I need help creating a discrete DAC for an old project - Page 1
Forum section: electronics / projects

author: DaveHardy
author_rank: Regular Contributor
timestamp: May 27, 2023, 10:17:47 pm
content: Hello, I'm trying to recreate a Burr Brown DAC 71 CSB I for a project I'm in the middle of. The DAC itself is relatively easy and straight forward. The problem is that this old DAC doesn't require a clock / latch instruction to output data, and I'm having trouble finding a primitive replacement or one that would allow me to tie the latch low for constant output.Any suggestions or comments will be appreciated.

author: moffy
author_rank: Super Contributor
timestamp: May 27, 2023, 11:24:20 pm
content: The DAC71 doesn't have a latch on its input, so whatever digital word is present on its input is converted to an analogue output. If you want a fixed value just provide a 16 bit latch or 2 eight bit latches. What logic are we talking here, 5V TTL/CMOS, 3.3VCMOS, FPGA etc.

author: Benta
author_rank: Super Contributor
timestamp: May 27, 2023, 11:44:10 pm
content: Perhaps you should read the OP's question again...

author: moffy
author_rank: Super Contributor
timestamp: May 28, 2023, 12:15:07 am
content: Quote from: Benta on May 27, 2023, 11:44:10 pmPerhaps you could help the OP since you understand the question better than I do.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 28, 2023, 01:32:21 am
content: Quote from: DaveHardy on May 27, 2023, 10:17:47 pmIf that settles in 5us, can you not simply clock almost any suitable DAC at 200kHz or above, to emulate the same action ?

author: DaveHardy
author_rank: Regular Contributor
timestamp: May 28, 2023, 01:55:23 am
content: Thanks for the comments everybody! Regarding clock timing, I don't want to go that route unless I absolutely have to. I found this IC. It's V out instead of current out. I think I can make it work in this circuit. According to the datasheeet, holding everything low will create a pass through device. I'll post the circuit next.https://www.analog.com/media/en/technical-documentation/data-sheets/1657lfa.pdf

author: DaveHardy
author_rank: Regular Contributor
timestamp: May 28, 2023, 02:10:03 am
content: Gain adjust is supposed to be calibrated for 5V + offset from zero at pin 6 of U347. I could install an op amp on a minature board with the DAC and a gain trimmer to get V out and omit U347. Is this a good idea? What could go wrong?

author: DaveHardy
author_rank: Regular Contributor
timestamp: May 28, 2023, 02:25:43 am
content: This might be an even better part! This is a current DAC.https://www.analog.com/media/en/technical-documentation/data-sheets/AD7538.pdf

author: moffy
author_rank: Super Contributor
timestamp: May 28, 2023, 02:30:15 am
content: You have a latch signal that feeds into the 74174, if you wanted to you could forward that signal to the DAC and use anyone you liked.

author: DaveHardy
author_rank: Regular Contributor
timestamp: May 28, 2023, 02:37:26 am
content: Those are separate latches for upper and lower byte. If some parts in the program alternate high and low latches, I'm screwed. I'd rather avoid all complications first.

author: moffy
author_rank: Super Contributor
timestamp: May 28, 2023, 02:45:43 am
content: Quote from: DaveHardy on May 28, 2023, 02:37:26 amNasty. There is the possibility to forward both latch signals and combine them into a single latch signal that updates on any change.

author: bobcherry
author_rank: Newbie
timestamp: January 02, 2025, 10:15:09 pm
content: Hi Dave, Hope you don't mind me picking up on this post but did you have success with creating the DAC for the P5? I seem to be in a similar situation (P5 Rev3.0)I'd appreciate any thoughts/advice. MTIA. Rob