AS3364 CV response revisited

 I've measured the CV response of the AS3364 quad VCA before, but I still had a feeling I'm not getting unity gain so I decided to do it again.

I'm using a resistor voltage divider to drop the input CV from 0-5V to 0-2V, as max gain is at 1.8V to 2V according to the datasheet.

In my test, I've connected the audio inputs to a 5V reference voltage through 47k resistors. The output goes directly to the negative input of an op amp, with a 47k feedback resistor. According to the datasheet, this should give us unity gain.

The reference resistor is measured to 98.4Ohm (I later tried a 100Ohm here but nothing changed).

The response of a 0-5V input is shown as Out 1 and Out 2 (I tried three chips and they all respond the same). 

As in my previous tests, there is a deadband at each end - An input CV of 250mV at the chip (640mV before the resistor divider) is necessary to get the chip going. It tops out when the CV is 1.8V at the chip (4.6V before the divider).

Input CV must be > 640mV before the VCA opens

 

Max gain when Cv (at chip) is 1.8V, not entirely unexpected

The max output is 4.9V, slightly less than output gain. By increasing the output op amp feedback resistor to 48.8 (adding a 1.8k resistor) I am able to get 5V out. 

47k + 1.8k output feedback resistors

47k + 3.3k

Increasing the resistance to 50.3k gives an output of 5.18. Choosing a 50k output resistor will be a simple way to ensure that the output is at least unity.

 

Consequences

For the "normal" usecases, the offsets and early max-out of the CV just means that there are parts of the CV range that is not doing anything. It can be trimmed out in software if that is necessary. As there may be chip and/or temperature variations, I don't think I'll try modifying it. It's better to have a deadband at the start than losing the ability to completely shut the VCA off.

As for the wave mixers, the deadbands mean that there are parts of the pot travel where the waveform doesn't change much. I'm not too worried about this either, I haven't noticed in my prototype. 

The final place where this has an effect is in the panner and wet/dry pots. These will have deadbands at both ends. Again, not sure it matters much.