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ADC design with the ADC124S101
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antonmi97:
Hello,

I want to make a embedded true RMS current meter and I want to use the ADC124S101 as my ADC. Now, I've never designed my own ADC circuit and in the datasheet http://www.ti.com/lit/ds/snas283d/snas283d.pdf I wasn't able to find out how the adc's output is mapped.
I know it's a 12 bit ADC (0 - 4096) and that the maximum analog input it VCC+0.3V (from the datasheet).

Does that mean that 0V == 0 & VCC == 4096 with a linear distribution in between?

Any advice for a novice is very welcome and appreciated!   ;D
mariush:
See page 18 in datasheet: http://www.ti.com/lit/ds/symlink/adc124s101.pdf

Because the reference for the ADC124S101 is the supply voltage, any noise on the supply will degrade device
noise performance. To keep noise off the supply, use a dedicated linear regulator for this device, or provide
sufficient decoupling from other circuitry to keep noise off the ADC124S101 supply pin. Because of the
ADC124S101's low power requirements, it is also possible to use a precision reference as a power supply to
maximize performance. The four-wire interface is also shown connected to a microprocessor or DSP.

You could use a 4.096v voltage reference to power the chip. and that way your output will be exactly 1mV per step ...
Voltage references aren't expensive, here's one that's cheap but not super great when it comes to tolerance and temperature coefficient : https://www.digikey.com/product-detail/en/microchip-technology/LM4040CYM3-4.1-TR/576-2568-6-ND/1858327
this one's slightly better: https://www.digikey.com/product-detail/en/microchip-technology/MCP1501T-40E-CHY/MCP1501T-40E-CHYDKR-ND/5844654

anyway... digikey has only 477 voltage references with fixed 4.096v in stock  : https://www.digikey.com/short/pphwnq

antonmi97:
Thank you very much for your answer. Helped me a lot!

This is what I have in mind for now:
Power the ADR443BRZ precision 3V reference from my 5V SMPS (with all the recommended passives), use the 3V reference to power my ADC124S101 12-bit 1MSPS ADC and with that I should get a resolution of 0.732 mV/bit.





Another thing I would like to know is if I'm right with my memory calculations. So I'm hoping for a 1MSPS rate and I'll be sampling for about 166 milliseconds (~10 mains cycles 60Hz). With that I should get 166 666 samples of 12-bit values which I'll have to store in 16-bit values. That will use about 2.7 Mbit or less than 350 kB of memory.

So my plan is to first take the samples, store them in memory, do some processing, spit out the result, clear the memory and repeat. Any takes on this? Have I made any mistakes with my calculations?
mikerj:
Your maths looks ok, but 1MSPS seems like an excessively high sample rate for the application.
MasterT:
Your choice of the adc isn't correct. For current metering the most important parameter is dynamic range. Math is simple, to have 1:1000 working input scale ( 1mA : 1A), and accuracy 1% at the lower end, dynamic range is 1:100 000. It demands 17-bits ADC as a minimum. Otherways PGA's, a lot of calibration at each steps of the pga, and error in fast varying current application. This is why sigma-delta adc architecture  is common for power metering,   great dynamic range and linearity for low price.
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