[sdiy] d/a, voltage references & multiplexing considerations

[Antti Huovilainen]

On Mon, 2 Jan 2006, Karl Ekdahl wrote:

> I'm after a d/a for audio use, high quality. I'm thinking of using a 
> 18-bit AD1861 with 0.008% THD (since i have them), is this good for 
> high-quality audio? I assume that i should use a good voltage reference 
> for the analog voltage in so i'm looking for suggestions on components

You'll also need stable jitter-free master clock. Unfortunately I have no 
idea how good your typical run of the mill crystal oscillator circuit is 
going to be at that.

> and circuits for that. Also i'm thinking that since it supports 16x 
> oversample i could multiplex it into a number of outs using sample and 
> holds, is this a bad idea?  Also, i'm a little confused as how to wire

This will be tricky. You will have phase offset between the channels and 
your sample & hold must be linear enough (100dB THD is not easy), fast 
enough and low feedthrough enough. I wouldn't do this.

> the 1861 for voltage out, the only thing i found on the net was someone 
> hooking up Iout to SJ and RF to Vout and then tie both together with a 
> cap of mysterious value.

The datasheet shows internal Rf between the pins, so a capacitor would act 
as simple lowpass filter (possibly needed for compensation too).

Antti - By no means an expert in AD/DA but has read lots about what can go 
wrong there

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova