DAC = thanks people for your help.
Had a look at the search results for the 12 bit ones Robert posted.
First / cheapest one is not cheap, almost 10 Euros a pop... in this day and age how can these things still cost so much money.... :'(
Anyway, it still requires many control signals, at least to latch the data (can't do on the fly conversion), as well external caps.
... funny part is, the datasheet says it uses an R-2R ladder inside... well then just cut the crap and just sell me your 12 bit bare ladder then, for cheaper, would make my day...
Next model after that, more expensive and requires a CPU, needs registers insise to be written to.
I think it all goes to choosing the 2 Euro R-2R ladders, at least for a first prototype.... will use "proper" DACs only if experiments show it is strictly necessary...
Hi Vince, in case you'd still want to DIY after seeing those, don't bother with 0.1% resistors for the "additional bit" on the R2R networks - rather try to match the (2% tolerance!) value inside the network (the "flat blue box" resistors might have a nice even spread, at least mine do; so they should have a good selection to match some odd value)
I am surprised you remember about that box of resistors, you have quite an amazing memory...
As mentioned earlier, the 2% tolerance stated in the datasheet makes no sense for an 8 bit DAC.... so I am thinking, the only way this can work, is if all the resistors are trimmed to be close to each other, much tighter than 2%. That's the only way. So for example, all resistors in the ladder would be say at -2%, or all at +1.25%, or whatever...
So if I buy say a 10K ladder, the resistors can be anywhere between 9.8K and 10.2K, but if one is say 10.15K then they would all be damn close to 10.15K (and 20.30K of course, for the 2R ones...)
So what I need to do, is to measure what the actual resistance is, then match it a multi-turn trimmer in series with a fixed resistor.
The ladders are available in DIP packages. takes more board space than SIL packages, but it does give you probe point to let you tap all the "R" resistors of the ladder, individually.
The "2R" one can be easily measured with either package type, between the output pin and the MSB pin. That would do it for a prototype.
Then once I know what value I need, I can replace the trimmer and fixed resistor, with a single precision one.
Today at work I had nothing to do at all all afternoon, so I started working on my design.
Turns out my original envy of using only discrete logic chips could spiral into a mess...so I am now steering towards a single chip/MCU solution.
The data stream I have to process runs at about 15kHz, with pulses about 8µs wide.
I use AVR MCU's, have an old ATmega32 in DIP package in stock from my last project....15 years ago.
It can run at 16MHz, so an instruction time of 0.065µs or something if I have my maths about right. single cycle instructions mostly, by the time I get a rising edge from the pulse, I can execute quite a few instructions long before the pulse neds, never mind before the next pulse comes in. I don't need many instructions to do what I need to do.
So at least back oif the envelope, it possibly could work, so worth a try. Using a micro would also allow me use easily any "real" DAC, and let me modify / fine tune the design much more easily/faster/cheaper than a H/W discrete solution....
I could maybe even get rid of the ladders and just use the internal PWM channels as a DAC.... but that would be a bit too optimistic I think. I would need a very low RC time constant to get the required responsiveness I guess, hence would need the PWM to run at a very high frequency that the MCU could not provide I think. I mean if it runs at 16MHz, and would use IIRC a 10 bit PWM register, then at best the PWM would run at 15kHz.... piss poor, won't cut it.
So I guess I need to dig out my 15 year old "Dragon" USB AVR programmer..... 15 years.... I hope I can remember how to use all the tool chain and program these things...
Might use the ATmega128 collection that Zoli sent me. More I/Os .... and a much more compact quad SMD package.
@Factory : OK so I guess I can finalize my part of your Mouser order then... what I listed previously... + a few R-2R ladders....