Variable Low-Pass Filter

033 VARIABLE LOW-PASS FILTER The Type SSM2045 lc from Pmi is an active low- pass filter whose order, Q-factor, cut-off frequency and amplification are set with the aid of control signals. A possible design for use in electronic music sys- tems is shown in the diagram. To prevent distortion, the level of the input signal is reduced by R1 and a resistor on board the chip to one not exceeding 150 V peak to peak. Outputs 2 POLE and 4 POLE are each connected to an internal voltage-controlled amplifier (vcA), mixl and mix2 respectively. For optimum off-set and control rejection, these connections are made via resistors R4 and R5. The gain of the VCAS is set by P2, which con- trols the current that flows through the amplifiers via pins 15 and 16. The maximum current at these control R14 inputs is 250 p.A. The balance between the VCAS, and 018 P4 2k2 R18 P1 1k R19 88 CO 00n 15V .28mA 0 C 15V .19mA with it the order of the filter, is set by P4 via pin 14. The voltage at this control input ...