[sdiy] Multiplying DACs

[Ingo Debus]

Am 29.01.2011 um 03:18 schrieb Matthew Smith:

> Quoth Olivier Gillet at 28/01/11 21:51...
> ...
>> Also, check your datasheet, I doubt the Vref can swing to negative
>> voltages (on the MCP492x it can't). I think there are better VCA and
>> ring-modulator than the one-quadrant multiplier you can get out of
>> messing with Vref.
>
> Thanks, understood. Just wanted to make sure there was nothing cool  
> I could do with that external Vref - thought it sounded too simple!

You can use many multiplying DACs as programmable attenuators or as  
programmable integrators to digitally control the cutoff frequency of  
state variable filters. I've built both in the past, it worked very  
well. You can find such a filter for instance on page 13 of the  
datasheet of the Analog Devices DAC8408:

<http://www.analog.com/static/imported-files/data_sheets/DAC8408.pdf>

The DACs can be viewed just as switchable resistors that are  
connected to the opamps' virtual grounds.

Downside of these integrators is, while the resistor ratios of the  
DAC are pretty accurate, the absolute resistor values are not. You  
probably have to select the capacitors of the integrators by hand to  
achieve a given cutoff frequency range. For the attenuators (used for  
gain and Q in the above filter) this no problem since the feedback  
resistor is on the DAC chip too.

Ingo