MCP4922 DAC AGND and VDD confusion - Page 1
Forum section: electronics / beginners

author: nemail2
author_rank: Regular Contributor
timestamp: November 19, 2018, 10:22:09 pm
content: Hi!I'm a bit confused how to connect the MCP4922. I have got a digital power rail and an analog one and digital ground as well as analog ground.SPI signal for the MCP4922 DAC of course comes from the digital power rail (i.e. from the MCU).The chip only has an AGND pin, no DGND pin. Where should I connect VDD to? Should I try to get a VDD powerrail which is from the analog part?My issue is, that when I power the DAC from a digital power rail, the ground path to AGND would be huge because everything is nicely seperated and the ground star point is quite far away.So, connect DAC to:- a digital power rail + digital ground (short path from VDD to VSS, but noisy)?- a digital power rail + analog ground (long path to ground because star point is far away)?- an alaog power rail + analog ground (would need another voltage regulator because I don't have the 3,3V I need for the DAC in the analog part + SPI comes from the digital part -> noise issues?)?by digital power rail i mean a power rail which is above digital ground and from the digital part of the PCB.The MCP3204 ADC for example has an AGND and a DGND pin, much less confusing story...Thanks!

author: nemail2
author_rank: Regular Contributor
timestamp: November 20, 2018, 07:30:15 am
content: Anyone? Too stupid/confusing question? :-)I guess it would be best to keep the path from VDD to VSS as short as possible but wouldn't then be the noise on this particular VDD rail (which is on the digital, noisy part of the board) an issue?

author: iMo
author_rank: Super Contributor
timestamp: November 20, 2018, 08:28:55 am
content: Since the chip has got only Vdd and Vss I would connect its Vdd to the analog power rail and Vss to the analog gnd. You may use 100ohm resistors in SPI signals to limit the mcu noise, for example.

author: nemail2
author_rank: Regular Contributor
timestamp: November 20, 2018, 08:35:53 am
content: Quote from: imo on November 20, 2018, 08:28:55 amThat would mean that I'd have to put in an extra 3.3V regulator in the analog power rail cause I don't have 3.3V there yet, unfortunately.So it is worth the effort doing this instead of powering it from the noisy 3.3V digital power rail?

author: iMo
author_rank: Super Contributor
timestamp: November 20, 2018, 08:42:58 am
content: In case you want to use the 12bit DAC for generating noise you may power it from the "noisy 3.3V digital power rail". The chip is 2.7-5.5V one (datasheet), and you need an external Vref as well..

author: nemail2
author_rank: Regular Contributor
timestamp: November 20, 2018, 08:48:29 am
content: Quote from: imo on November 20, 2018, 08:42:58 amnah i don't want it to generate noiseI have to power it with 3.3V because the MCU is at 3.3V. Otherwise the SPI interface won't work correctly.A VREF I already have (2.048V).ok so one extra 3.3V regulator for the DAC... well that is unfortunate but apparently there is no other option.Thanks!

author: mvs
author_rank: Frequent Contributor
timestamp: November 20, 2018, 09:48:20 am
content: Quote from: nemail2 on November 20, 2018, 08:48:29 amHigh frequency noise from digital circuits is not always bad for low frequency analog stuff. If it does not affecting circuit performance, you should not care about it.By trying to separate 2 grounds and 2 power rails on 2 layer board you may introduce more harm, then benefit.If HF noise really matters, you can use some local filtration with ferrite beads and decoupling capacitors, both on noise source and noise sensitive circuits.

author: exe
author_rank: Unknown
timestamp: November 20, 2018, 10:58:31 am
content: It's a common problem, there are several application notes on it. E.g., this one:https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html.In short, there are a few misconcepts "urban legends". Like, how to connect IC's with separate grounds. To my surprise the answer was to connect them together and to tie to analog ground (if you have one). The article I mentioned above explains why.

author: nemail2
author_rank: Regular Contributor
timestamp: November 21, 2018, 09:36:02 pm
content: Thanks for your help!I have the analog and digital part (power rails + ground plane) completely seperated as described in exe's link so I ended up putting in another 3.3V regulator with a ferrite bead at the output (sugar topping) where the DAC gets powered from. Both 3.3V + GND connected completely to the analog portion.The SPI lines have small resistors directly at the MCU to reduce the spikes.btw @exe, very very interesting link, thanks for that. some of that I was already aware of but definitely a must-read and many new information for me to assimilate.