How to choose R and C to reduce noise on analog input of arduino?

Question

I'm not terribly proficient when it comes to hardware, so please bear with me.

I've tried to google around to figure out how to choose values for a low pass filter to reduce noise on a potentiometer. Most of the articles I have read explain how to choose values based on frequency characteristics. That is a reasonable approach when you have that information.

In this situation, my digital input on an arduino is modulated by a potentiometer and I am seeing the values bouncing around a lot between 1 and zero when it's around the threshold. I would like to implement a low pass filter, but I am not sure how to choose my values since it's not clear to me what the frequency band I want to filter is (I do not have an oscilloscope).

This problem feels fairly simple, and so far i've seen a bunch of arduino tutorials to 'just implement a low pass filter'. But what I'm looking for is, how would an electric engineer approach choosing the right values for R and C given they are seeing a noisy analog input?

Also, this is not a question about any specific circuit I've built, this is more a general question about how to build low pass filters in practice.

Edit: Here is a bit more information.

I'm using an LM35 sensor, which operated in the \${mV}\$ range. The temperature from the sensor is \$\frac{output(mV)}{10mV}\$ so 400mV would be 40.0 degrees C. I wanted to use the potentiometer to modulate a 0-450 mV value to feed to a comparator (LM393) to determine whether the temperature is greater than some adjustable threshold. In order to set the Vcc pin of the potentiometer, I used a voltage divider \$\frac{10kΩ}{100kΩ + 10kΩ} * 5V = .454V = 454mV\$

The circuit looks like this, and \${V_{out}}\$ is going into one of the arduino digital inputs. I also feed the inputs of the comparators to analog inputs for inspection, and the potentiometer input bounces around quite a bit.

Answer 1

• The ADCs will have a specific maximum input impedance mentioned in the datasheet (for most of the MCUs). Even the recommend series resistor and the capacitor can be found.

• Since the signal is from a potentiometer, it is a slow changing signal. A capacitor of value 10nF or even 100nF at the input of the capacitor will help reduce the sampling glitch. The internal sample and hold capacitor will then be easily charged with the external capacitor.

• The internal capacitor wil be of a very small value in 10s of pF. So, a 10nF or a 100nF External capacitance will not see any loading when the sample and hold capacitor is made to come in contact with the ADC line

• In software, you can employ, moving average as well to reduce the noise further.

• The external R and C together define the time constant. Waiting for several RC time constants (10x or even higher) before reading the value of ADC register yields more accurate result.

• If the potentiometer adds higher input impedance than the ADC expected value, opamp in a voltage follower mode can be used, I'd feasible, bringing down t output impedance of the signal source.

Answer 2

In this situation, my analog input on an arduino is modulated by a potentiometer and I am seeing the values bouncing around a lot. I would like to implement a low pass filter

Given the schematic you posted, it seems unlikely that the potentiometer output is so noisy - unless you have some troubles with the power supply. You could anyway put a capacitor at the output.

It is not clear when you say "the values bouncing a lot": do you use the arduino to read from the potentiometer, or to read the comparator output? And are you sure your routine is correct (let enough time to sample the input and enough time to the ADC to make the conversion).

That said, I think comparators are very sensitive to noise, may be you could use your arduino to read directly the output of the sensor, perhaps amplified via an operational. Doing so, you can make a filter in software which will grant you much more control; it should be easy because normally temperature varies very slowly, so the software should have a lot of time to filter. And you can also add some hysteresis; or some prediction: if you see the temperature is raising quickly, you can predict - ahead of time - it will reach the setpoint or whatever (there is always some inertia).

Answer 3

1Kohm and 1uF should work fine. Why?

Consider the equivalent input current of the ADC.

I = frequency * charge_per_sample

I = F * (C * V)

At 1 MHz sample rate , 10pF Csample(inside the ADC), you have

I = 1e6 * 1e-11 * 5volts = 5 e-5 = 50 uA

50 uA can be inverted, to find the Rerror is 20,000 ohms per volt. Thus for 1 millivolt drop, the series R can only be 20 ohms.

However, you will be adjusting the pot, so there is no accuracy requirement.

And your voltage is not 5 volts, but 0.5 volts.

Just place a 1Kohm in series with ADC input, and place 1uF from ADC input to ground.

This provides 159Hz input filter, plenty to remove most AM radio and black-brick switch-reg trash.