Electronics > Projects, Designs, and Technical Stuff
Understanding this ADC resolution calculation
Red_Micro:
From this article the author has a 1000 turns CT and a 100 ohms burden resistor. He states:
--- Quote ---A simple 12bit ADC will give us 2^12, or 4096 possible values. Applied to a 3300mV range, this gives us a resolution of about 0.8mV. The load pictured above has a total peak-to-peak voltage span of 1219mV, and should be about a 4.5-4.7A load. The 1219mV give us a possible 1513 values for the voltage displayed at our calculated resolution, which translates to about 3mA per ADC value when using our 100 ohm load resistor (remember: it’s RMS!). That’s more than enough for this 10 amp sensor. However, as you start reading larger and larger amperages or use a current transformer with many more turns than the AC1010, this may fall short of your requirements.
--- End quote ---
He says the resolution translates to 3mA per ADC count. If I do calculations I get:
3 mA /1000 = 3 uA
3 uA * 100 = 300 uV (rms)
Vburden_peak = 300 uV * 1.41 = 424.26 uV
Vburden_pkpk = 424.26 * 2 = 0.84 mV
As you see, for a 12 bit ADC, 0.8mV is one count. However, the author references one count to the peak to peak voltage. So when sampling, the ADC would read 1.65V + 0.4 mV for positive peak, and 1.65 - 0.4 mV for negative peak.
Is my reasoning right? Is this method good to find the peak of a sine wave? Why one ADC count refers to the peak to peak voltage instead of just the peak?
PartialDischarge:
That article is plagued with errors, remove it from your mind. It's so bad that a college professor made a few videos about it:
Karel:
Also good to know:
"Coding Schemes Used With Data Converters"
http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=sbaa042
Kleinstein:
When calculating the RMS value one usually uses a lot more than 1 ADC reading. So one has quite some oversampling and the actual resolution for the RMS value can be quite a bit higher, going up with the square root of the number of readings used. There is a little lost for the suppression of the ADCs DC offset though.
Red_Micro:
--- Quote from: Kleinstein on April 18, 2020, 07:13:21 am ---When calculating the RMS value one usually uses a lot more than 1 ADC reading. So one has quite some oversampling and the actual resolution for the RMS value can be quite a bit higher, going up with the square root of the number of readings used. There is a little lost for the suppression of the ADCs DC offset though.
--- End quote ---
In my application I don’t need to calculate RMS strictly. Finding the peak would be enough. I’m thinking of taking 100 samples every 200us and finding the peaks. But I’m not sure, as I’m a bit confused about the relationship between the current and the ADC count.
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