Noisy analogRead

Question

I'm trying to read voltage using Arduino, here's the code:

float r1 = 99700;
float r2 = 10004;
float adc, voltage;
unsigned long analog;
void setup() {
  Serial.begin(9600);
  analogReference(EXTERNAL);
  pinMode(A0, INPUT);
}

void loop() {
  for (int i = 0; i < 64; ++i) {
    analog += analogRead(A0);
    delay(1);
  }
  analog = analog / 64;
  adc = (analog * 2.441) / 1024;
  voltage = adc / (r2 / (r1 + r2));
  Serial.print(analog);
  Serial.print(", ");
  Serial.print(adc);
  Serial.print(", ");
  Serial.print(voltage);
  Serial.println(" ");
}

Had a VERY noisy reading and what I did was adding an external voltage reference (TL431) and also connected a 9V switching adapter to Arduino UNO. But I still had some noise in analog readings so as you can see in the code I'm taking 64 sample and averaging them.

After doing all those noise reduced a lot but not completely:

What else should one do to get an steady reading?

_{By taking 1024 sample noise will be removed but then I'm sacrificing performance...}

Answer 1

I solved the problem by adding 3 capacitors:

1. 220uF on the Arduino power supply.

2. 100uF on TL431 output.

3. 1uF on A0 input.

I also increased the sampling rate to 128 times (and removed the delay).

Here's the result, as accurate as my multimeter:

There's still 2mV difference in higher voltages which is because of my 2.5V reference voltage and Arduino ADC 10bit resolution (2.5 / 1024 = 0.002).