Hi,
From ADC theory my Full_scale is 3.3V so LSB=3.3/2^18=0.0000125885009765625
No. It depends on the used ADC. Thus there is a datasheet.
For your ADC the datsheet says: VRef is 2.048V nominally and it has a decodable input voltage range of +/-VRef which is +/-2.048V.
(assuming G=1)
so the total input range is 2x 2.048V = 4.096V which corresponds to 2^18 as digtal output value.
One LSB represents 4.096V / 2^18 = 15.625uV
btw: clearly stated in the datasheet Table 4-1:
***
Quantisation noise and all other ADC noise and unlinearities will be ignorable compread to the VCC noise and fluctuations.
ADC is a precision part, but a power supply is something unprecise.
Compare the "ADC" with the steel ball of a bearing. Clean, shiny, almost perfectly round...wone ball has almost exactly the same size as the other.
Compare the "power supply" with random stones. Dirty, different colors, different sizes, different form, different, some are strong, some are weak.
Where the ball bearings are good for a bearings --- the stones a good to build roads. (no one will do it the other way round)
***
You compare "noise" with a fixed deviation from 1.50V to 1.49V.
But this we call accuracy error.
"Noise" never is fixed. It will move all the time.
Thus the noise of your ADC is given with 1.5uV RMS .. but don´t forget the "RMS".
With a rule of thumb you may expect a 1.5uV x 6 = 9uVpp noise. (which for me seems unrealistic on a ADC with 15uV resolution).
This is ADC noise. So you may expect it to be around 1 LSB.
But the 5V supply easily jumps by 100mV in your case. divided with your voltage divider it still is 50mV.
50mV means 3200 LSB. Which means more than 11 bits of noise. Without filter.
Now - good thing for noise - the ADC has an integrated low pass filter. It´s cutoff frequency is around 2Hz. This will reduce ouput noise significantly.
Still I expect more than 256 LSB noise. (depends on the NODEMCU current behaviour when WiFI and bluetooth is active)..this is just an asumption - maybe I´m mistaken.
So from the 18 bit there maybe are 11 true informative bit remaining.
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A capacitor will filter. But you may additionally add an inductor.
It´s not only high frequency, even the low frequency pulsing caused by the WiFi and bluetooth operation.
I´m not speaking of the GHz frequencies. I´m speaking of the modulation frequency.
Use a scope and see what happens.
Or look at the ADC values when you set to 12 bit mode and a data rate of 240Hz.
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What you need to take car of:
* GND bouncing or GND_Reference bouncing. When the ADC "sees" a different GND voltage than the sensor it will lead to errors.
* analog value filtering: use a suitable C connected to ADC_GND and ADC_input
* Don´t use VCC (which you currently do with the voltage divider) as reference at all. Use a separate regulator, a zener, a true reference... when you need high quality measurement values.
And not to forget: Read the ADC datasheet thoroughly. MICROCHIP also provides additional documents: Application Notes and User Guides.
They tell you how to use this ADC. And they are for free .. made for circuit designers...
Klaus