(Uno r3) voltage of PWM from pin 5, 8

I'm monitoring the PWM signals from pin 5, 8 with the following code
http://withinspecifications.30ohm.com/2014/02/20/Fast-PWM-on-AtMega328/

They are fast PWMs with high frequency and now my oscilloscope read 3.3v for both. But I wonder if the measured voltage is correct. I expected that the output voltages would be 5v.

For oscilloscope setup, I set input impedance to 50Ω. When I set it to 1MΩ, the voltage was around 5v. I don't understand why the voltage is read differently.

50ohm setting is loading the output. 1Mohm does not.

Weedpharma

weedpharma:
50ohm setting is loading the output. 1Mohm does not.

Weedpharma

@weedpharma
could you please explain little more? I'm sorry that I do not totally understand it.

The 50 ohm setting is the same as putting a 50ohm resistor across the output. This draws a power out of the pin and lowers the voltage.

The 1M ohm setting is a high resistance so draws virtually no power from the pin so the voltage does not drop.

The CRO 50 ohm setting is used on radio frequencies not on the type of readings you are making.

Weedpharma