couplingoscilloscope
I find many information about what AC coupling is about as following:
But not a single of them mentions the typical value of the blocking capacitor used in a scope's AC coupling. No quantitative information.
Considering a scope in 1x setting, input resistance of a scope is typically 1Meg. But what is the typical value of the scope's blocking cap. used in AC coupling? And why?
The current thru a capacitor is the rate of change of the voltage times the capacitance. At the same voltage, higher frequencies have a higher rate of change (derivative), so a capacitor conducts more current at higher frequencies, all else being equal.
Another way to look at this is to imagine a capacitor driven with different frequencies, all with the same peak to peak voltage. That peak to peak voltage dictates how much charge is sloshed around each ½ cycle. The higher frequencies will cause the same charge to be sloshed more often. Charge moving per time is current, so the higher frequency voltages cause a higher current thru the capacitor.
Don't get so hung up on names of how a capacitor is used in a circuit. Not all uses have short names, so don't try to force one onto every occasion. Generally bypass capacitor is used for the relatively small and high frequency cap close the the point of use of power. It's job is to be a short term but immediately responsive reservoir to keep the voltage smooth as the current demand changes quickly.
You can call C1 a DC blocking cap. I wouldn't try to put a particular name on the use of C2.
Scenario 1: When the LM741 output voltage goes from a set value to a more positive value electrons flow into the LM741 energizing the capacitor as show in the image below:
This is not correct. The capacitor has already been 'energized' (charged up to half the supply voltage) by the op-amp's steady state output voltage. When a signal causes the op-amp output to go up or down the capacitor remains charged to half the supply voltage. It passes the signal through to the volume control by maintaining a fixed voltage between its terminals. Since the DC voltage at the pot is zero volts, at this point the signal will go above and below ground.
The capacitor is only able to pass the AC signal through because it does not become energized and de-energized as the signal goes up and down. As the frequency is lowered however, the capacitor will indeed start to charge and discharge, reducing the output signal level.
If you feed in a low frequency square wave you will see the output droop towards ground as the capacitor charges and discharges exponentially (with a time constant of 10uF * 10kΩ). The trace below is an example of what the input and output waveforms look like when a low frequency square wave is passed through a capacitor.
Best Answer
If you were designing a scope and you wanted less than say 1% error measuring 60 Hz into 1 MOhm impedance, yet not have to deal with slow decay times from probing DC levels with small AC signals you would want the response time as quick as possible.
So what is that optimum frequency?
I would choose f-3dB slightly less than a 10 Hz HPF breakpoint for AC coupled Scope input.
Thus you can figure this out yourself, or not... ;)
C = 1/(2πfR) = 1/(6.28*10*1M)= 0.0156 uF or slightly more 0.02uF as in agreement with Sam and Barry. But it depends on th designer's two criteria I defined up front. decay time and attenuation error at 50/60Hz.