For the life of me can't figure out how to get a NE5007 DAC to work - Page 1
Forum section: electronics / projects

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 15, 2018, 08:23:14 pm
content: I have a bunch of old NE5007 current steering DACs in my partsbins, so I decided to try and use one of them in an experiment. However, for the life of me I can't get it to do anything usefull.This is what I tried on the breadboard. In the first situation, I just tried to connect the digital pins to V+ or V- by hand. I've also tried adding a 5k resistor to ground.In the B schematic, I just wired up a multimeter to measure the output current. This worked to some extent, but I didn't get nice behavior out of it.In the A schematic, I replaced the multimeter with a opamp wired up as a transimpedance amplifier. But here the opamp just jumped to 8V output. Changing any of the bits would change that (to 7.8V) but it would always be 7.8V, regardless of which and how many bits I change.I have a feeling I'm doing something really stupid, but this has got me stumped. I've tried a few different ones, and they all do the same.

author: danadak
author_rank: Super Contributor
timestamp: January 16, 2018, 12:07:18 am
content: Value of Rf and what is the power supply on the OpAmp ?Regards, Dana.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 16, 2018, 06:45:35 am
content: I have tried various sizes of Rf, from 50 Ohm to 18 k. Opamp is fed with same supplies as DAC, +- 12 V.

author: danadak
author_rank: Super Contributor
timestamp: January 16, 2018, 10:50:06 am
content: What OpAmp are you using ?Terminate DAC in just an R and measure its output to confirm its a good part.Test at 1 bit, then 10 bits FS, to see you are getting ~ accurate result.Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: January 16, 2018, 11:15:35 am
content: Check out theDAC08 datasheetfor how to use the NE8007. Pin 16 requires a compensation capacitor to ground.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 16, 2018, 11:39:47 am
content: Quote from: David Hess on January 16, 2018, 11:15:35 amThat datasheet is a great resource. I could only find poor quality scans of the original signetics datasheet. From what I could tell there, the compensation capacitor to ground was optional. I guess I can try a bit more later with it connected.Quote from: danadak on January 16, 2018, 10:50:06 amI tried various ones. OP284, AD544, LM324.I'm gonna read through that DAC08 datasheet later and see if I find something. It has a lot more info so I will report back once I tried again.

author: David Hess
author_rank: Super Contributor
timestamp: January 16, 2018, 12:18:33 pm
content: Quote from: TheUnnamedNewbie on January 16, 2018, 11:39:47 amWithout the compensation capacitor, it may be oscillating. Also, the operational amplifier should also have a feedback capacitor to isolate it from the output capacitance of the DAC.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 16, 2018, 01:36:43 pm
content: Quote from: danadak on January 16, 2018, 10:50:06 amI would second that.With a 5V for Vref and Rref = 5k as in your schematics you should get 1mA full scale. Try a load resistor RL on pin 4 (no op amp) of 5k and see what you get voltage wise.Also, do not apply -12V on the logic inputs. According to the datasheet, if VLC is grounded, you should not get lower than -10V for V- = -15V. It should work for TTL logic levels (0, +5V) or lower Vhi since the input threshold is ~2V.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: January 16, 2018, 06:30:06 pm
content: Ok, update;Adding some compensation capacitance helped. However, I was back the the same issues I started with the other day - there was something happening when I switched, just not the right thing.Now I changed the logic levels to 5 V/ 0V and that fixed it. It seemed my pulling the logic so low was breaking things.Thanks for the help everyone!