Fluctuating ADC with stabilized source

[supersjimmie]

I supply the ADC with a stable voltage, supplied by a separate LM1117 3.3V with 100uF capacitor a resistor voltage divider. The LM1117 will provide 3.3V regardless of the load of the ESP's own voltage regulator, the capacitor takes away any noise and the 2 resistors make a fixed voltage suitable to be read by the ADC.
(GND for both the ESP and the LM1117 are connected)

Then, based on this article, I tried to add more stability to the readings:
https://www.quora.com/Why-is-a-little-delay-needed-after-analogRead-in-Arduino
So I do subsequent readings and only use the last one. I even do it 3 times instead of 2, and then only use the last one because that one is supposed to be stable.

#define ANALOGPIN A0;
...
analog = analogRead(ANALOGPIN);
delay(1);
analog = analogRead(ANALOGPIN);
analog = analogRead(ANALOGPIN);
Serial.print(F("Stablized Analog pin: "));
Serial.println(analog);

Readings are done and stored every minute.

The ADC reading has 2 fluctuations, a slow drift and spikes.
This show the ADC output:

This has been tested with the mentioned 'external' 3.3V regulator and with the 3.3V output from the ESP itself. I also tried with or without the subsequent readings. I don't really mind the spikes because I can add some code to flatten those, but the "long" wave-like fluctuation makes it all useless.

(I need this to read an MQ135 sensor.)

Using current core stable and Arduino IDE 1.6.9.

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[supersjimmie]

Full code, when logging to the IDE's Serial Plotter:

#include <Arduino.h>
void setup() {
  Serial.begin(115200);
}

void loop() {
  int A;
  A = analogRead(A0);
  A = analogRead(A0);
  Serial.println(A);
  delay(60000);
}

I've had this code running for a few hours on a ESP with only the 2 resistors on the A0, one connected to Gnd and one connected to an LM1117 3.3v output.

[igrr]

I've set up a circuit to test this, so far I'm getting readings within 1 LSB of average. The thing will run for a few hours, and I'll post a graph by end of day.

Here's my schematic, for the record:

ESP8266 is mounted on a test board, which is powered by a SY8088 dc/dc step-down regulator. 3.3V to ADC input is supplied by another identical board, which doesn't have an ESP on it (so the regulator is only loaded by ADC divider).

[igrr]

No drift, just random noise mostly within 1 LSB:

[supersjimmie]

Strange, had it running on a separate ESP (NodeMCU), which is powered through USB, and the ADC powered like this:

Yesterday it kept going up and down about 10, currently it runs smooth.

[supersjimmie]

@igrr , I think I made some progress.
I've done several tests:

• with 2 different 'external' 3.3V power sources or with the 3.3V line from my NodeMCU board,
• with one or two subsequent analogRead calls,
• intervals ranging from 100ms to 60s,
• two different nodeMCU boards,
• with or without some WiFi related libraries loaded and connect/webclient calls.
  In all cases it kept jumping up and down with a difference of about 10.

Then I noticed that after a reset, it was stable for only a few seconds.
So that made me think about the fact that even without any WiFi library loaded and without any WiFi.begin(), the modules are still using their last save Wifi settings.
So I added #include <ESP8266WiFi.h> and in the setup I just called WiFi.disconnect() and then it was totally rock stable!

Then I added the WiFi.begin(..) again and it got unstable.

So this issue is related to WiFi.

[ibaranov-cp]

To me, this looks like the power draw from WiFi is distorting the onboard ADC.
This is honestly not too surprising, and unless there is an on-board external reference input, it is difficult to combat in software. Perhaps an I2C attached external ADC would serve your purposes if you need actual precision?
The onboard 10bit ADC is most likely a SAR type (simplest and cheapest).
So, if it is possible to control sample timing, giving the ADC more time to do its job may also help.

[supersjimmie]

Using an 'external' adc is not an option for my setup, simply because I don't have any empty usable digital pins left on the module. How could I control the sample timing?

[supersjimmie]

Until now I have this as a work-around to do a reading every minute:

void loop() {
  unsigned long starttime = millis();
  WiFi.disconnect();
  delay(10);  // allow a short time to disconnect wifi
  Serial.println(analogRead(A0));
  WiFi.begin(ssid, password);
  while ((WiFi.status() != WL_CONNECTED)) {  // wait for connect wifi
    delay(500);
  }
  delay(60000-(millis()-starttime));  // make loop time 60 sec
}

Only this introduces another issue, that the ESP doesn't always (re)connect to wifi causing it to hang.

[supersjimmie]

I have the issue with 3 NodeMCU boards, so I tried to find out if it was perhaps related to the small power regulator on those board. Instead of powering over usb or with an external 5V powersupply to the Vin pin, I now used a LM1117 3.3V reguator to supply to the 3.3V of the board. (thus bypassing the small onboard voltage regulator)
This still gives the same jumpy values.

[Gorkde]

Same problem for me see:

http://bbs.espressif.com/viewtopic.php?f=66&t=2286&p=7367#p7367
https://www.mikrocontroller.net/topic/399791#4616103

Even tried battery powered and entire Board shielded with foil.
Definitive the transmitting is causing this

After wifi.disconnect() the reading is ok!
In the Beginnin good readings then fluctuating

[Gorkde]

Btw as for me I also have no pins for external ADC which probably would be the best choice.

[Gorkde]

Isn't there a way to check if wifi is transmitting and poll the ADC when it's not?

[arizzi]

same problem here... is there a way to "pause" the wifi radio without really disconnecting?