[sdiy] DIY Parallel DAC

[ASSI]

On Sunday 31 July 2011, Matthew Smith wrote:
> Due to the lack of sensibly-priced parallel load DACs (and the fact I
> don't want to muck around with serial timing for waveform generation) I
> am looking to drive a R-2R ladder DAC directly from an FPGA.

I'm not sure why you'd absolutely need a parallel load DAC if you already 
have an FPGA running the show, but the AD5556 (single) and AD5557 (dual) are 
parallel load 14-bit, 4-quadrant multiplying DAC with 2MHz update rate and 
good multiplying bandwidth in TSSOP.  There's a 16-bit variant of those too 
(AD5546/AD5547), but the 14-bit is more cost-effective and plenty good 
enough.  They also have an input register and a "load" signal to synchronize 
the DAC update.  Lastly they integrate the feedback resistors you will need 
to produce bipolar output.  There's also an SPI version of these DAC and due 
to the LDAC signal really doesn't have any more or less problems with timing 
of the DAC output.

This is just one example because I've downloaded the parametric table from 
AD.  You can do the same search on other manufacturers' sites (TI, Linear, 
etc.) and then hit your preferred distributor with the results.  Getting 
samples from the manufacturer is one thing, but what the distributor has in 
stock (and at what price) is something entirely else.
 
> Other than buffering the output and using half-decent resistors (looking
> at some Vishay 0.1% at $1.52 a piece, in easy-to-solder 0805 packages)
> is there anything I should be aware of? (And can I get away with lower
> precision/cheaper resistors?)

Actually there are trimmed R2R resistor networks that should work out much 
better than using individual resistors if you insist on rolling your own 
DAC.

http://www.irctt.com/pdf/laddernetworks.pdf
http://www.bitechnologies.com/pdfs/resistorladder.pdf
http://www.bourns.com/pdfs/r2rap.pdf

> Working on a 3.3V reference, 1k and 2k resistors.

You'll be lucky to get much better than 8-bit effective resolution and it 
won't be cheaper than a DAC proper.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Samples for the Waldorf Blofeld:
http://Synth.Stromeko.net/Downloads.html#BlofeldSamplesExtra