[sdiy] Suitable DACs for Demultiplexing

[Tom Wiltshire]

Hi All,

I've just been messing about with the dsPIC 33FJ128GP802. I've used  
the dual channel DAC on this chip for modulation sources before with  
no problems. It occurred to me that since the DAC can run up to  
100KHz, it wouldn't be too difficult to demultiplex the output and  
get 8 signals instead of two.

I set this up using a DG409 dual 4-to-1 switch with one DAC channel  
to each switch. The uP controls the DG409 address lines and Enable  
pin. The Switch outputs go to the usual cap-and-op-amp-buffer that  
you see on numerous synth schematics from the 80s and after.

However, I ran into a problem. The DAC is a Sigma-Delta type and uses  
an upsampling digital filter to produce a pulse train at 256 times  
the sample frequency. What this seems to mean is that the output is  
filtered. Instead of stepping abruptly from one sample to another, it  
glides smoothly between them. Obviously for audio (the DAC's intended  
purpose) this is ideal behaviour, but for demultiplexing it's a  
nightmare. The output sample has only just reached the level you  
wanted it to have when it is time for the next one to be sent. The  
effect on the demultiplexed outputs is for an 'echo' of the previous  
channel's signal to break through on each output channel.

Can anyone give me any guidance about which types of DACs are good  
for this application?

I've got a Microchip MPC4922 dual channel resistor string DAC that  
I'm currently experimenting with instead, but it is only 12-bit. I'd  
like a 14-bit chip if possible, since the internal modulation sources  
are 16-bit and it seems stupid to chuck away 4 hard-won bits of  
accuracy with a cheapo DAC at the very end of the process. 2 wasted  
bits I can live with!

Thanks,
Tom