[sdiy] DIY Parallel DAC

[Matthew Smith]

One of the reasons I wanted to go the DIY DAC route was to learn - I 
believe that to truly understand something, I need to build it from 
scratch.

For all your responses, I am truly grateful. That's a LOT of information 
and ideas - I'll attempt a synthesis (ha!) over the next couple of days.

Now to address the various questions/comments, in no particular order:

* Why am I looking for a parallel solution?  Because my original design 
is a clock driving a counter that drives the address lines of an EPROM, 
the data from which drives a DAC.  The evolution of this is to replace 
the EPROM with a lookup table in the FPGA, this driving the DAC. With 
FPGAs, you can have arbitrary data widths - no having to send 12 bits as 
2 x 8-bit words.  If I want a 12-bit DAC, I just have 12-bit outputs 
from my lookup tables, and present these to the output pins. Pure 
simplicity.

* Why not serial? I am using 1024 bit samples, clocked at the 
appropriate rate for the frequency required. Nearly all the serial DACs 
from my regular vendor (Element14) seem to rely on SPI for timing. I 
like to treat SPI as asynchronous, when dealing with microcontrollers. 
Send stuff out fast, but allow other tasks (from interrupts) to take 
priority. Resetting the SCK rate all the time also lacks charm.

Having said that, I have put an SPI DAC into my ADSR design - can't 
remember the part number, but it's a Microchip device which has a load 
pin so that conversion can be timed precisely, but still with an 
asynchronous SPI.

[Aside: I have avoided SPI up until very recently as I simply hadn't 
used it before. All my microcontroller work has been very simple, all 
communications being via U(S)ART. Tried to use SPI from an ATTiny2313 to 
control an LTC6903, which I am trialling for clock generation for Mark I 
wavetable system. Had so many problems with the restrictive 
implementation that I ended up having to big-bang the SPI rather than 
use the integrated peripheral. Worked like a charm :-)  Now that I "get" 
SPI, working on my own, simple, Verilog implementations in CPLDs.]

But I think Veronica has answered my issue with changing SCK rates - put 
the value in a register and push samples out at a fixed rate. With the 
parallelism you can get from programmable logic, this should work well - 
just need to push it out sufficiently fast that no jitter occurs from 
the timing of register updates versus SPI transmissions.

* Which parallel DACs have I looked at? Not many - either Element14's 
search isn't up to much (I've had issues finding stuff before,) or they 
have limited lines. The reason I'm only searching on this vendor is that 
I have to get the stuff from somewhere. Digikey & Mouser are out of the 
question, as they have arm-and-a-leg freight for anything under $200.

* Picked resistors - yes, this is a one-off. It's a "how does it work" 
and a "because I can." Whilst I like to work with SMD devices, as it 
keeps PCBs small (and cheap) I do have a box of 1,000 1k ex-military 
resistors. I think I may well do some measurements and build a ladder on 
matrix board. I only have the one value, but then I can match pairs for 
series resistance for the 2R part.  As far as I can make out, it's the 
MSB end that is most critical on values.

I would not consider using home-brew DACs in a commercial product unless 
it were the audiophile market - in which case I'd buy the cheapest 
Chinese resistors on eBay, dip them in gold paint mixed with snake oil, 
and sell for $2k a piece ;-)

* Variation in output levels - interesting point. I've got a Spartan 3E 
demo board - think I'll set it up to pull a load of pins high, dump a 
load on them and measure the voltage, just to see what sort of variance 
   I get.

* For the purposes of what I was looking to achieve, I will probably go 
with an SPI DAC, as the parallelism of an FPGA and Veronica's 
observation should mean that it will work just fine. HOWEVER, I still 
want to build a discrete DAC and, since this is for the sake of doing 
it, will take up Dave's suggestion and attack my box of 1k resistors and 
see what I can put together.

* I will read through Robin's notes on the Casio DACs with great care. I 
knew there had to be prior art I could use for inspiration :-)

Thanks again, all. Your input is really appreciated.

Cheers

M