T he SSM 2000 has been designed to minimize control feedthrough. Control feedthrough may cause an audible output as the internal control lines ofthe VCAs and VC F s are changed rapidly. T his feedthrough is the cause ofmany ofthe unpleasant artifacts prevalent among noise reduction systems and is ofien due to parasitic capacitance and mismatches within the IC. T his speci- fication is important for the VCA and VC F, but the VC F is the most susceptible because it operates at constant dc gain. The solution incorporated into the S SM 2000 to reduce control feed- through has been to convert from single-ended to full differen- tial at the signal input and convert back again at the output buffer. Because the audio path through the VCA and VCF are fully differential, control feedthrough is determined purely by mismatches with no systematic errors. The slight penalty to be paid for a fully differential system, besides extra complexity, is that the externalVCF capacitors (1 nF recommended) now re- quire two pins each instead ofone. Careful attention to VCF capacitor matching and layout symmetry reduces control feedthrough to better than 40 dB through the signal path.
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