PC-configurable RLC resonator yields single-output filter
Edited by Bill Travis
Saurav Gupta, New Delhi, India -- EDN, August 7, 2003
This Design Idea presents a versatile filter circuit for low-power-consumption instrumentation that you can program from your PC using the parallel port. The circuit uses analog switches and latches instead of digital potentiometers for the digital control (Figure 1 and Figure 2). By running simple software code on the PC, you can configure a single robust design to work as a lowpass, highpass, or bandpass filter, and you can also select the desired center frequency, ω0 (Listing 1). Unlike a similarly controllable design (Reference 1), this design is a single-output-at-a-time filter. Many power-sensitive systems do not require simultaneous-filter functions.
The design exploits the fact that a series RLC resonator can provide various filter functions with its elements. Because the design is based on an RLC section, it is trivial to convert the design into a PC-controlled resonator. In Figure 1, the inductor, LP, is implemented as a PC-controlled synthesized inductor. The value of the inductor is LP=C2RPR3R5/R2. Here, RP can assume any of 15 possible values, depending upon the state of switches S1 through S4 (determined by PC-port data bits D2 through D5). The expression for the frequency is ω0=(R2/C1RPR3R5)½. You can thus effectively select 15 frequency values. (This design uses 12 values of practical interest.) Data bits D6 through D9 from the PC's parallel port set the state of analog switches S5 through S8. The state of the switches determines the type of filter.
Figure 3 shows the software-generated display for the circuit. This design uses a 9.93-kHz bandpass filter for demonstration and testing. Increasing the number of analog switches can provide a wider range. Moreover, you could use additional switches for gain programmability. The 74573 latch provides the interface to the PC. Table 1 shows the port/switch settings for a few frequency and filter-type selections. Note that the analog switches (DG308) have a finite operating on-resistance of approximately about 110Ω; you must take this resistance into account when you calculate the center frequency. For precision instrumentation, other switches are available with operating on-resistances as low as 30 to 50Ω.
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