Electronics > Projects, Designs, and Technical Stuff
Deviation MCP3914 ADC
Freddy:
The 30 mV is measured with a multimeter directly across a differential input pair eg. between CH5- and CH5+. An insulated from the circuit 30 V power supply was connected between inputs V13 and V14.
uer166:
Do you measure both with DMM and ADC at same time and compare? The DMM will load the front-end with its' own impedance (10meg or something?), reducing the voltage seen by ADC a bunch.
T3sl4co1l:
--- Quote from: Freddy on November 25, 2019, 10:34:32 pm ---The 30 mV is measured with a multimeter directly across a differential input pair eg. between CH5- and CH5+. An insulated from the circuit 30 V power supply was connected between inputs V13 and V14.
--- End quote ---
Hmm, do you know if the power supply may be introducing AC noise (most likely, common mode RF) this way? Shouldn't be, since it's behind the filters anyway...
Huh, also, why double filter the signals, why not share each consecutive +/-?
...Does the converter load the inputs differentially at all?
(Does help for probing individual channels, though.)
--- Quote from: uer166 on November 25, 2019, 11:53:23 pm ---Do you measure both with DMM and ADC at same time and compare? The DMM will load the front-end with its' own impedance (10meg or something?), reducing the voltage seen by ADC a bunch.
--- End quote ---
Nahh, out of a ~20k Thevenin source, that can only account for a ~0.2% change.
Tim
iMo:
Doublecheck the ADC settings. It could be the 3914 has got some different config bits there against 3911.
Also they use "1.5" coefficient in the math at several places in the DS.
Freddy:
First I have done some double check measurements, now with four 9 volt battery (36.44 V) in serie to avoid common mode noise. The ADC outputs 391690 without multimeter load. With MM load on the differential input it outputs 390970 (little difference) and the MM reads 36.0 mV.
I'm not sure if I read the question about the filter configuration right, but if I connected the CH- and CH+ of neighboring channels to each other (and use a single filter for the common node) then neighboring channels will influence each other because there is a gain depending input impedance of the ADC.
imo, yes, that must be it, I totaly forgot the 1.5 factor in my calculations. With this in mind the ADC result lays within the gain error margin listed in the datasheet. And yes, i'm aware of the differences between the registers of the 3911 and 3914.
Thank you all for looking with me at this problem.
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