# oscilloscope and load limit

#### ranianya

Joined Jan 25, 2024
5
The vertical amplifier of most oscilloscopes typically has an input resistance of 1MΩ and a calibration accuracy of 3% of full-screen deflection. You want to use a scope to measure voltages in a variety of circuits yet limit loading by the scope to 3%. What is the highest output resistance of the circuits that you wish to measure that will remain within this load limit?

do I load it in series or parallel?

#### MrChips

Joined Oct 2, 2009
30,814
The vertical amplifier of most oscilloscopes typically has an input resistance of 1MΩ and a calibration accuracy of 3% of full-screen deflection. You want to use a scope to measure voltages in a variety of circuits yet limit loading by the scope to 3%. What is the highest output resistance of the circuits that you wish to measure that will remain within this load limit?

do I load it in series or parallel?
The oscilloscope appears as a load in parallel of your circuit under test.

#### ranianya

Joined Jan 25, 2024
5
The oscilloscope appears as a load in parallel of your circuit under test.
thanks! this gives me R || 1M = 0.97R so about 30k

#### WBahn

Joined Mar 31, 2012
30,063
Since the question is specifically talking about the output resistance of the circuit being measured, you need to consider your circuit as being a Thevenin equivalent resulting in a voltage divider.

#### crutschow

Joined Mar 14, 2008
34,454
limit loading by the scope to 3%
That could give a max total error of 6%.

Best to use a 10:1 probe with the oscilloscope for most measurements, as that increases the input impedance to 10MΩ in parallel with <10pF.

#### ranianya

Joined Jan 25, 2024
5
Since the question is specifically talking about the output resistance of the circuit being measured, you need to consider your circuit as being a Thevenin equivalent resulting in a voltage divider.

wouldn’t the voltage divider only work if I connect the load in series?

#### ranianya

Joined Jan 25, 2024
5
That could give a max total error of 6%.

Best to use a 10:1 probe with the oscilloscope for most measurements, as that increases the input impedance to 10MΩ in parallel with <10pF.
I’m not sure I understand you

#### crutschow

Joined Mar 14, 2008
34,454
I’m not sure I understand you
The probe is in series with the output impedance of the source being measured.
So the higher the oscilloscope probe impedance, the smaller the voltage drop in the source impedance, due to the probe current, and the smaller the error.

Make sense?

#### MrChips

Joined Oct 2, 2009
30,814
The rule of thumb is the measuring instrument should have input impedance that is ten times higher than that of the circuit being measured. That limits the loading effect to 10%.
If you want to limit the error to 3%, then the impedance of the measuring instrument should be 30 times higher.

#### ranianya

Joined Jan 25, 2024
5
The probe is in series with the output impedance of the source being measured.
So the higher the oscilloscope probe impedance, the smaller the voltage drop in the source impedance, due to the probe current, and the smaller the error.

Make sense?
ohh is it meant to be in series? MrChips says parallel haha

#### WBahn

Joined Mar 31, 2012
30,063
That could give a max total error of 6%.

Best to use a 10:1 probe with the oscilloscope for most measurements, as that increases the input impedance to 10MΩ in parallel with <10pF.
It's a homework problem.