design.”

The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen, under the “Filter Topology Specifications” section. Change to Capacitor seed value to 15 e-9 or 15 nF, then press the “update” button. When this capacitor seed is set, the software changes the resistors in the circuit to appropriate values.

To produce the remaining filters in Table 1, return to page one and set the conditions for the next filter. As you do this make sure that fs is ten times higher than fc.

Digital potentiometer program setting for Butterworth filter
Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF, C2 = 15 nF).

In this design, you have learned to quickly produce an adjustable analog filter. This is one technique to design a programmable anti-aliasing filter. Can you think of another way to do this? Send in your ideas and we can discuss them!

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