50ohm Oscilloscope Input Impedance DC or AC?

[JohnLai]

Dear Friends,

A high-speed Oscilloscope has a nominal input impedance of 50ohm. Is this 50ohm just for AC(RF) signals? For DC biasing, is this 50ohm applied as well? If in DC, the probe is 50ohm, then the circuit biasing will be disturbed?

Another thought is, for DC biasing, the oscilloscope probe is 1Mohm. But is it right?

Can somebody explain. Thank you.

 

[biff44]

often there is a switch to choose between 1 M ohm and 50 ohm input. There is often another switch to choose between ac and dc coupling.

If you try to measure 100 v dc with the switches in DC and 50 ohms, there will be smoke!

 

[w2aew]

In general, lower BW scopes (< few GHz BW) will have a 1Mohm input impedance, which can be switched between AC and DC coupling. For medium BW scopes (300MHz-4GHz typically), the input can often be switched between 1Mohm or 50 ohms. Many of these ONLY offer DC coupling when the 50 ohm termination is selected. Higher BW scopes (>4GHz) are typically 50 ohm, and usually DC coupled only. Some models offer the ability to change the termination voltage - so that is looks like 50 ohms to xx voltage (instead of ground), but this is the exception, not the rule.

 

[JohnLai]

Thank you all. I kind of got it now.

 

[bluesy]

In general, lower BW scopes (< few GHz BW) will have a 1Mohm input impedance, which can be switched between AC and DC coupling. For medium BW scopes (300MHz-4GHz typically), the input can often be switched between 1Mohm or 50 ohms. Many of these ONLY offer DC coupling when the 50 ohm termination is selected. Higher BW scopes (>4GHz) are typically 50 ohm, and usually DC coupled only. Some models offer the ability to change the termination voltage - so that is looks like 50 ohms to xx voltage (instead of ground), but this is the exception, not the rule.

I would like to as why DC coupling is offered only when the 50 ohm termination is selected? Is that any effect on the signal if i am measuring low frequency with 50 ohm input termination?