Low pass filter connected to PWM not outputting correct value

Using Arduino General Electronics
1

Hello,

I have attached a low pass filter consisting 1MOhm resistor and 100nF capacitor to pin D13 on my Arduino Mega. Using Matlab I told Arduino to output 2.5V to pin D13( writePWMVoltage(a,'D13',2.5) ). So I thought measuring the voltage across the capacitor will give me smooth 2.5V however both my DMM and Analog read of Arduino are reading me ~2.1V. Why is that?

This might be a dumb question but I could not search answer for my situation
so I gave a go :slight_smile:

Thanks in advance.

2

I told Arduino to output 2.5V to pin D13( writePWMVoltage(a,'D13',2.5) )

And will that work? What actual value are you sending to D13?

Also 1M is a bit high, approaching the value of the input impedance of your meter. Plus check the 5V rail of your computer, what is it?

3

The filter is off. Your cutoff is about 1.6 Hz. You need higher cutoff freq for PWM. Try again but set cutoff about 1 kHz.

I think I understand what you’re trying to do. Filter the PWM to output a steady DC. Interesting. Play with the filter.

4

wolframore:
The filter is off. Your cutoff is about 1.6 Hz. You need higher cutoff freq for PWM. Try again but set cutoff about 1 kHz.

This is totally incorrect. It is a low pass filter the break frequency only affects the speed that the voltage can be changed, not the DC value.

5

Corner frequency must be low to remove ripple.
But as Mike said, a (10Megohm?) DMM or other circuit connected to a filter loads it, and you end up with a lower voltage.
Especially if you have choosen a very high resistor for your filter.

Try 10k/47uF, and/or buffer the output with an opamp.
Leo..

6

Grumpy_Mike:
This is totally incorrect. It is a low pass filter the break frequency only affects the speed that the voltage can be changed, not the DC value.

Hey Mike you're right... I didn't get what he was trying to do until after I posted... He's doing 50% PWM to try to output 2.5v after filter. Looks like the filter is dropping voltage.

7

I checked my 50% duty cycle output (2.5V) and 100% duty cycle output with my (5V) oscilloscope and it is giving me value of 2.48V and 4.96V so Arduino looks fine,

I also tried 100k resistor instead of 1M, and now I am getting ~2.28V at 50% duty cycle

8

Seems like impedance of the filter was the problem, Now I'm getting 2.48V with 10KOhm 100nF filter. Thanks everyone!

9

Not sure what you're trying to do, but 10k/100nF is not enough to remove the PWM component.
Try a 10-100uF capacitor.
Leo..

10

Hi,
Welcome to the forum.

Please read the first post in any forum entitled how to use this forum.
http://forum.arduino.cc/index.php/topic,148850.0.html.

What will you be using the analog level for?

You may need to add a unity gain buffer to prevent the filter from being loaded down.

Tom... :slight_smile: