# Why does my O-scope show capacitor charge starting negative?

My wave generator has a 50 ohm output impedance. I'm using a 10kohm series resistor and a 100nF capacitor. I calculated the RC time constant to be (10050)(100E-9)=1.005ms. A safe value for charging and discharging should be 5*(time constant), so 5.025ms. My function generator has a square wave with a duty cycle of 50%, so I want the full period to be 2*5.025ms=10.05ms. This corresponds to requiring a frequency < 1/10.05ms, or f < 99.5Hz.

So I set my wave generator to: Square, 50% Duty, 99Hz, 2.5V Amp, 2.5V Offset:

Now when I look at the voltage across my capacitor on my oscilloscope, instead of it starting at 0v and charging up to 5v, it shows that it starts at -2.5v and charges up to 2.5v. The circuit simulator I compared it against, however, shows that it goes from 0v to 5v like I expected, with the same source of 2.5v amplitude and 2.5v offset.

So why is my o-scope showing -2.5v to 2.5v across my cap instead of 0v to 5v?

• Did you set that 'scope channel to AC-coupled or DC-coupled? Oct 25, 2016 at 1:51
• It's set to AC coupled Oct 25, 2016 at 1:55
• AC-coupled means it essentially puts a capacitor in series with the 'scope's input, and the average voltage of the signal measured at the sampler will be 0 V. So you got exactly what you asked for when you selected that setting. Oct 25, 2016 at 2:18
• But if I set it to DC coupled then I just see 5V DC. What do I do to see both the waveform and the offset? Oct 25, 2016 at 2:29
• oops, looks like that was unintended behavior and I just needed to reset my O-scope. Works now, thanks. Oct 25, 2016 at 2:57