[TW#12287] ESP32 ADC accuracy

[nassks]

Hi,

I'm testing the ESP32's ADC on the SparkFun ESP32 Thing.
Currently my measure seems very noisy, it constantly varies on the 5 first bits, and the zero value is around 90mV.
Is there a way to achieve better accuracy?

My test code :

adc1_config_width(ADC_WIDTH_12Bit);
adc1_config_channel_atten(ADC1_CHANNEL_7, ADC_ATTEN_0db);

while (true) {
        vTaskDelay(300 / portTICK_PERIOD_MS);
        printf("ADC value : %x\n", adc1_get_voltage(ADC1_CHANNEL_7));
}

Thank you in advance for your help.

[projectgus]

The ADC has a non-linear response, and we are working on producing characterisation data which will be added to esp-idf to produce a linear output.

When you say "noisy", are you getting noisy readings on a constant voltage or is this noise caused by the non-linearity?

There's some more in this related Arduino bug: espressif/arduino-esp32#92

[projectgus]

@CarlosGS : for issues, it is much better to start a new one than to post off-topic in an existing one.

For questions like this, the http://esp32.com forum may be an even better choice.

[michapr]

@projectgus : I think nassks means the "noise" within same values - additional I have seen that there is a "noise" between the inputs.
put 1 Volt (with capacititor) on inputs CH4 ..CH7 I get such results:
(using 0bd and 11db for the values)

ADC1 CH4 value: 3719 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3889 @ 0db, 1052 @ 11db
ADC1 CH6 value: 3883 @ 0db, 1052 @ 11db
ADC1 CH7 value: 3891 @ 0db, 1052 @ 11db
-------
ADC1 CH4 value: 3867 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3885 @ 0db, 1053 @ 11db
ADC1 CH6 value: 3887 @ 0db, 1040 @ 11db
ADC1 CH7 value: 3886 @ 0db, 1057 @ 11db
-------
ADC1 CH4 value: 3894 @ 0db, 1056 @ 11db
ADC1 CH5 value: 3855 @ 0db, 1061 @ 11db
ADC1 CH6 value: 3887 @ 0db, 1049 @ 11db
ADC1 CH7 value: 3875 @ 0db, 1053 @ 11db
-------

Difference is up to 5% - maybe it is normal?

But for 0V - I get a value "0" ... without "noise" - it is ok for me

Michael

[michapr]

Do not know why the crazy style came up before ;)

Code snippet for reference:

...
adc1_config_width(ADC_WIDTH_12Bit);
...
adc1_config_channel_atten(ADC1_CHANNEL_4,ADC_ATTEN_0db);
int val=adc1_get_voltage(ADC1_CHANNEL_4);
adc1_config_channel_atten(ADC1_CHANNEL_4,ADC_ATTEN_11db);
int val1=adc1_get_voltage(ADC1_CHANNEL_4);
printf("ADC1 CH4 value: %d @ 0db, %d @ 11db\n", val, val1);
....

Michael

[marcmerlin]

So, in case anyone finds this ADC bug, I've found another ADC issue apparently where it will return data between 0-50% input, and cap out at 4096 from 50 to 100% input, but only after I try to use RMT.
Details here: #462

[marcmerlin]

So, the lastest arduino-esp32 fixes the issue I just reported when RMT is turned on, analogread values would get damaged.

[FayeY]

Hi nassks, is this problem solved?

[igrr]

@FayeY not yet, @Spritetm is working on it.

[dmody]

Just leaving my two cents on this...
I was hoping to use an inexpensive Voltage Reference IC (LM4040C30ILP) to help increase the accuracy of measured values. I too found the ADC to be non linear and have included my data below for anyone interested.
voltage, ADC
2 2222
3 3730
1 1033
1.5 1629
0 0
0.5 422
2.5 2840
2.8 3300
3.1 3980
3.007 3750

[dmody]

took another crack at this, and assuming the ADC readings are related to the 3.3V regulator voltage (mine is at 3.275 V on a developer board). Here's what I found.

[negativekelvin]

@dmody interesting, have you tried to fit curves to the upper range and lower range separately?