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| MCP4922 DAC AGND and VDD confusion |
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| nemail2:
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! |
| nemail2:
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? |
| iMo:
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. |
| nemail2:
--- Quote from: imo on November 20, 2018, 08:28:55 am ---Since the chip has got only Vdd and Vss I would connect its Vdd to the analog power rail and analog gnd. You may use 100ohm resistors in SPI signals to limit noise,for example. --- End quote --- That 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? |
| iMo:
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.. :) |
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