// Seeed XIAO RA4M1 ADC Test
// Michael Koch, September 2024
double x, mean, sigma;
double sum_x, sum_xx;
int n;
// the setup function runs once when you press reset or power the board
void setup()
{
analogReadResolution(12);
Serial.begin(115200);
while (!Serial)
delay(10);
}
// the loop function runs over and over again forever
void loop()
{
n = 10000;
sum_x = 0;
sum_xx = 0;
for(int i = 0; i < n; i++)
{
x = (double)analogRead(A1);
sum_x += x;
sum_xx += x*x;
}
mean = sum_x / n;
sigma = (sum_xx - n * mean * mean) / n;
Serial.print("Mean: ");
Serial.print(mean);
Serial.print(" Sigma: ");
Serial.print(sigma);
Serial.println();
delay(500);
}

The mean value is as expected, but there is a standard deviation of about 135.
The same software with a SAMD21 module has a standard deviation of about 4 (in the same test circuit).
Why does the RA4M1 have more noise by a factor 30? The ADC is almost unusable.

I did already try to use an external reference instead of the 3.3V reference (after removing R5 from the module) but it didn’t help much.

I forgot the square root in the formula for the standard deviation:
sigma = sqrt((sum_xx - n * mean * mean) / n);
But the problem remains the same: RA4M1 has much more noise than SAMD21 in the ADC values.