AN11| Application Note
Maxim > App Notes > FILTER CIRCUITS (ANALOG)
TEMPERATURE SENSORS and THERMAL MANAGEMENT
Keywords: Programmable Universal Filter Implements C-Message Weighting Function
Jul 09, 1998
APPLICATION NOTE 11
Programmable Universal Filter Implements C-Message Weighting Function
The C-message filter, which simulates the frequency response of the human ear, is a commonly specified test and measurement filter for voice, audio, and telecomunication applications in the U.S. In Europe, a close relative is a psophometric noise-weighting filter. You can construct either type by cascading three 2nd-order bandpass sections with a 2nd-order lowpass section. The C-message filter, for example, is shown in Figure 1
Figure 1. Cascaded, 2nd-order universal filter sections implement a C-message filter. Dual universal, 2nd-order IC filters provide a compact and efficient means for implementing the circuit of Figure 1. If the IC filters are programmable, switched-capacitor types as shown, you can rapidly implement a Cmessage, psophometric, or other test filteres on demand simply by loading the chips with different sets of coefficients. These coefficients set each 2nd-order section's filter mode, Q, and cutoff or center frequency f0. The C-message filter has poles only, which are specified by the IEEE Standard 743-1984: Table 1. Pole Value in rad/sec Value in Hz (f0) Q BP#1 -1502 j1267 BP#2 -2439 j5336 BP#3 -4690 j15267 LP#1 -4017 j21575 312.741 933.761 2541.886 3492.778 0.6540 1.2027 1.7026 2.7316
Figure 2 shows the external connections that configure two filter ICs in the architecture of Figure 1, along with a table of decimal equivalents for the digital coefficients assosiated with each filter section. These 2nd-order sections establish pole locations in accordance with the f0 values listed. Each section contains two continuoustime Chebysheff filters whose center frequency can be digitally programmed in 128 steps over the range 1 to 25kHz. Passband ripple is 0.1dB. For maximum signal-to-noise ratio, the signal amplitude at each section output should be as high as possible.
