[sdiy] Query about SSM2164

[Neil Johnson]

Tom,

> In the datasheet (below) there's a schematic of a basic Quad VCA:

Mmmm... "datasheet" == marketing document.

> Now, they say that the control constant for this chip is -33mV/dB.  
> Am I right in thinking that this means that +5V control voltage  
> gives -151dB of attenuation?

In theory, yes.  But the limit is -100dB, so anything after that is  
dead space.

> The reason I ask is that they also say that the chip has 120dB gain  
> control range, from +20dB to -100dB, so the choice of a 0-5V  
> control voltage here seems a bit odd. 3.3V gives you the maximum  
> -100dB attenuation, so why go to 5V? Why did they not put a 51K  
> resistor above the 100K pot?

As I said -- this is a marketing document first and foremost.  Adding  
a 51k resistor would add unnecessary complication to the diagram.   
Perhaps they use +5V because it is used more widely in audio circles  
than 3V3?

Also note that the 5k input impedance of the control pin will bend  
the 100k control pot's curve into a pseudo-log curve -- not  
necessarily what you want for a log-control input.

Neil
--
http://www.njohnson.co.uk