ESP32 Forum

Any update on this?

I'm also interested in an update. I have both a NTC thermistor and an analog thermistor. I'm afraid to use the internal ADC going forward but would prefer to use the internal ADC vs. adding a $1+ ADC to the board.

Please take a look at the following link
https://esp-idf.readthedocs.io/en/lates ... alibration
Calibration for 11db attenuation is not very good as 11db attenuation does not have a linear response. Adding a 0.1uf capacitor can also help with the noise.

As far as I understand:
1. we have to route the vref to a gpio internally
2. measure it manually
3. save that value somewhere and replace the #define V_REF 1100 with the saved value

This is kinda produces unexpected steps in factory process and it is bad too since we already used all the GPIO's, connecting a reference to electronic circuits might bias the measuring and also produce unexpected behavior ... Doing it before is soldered is also a pain ...

ESP_Sprite said this process might happen for any new module in ESP factory - is the date known from which any new ESP32 bought will have this value measured, stored in EFuses to not be bugged with this additional step?

High offset levels needs fixing.

The high offset levels of the ADC makes it difficult to read lower voltages. As the ADC lowest range starts from zero at appr 70-100 mV you need external signal conditioning to lift the signal into "view".

Also noticed the ADC 12-bit, Range 3 is exremely unlinear. See image..

The light blue line is an RC filtered 12-bit 3.4V PWM to DC voltage and ADC values are averaged from 100 readings at each point.

A closer look at the large ADC offsets

About the offset you can see the PWM to DC signal has the ouput at 150 ticks before the ADC Range=3 reading bcomes > 0.

A closer look:

To show the PWM is not unlinear a 0.5X voltage divider using 220k resistors feeding the ADC input. As seen the lower half scale in Range 3 is pretty straight.

And this is also serves as a demonstration of the 12-bit ADC used for 6.4V full scale input, where the ADC does not producing readings >0 until >0.12 volts.
Reading a linear sweep 0..3.4 V with a 0.5X voltage divider usiing 12-bit ADC, Range 3.

Maybe you may want to test:

esp32.com/viewtopic.php?f=19&t=2881&start=30

Posted: Sat Aug 10, 2019 12:30 pm

You may calibrate your own ESP32( the nonlinear 11DB curve) with this program.<br>
All you need is a wire from pin 25 to pin 35 and a normal voltmeter.<br>

Calibration is done automatically.<br>
It produces a LookUpTable[4096] to correct your values like: <br>
CalibratedValue = LookUpTable[Read Value];<br>

I do NOT use a fitted curve but a real LookUpTable with 4096 values !

I get correct values +/- 0.01V compared to the voltmeter.

I don't think it helps with the lower-end dead offset between 0-70 mV, does it?