SV: Re: [sdiy] 16bit DC, DAC, SPI!

[Adam Schabtach]

> What are the general criteria for these applications? 

Well, Karl started the thread, but I'm looking for a good DAC for a
microprocessor-based CV sequencer. Essentially my requirements are the same
as they would be for a MIDI/CV converter. (In fact, I'll probably build
both, eventually.)

> * Large packages for easier DIY

Sort of. I don't mind doing SMT--in fact, I prefer it--but I'm not sure that
I can etch a PCB with the TT method with fine-enough pads for a MSOP
package. I haven't tried it yet, though. Clearly I need to etch a test
board.

> * SPI interface

Yes. Anything serial, really--I could do I2C also.

> * resolution >= 16 bits

16 should be enough. I want 1-cent tunability (with usual 1V/Oct control
convention) because I use just intonation sometimes.
 
> * Gain & offset accuracy > ?

Well, the smaller the better. :-) I'm likely to need external analog
trimming anyway, yes?

> * Small qty cost < $10

That would be great, but I could handle more than that. I only need a few;
I'm not planning to make commercial products.

> * Sample rate ?

Low. I'm generating CV not audio.

> * Conversion type != sigma-delta (charge transfer or resistor ladder?)

Doesn't matter to me as long as it can hold a DC value.

> * Supply voltage (3.3V, 5V, +/-15V)?

5V and/or +/-15V is fine; I don't have 3.3V in my modular cabinet.
 
> I went through a similar search a while back and found that 
> cheap SPI DACs in the 16-bit range are pretty scarce, mostly 
> because none of the usual suspects stock them. 

I found exactly the same thing. I assume we're still dealing with the RoHS
fall-out. Several times recently I've gone to order parts for which I
previously obtained samples only to find that they're no longer available
from the usual suspects. 

> The LTC1655 looks pretty nice. I've also got an application 
> that uses the LTC26x4 quad SPI DAC. They're around $5-$10 
> (non-stock @ Digi-Key
> however) depending on resolution (12, 14, or 16). Small 
> package, but the SPI clock rate is 50MHz so you can use them 
> for fast sample rates. Plus the DNL is better than 0.2LSB on 
> the 16-bit part!

That sounds good; thanks. I'll look into it.

> Not sure how good LTC is with samples, but if you register 
> with a company email you might get lucky. ADI and Microchip 
> are famously generous with this (more companies should be - 
> I'd guess it's a really effective way of getting good buzz).

I've had some luck with TI also, and I was looking at some Burr-Brown DACs
which are now owned by TI, but again availability is very spotty.

--Adam