AnalogRead - random values teensy 3.6

[RomanP]

Hi,

I'm trying to get readings on the voltage of my battery. Since the voltage of the battery exceeds 3.3V, I'm using a voltage divider with two 25 kohm resistors. See the attached picture for the schematic.
Before using the battery, I tried with a benchtop power supply so I can play with the voltage. I plugged the output voltage to the analog pin A16 (pin number 35) on my teensy 3.6 but when I use the analogRead() function, I keep getting random values. I then tried to put a 0.1 uF capacitor in between the + and GND but that did not resolve the problem.

Here is the simple code I used. And I keep getting random values. Can someone help me?

Thank you!
Roman

PS: I tested the voltage output with a multimeter and I had a very stable voltage of 3V after the divider, which should give me readings near 1000.



Code:
void setup()
{
Serial.begin(9600);
}

int val;

void loop()
{
val = analogRead(A16);
//val = analogRead(35);
Serial.print("analog is: ");
Serial.println(val);
delay(250);
}

[mborgerson]

It would help a bit if you defined what you mean by "random values". Do you get values from 0 to 1023 (the full scale for 10-bit resolution), or random over some smaller range near the expected count.

Two suggestions:

1. put the 0.1uF capacitor between the ADC input pin and ground, not between + and ground. Put the capacitor as close to the Teensy pin as possible.
2. Add "pinMode(A16, INPUT_DISABLE)" to your setup code. This disconnects any keeper resistors that might affect the analog input.

Even after you've done these things, you can still expect 4 or 5 counts of variance in your values. If you want a more stable output, like your multimeter,
you need to average the signal over several tenths of a second.

[mlu]

Do you have a ground connection, there is none on your diagram ?

[RomanP]

Dear mborgerson and mlu,

Thank you so much for your answers. I finally managed to solve the problem. My Analog GND pin was indeed not grounded!
Now, I have a very clear and stable signal.

Thank you!
Roman