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.

Testing bus mixer

I've spent between 50 and 100 hours laying out the voicecard mainboard lately, but finally I had some time to test the bus mixer.

Everything works as it should:

- 16 switches

- 12 VCAs

- Summing on both wet and dry busses

- Summing of wet AND dry

The only thing to worry about is the switching noise. I get huge peaks (>5V when switching. Some are more prominent than others, so not entirely sure what it is caused by. The peaks are there even when the switch has a 0V input on both channels:

Here is a sceenshot of summing two multiple 5V inputs (the output from the VCAs is 4.5V btw...)

Oh - and I had an initial scare, it looked like we had some capacitance somewhere, and serious crosstalk between busses. Turns out the logic probe ground lead had disconnected itself so the output showed the average of whatever it measured:

AS3364 gain > 1

Since my juno filter might output less than unity gain, I wondered if it is possible to make up for this in the output VCA mixer, effectively making gain > 1

Normally, I use a 47k input resistor and a 47k output resistor, as per the AS3364 datasheet.

I tested this quickly and got a sligtly more than 8Vpp output (I didn't measure the input, it may have been as high as 10Vpp but could also be lower).

5v/cell, approx 8Vpp

I then replaced the input with a 22k resistor. The resulting output was 17-18Vpp.

Still 5v/cell, 17Vpp

Changing the input resistor would increase the input current by 47k/22k = 2.14. 

8 * 2.14 = 17.12, so this looks like it is doing exactly what we want.

The datasheet says that maximum signal input current is between +/-300 and +/-500uA.

With a 47k resistor and max input 11.5V, the input current is 244uA. Reducing to 22k, the max is 522uA. I'm not sure if going higher actually kills the chip, but it is something to think about. It could be that it will just start clipping.

If we choose to use a smaller resistor we should also make sure that the input amplitude does not go too high. For example, if we go for the lowest maximum signal input:

max V input = 300uA * 22kOhm = 6.6V, or 13.2Vpp.

Oh - and to make up for a change from 100k to 230k input to the juno filter, use a 47k / 2.3 = 20.4k resistor (or just two 10k resistors in series)