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I'm designing an audio circuit which includes the V2164 VCA IC. This chip is a clone of SSM2164. I take a "Basic VCA Configuration" circuit which is supposed to give a unity gain with 0V control voltage level. The datasheet says the chip has "a ground referenced –33 mV/dB control port". I want to control my VCA in the range of full attenuation to a little above unity gain. Questions:

  1. Am I right that -100dB can be considered "full attenuation"?
  2. Do I understand right that in order to get -100dB attenuation I need -3.3V control voltage level?
  3. Why is there a minus sign in the "-33mV/dB"?

My input signal is going to be AC 5Vpp level. The signal source is a CODEC with 102dB dynamic range (if it matters), followed by an inverting op amp stage shifting it from 0V..+5V to -2.5V..+2.5V.

Volodymyr Smotesko
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1 Answers1

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Am I right that -100dB can be considered "full attenuation"?

Yes, for audio purposes, -100 dB is typically sufficient for "silence".

Do I understand right that in order to get -100 dB attenuation I need -3.3V control voltage level?

No, you've got things backwards. You are confusing attenuation and gain: you can't get -100 dB of attenuation because that is 100 dB of gain. By "-100 dB attenuation", you actually mean 100 dB of attenuation which is equivalent to -100 dB of gain. Higher gain means louder, higher attenuation means quieter.

If the slope of the VCA is –33 mV/dB and you want -100 dB of gain (100 dB of attenuation), then you need (-100 dB)*(-33 mV/dB) = 3.3 V. See the reference schematics, they show the control voltage port being fed by a positive voltage via a potentiometer.

Why is there a minus sign in the "-33mV/dB"?

Because that's the way they designed the chip: if 0V = 0dB, a positive slope in mV/dB would be inconvenient.

One final note about the SSM2164 and clones: take a look at Fig. 21: Gain Constant vs. Temperature. If you're hoping for a stable gain over a wide temperature range, this may not be the best choice: the gain constant isn't very constant...

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