6 ch digital controllable voltages needed. What should I get? - Page 1
Forum section: electronics / projects

author: Jelbert
author_rank: Contributor
timestamp: January 14, 2024, 01:44:57 pm
content: For a long term project I'm working on I need 6 channels of 0 to 30V for offset purposes so current is not important. The channels need to be controlled digitally.I did already build 7 linear psu's for this but they are controlled with a pot and that is not accurate enough.At this moment I use my Rigol DP832(A) but it has only 2 (3) channels. The rigol works fine and is accurate enough. I do consider buying an other one then I have 4 channels for the full range and two that only go to 5.3V, that could work too but is not ideal.A ready made solution would be best so I'm not sidetracked by designing and building my own equipment.Does anyone here have suggestions on a solution for 6 programmable 0 to 30V 1mV resolution voltage sources? They need to be independent so no common ground.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 14, 2024, 04:19:12 pm
content: Just get your checkbook. Standard fair in process automation. Just google "8 channel voltage output isolated plc dac" and you'll find things like:https://www.controltechnology.com/products/2500-Series-Classic/Analog-I-O/Outputs/2560-A-8-Channel-Isolated-Analog-Output-Module

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 14, 2024, 04:37:11 pm
content: There are plenty of cheap DAC's controlled by SPI, I2C or similar, and they often have 2 or 4 outputs. The I2C variants often have a settable address, and with SPI it's also easy to use some extra chip select wires.So then the question becomes, how much resolution, stability and linearity do you need?Another approach is to use PWM -> Filtering -> Switches (4053) -> Buffer caps -> Opamp with FET inputs.Advantages is that PWM has excellent resolution and linearity (Stability depends on the reference voltage used) there is a tradeoff between ripple and speed, but if you want to generate DC offsets, then speed is not a big issue and ripple can be very low. The switches act as a sample (track) and hold. this also reduces the ripple, or (more accurate) it replaces the ripple from the PWM to the drift rate of the track and hold circuit.Or a simpler approach, just use 6 PWM channels and filtering.

author: Jelbert
author_rank: Contributor
timestamp: January 14, 2024, 04:54:39 pm
content: Thanks,I had gotten stuck in the wrong train of thought. Now I found this onehttps://www.instructables.com/OpenDAC-Seekat-8-channel-16-bit-low-cost-laborator/and that is almost what I need. I only need to isolate and amplify the outputs.

author: dobsonr741
author_rank: Frequent Contributor
timestamp: January 14, 2024, 11:21:47 pm
content: QuoteReally need to have 6 or 8 independently referenced isolated output? It is going to be expensive/lot of work. Isolation should have occurred at the DAC level. If you really need it.

author: Jelbert
author_rank: Contributor
timestamp: January 15, 2024, 12:39:07 pm
content: This is what I'm planning to make:The EVAL-AD5780SDZ kit from analog devices x6 with on the digital side isolation and for each module a separate psu. For the psu I can use the modules I already made. Digital control with an arduino.The cost per channel will be about €80

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: January 15, 2024, 03:20:43 pm
content: Have you considered replacing the pots on the power supplies you already have with digitalPots? 10 bit I2C pots are about $6 ea.

author: inse
author_rank: Super Contributor
timestamp: January 15, 2024, 03:36:16 pm
content: 30V, 10Bits-> 30mV resolution, OP was asking for 1mV, he would need 15Bits

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: January 15, 2024, 05:15:12 pm
content: Quote from: inse on January 15, 2024, 03:36:16 pmOk, missed that. 8-bit are available in quad packs. Double up a couple 8-bit pots. He’ll only need three devices. Might need a buffer amp between the stages, but MSB to the first and LSB to the second with some basic math should work fine.

author: coromonadalix
author_rank: Super Contributor
timestamp: January 15, 2024, 05:26:06 pm
content: 1 mv of resolution is pushing the bar pretty high i would sayJust get more Rigols and do control them by Python Ni Labview or else ... if i recall they are floating between the channels ? no ?way simpler ...some industrial modules work with current loop at 0-20 ma or 0-5 vdc, witch is way to insufficient,creating some 12 - 16 - 24 Bit DAC will be more complicated, and some use half supply reference say 2.5vdc ... and you'll have to be "rugged" in a senseAnd some DAC speed may be to slow for youmy 2 cents

author: Jelbert
author_rank: Contributor
timestamp: January 15, 2024, 05:41:31 pm
content: Yes After diving into the analog and the isolation problems I came to the same conclusion. I already have a Rigol and adding an other DP832 is probably the best solution. Then I have a great manual user interface from the front panel and I can hook it up to the lan and control it with labview.Also building every thing by myself is adding a lot of work while I doubt that I can build better than what I can buy for that kind of money. Ex vat it will cost me €419.