PC-based configurable filter uses no digital potentiometers
Edited by Bill Travis
Saurav Gupta and Tejinder Singh, New Delhi, India -- EDN, January 23, 2003
Modern instrumentation requires digital control signals. These signals come from a central microprocessor or, in modern context, the popular parallel or serial PC ports. In recent times, digital potentiometers have eliminated the hassles from this interface for the analog section. Designers can replace the resistors of the analog design with digital potentiometers, thus providing the necessary digital control. However, digital potentiometers suffer more severely from temperature-sensitive performance drifts than their manual counterparts, and they exhibit finite wiper-resistance effects. The design in Figure 1 represents a multifunction, analog biquadrature design for automated mixed instrumentation. You can configure the design for both Q factor and center frequency via a PC's parallel port. The circuit requires no DACs or digital potentiometers. The circuit, based on a two-integrator configuration, provides simultaneous highpass, lowpass, and bandpass outputs.
By running simple code on the PC, you can choose from more than 150 programmable combinations of Q factor and center frequency (Listing 1). You can thus build a filter of desired parameters on the fly. The design uses quad analog switches DG308 from Maxim (www.maxim-ic.com) together with octal latches for programmability. A micropower precision op amp, OPA4242, from Burr-Brown (www.ti.com) makes up the analog-filter section. The software provides the data bits on ports pin 2 through 9, stored in the latch that controls the analog switches and, hence, selects the appropriate resistance combination to select the desired Q and f0 values. The center frequency and Q values are: f0=[1/C1C2RP6RP7]½ and Q=(1+RP2/R1)/3, where RP2, RP6, and RP7 are PC-programmable resistances.
Data nibbles from port pins D2 through D5 provide ganged-switch settings for RP6 and RP7, ensuring that they are always equal. Data nibbles corresponding to pins D6 through D9 control the value of RP2; hence, you use them for Q-value selection. For the given resistance values, Table 1 shows the data bits for different values of Q (ranging from 2.24 to 33.72). Table 2 shows the data bits for various center frequencies (159 Hz to 38.70 kHz). Figure 2 shows the software front-end screen to make the selection. The design uses an 8.11-kHz filter with Q-factor 2.9 for demonstration and hardware validation. Listing 1 gives the necessary back-end C code. The switches employed have a finite on-resistance, RDS, of approximately 150Ω, which the design takes into account. For higher precision, you can use better switches having on-resistances of approximately 35Ω. Note that more switches provide a wider range from which to select. You can choose the resistances to suit the application's bandwidth range.
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