Electronics > Beginners
Can I use a 50ohm scope as 1mohm by adding a series 1mohm resistor to the input?
Eth7an:
I just have a really nice 50ohm scope and I want to use it for general purpose.
tggzzz:
--- Quote from: Eth7an on July 04, 2019, 02:27:53 pm ---I just have a really nice 50ohm scope and I want to use it for general purpose.
--- End quote ---
If you are asking this question, you are probably in danger of damaging your scope - and that sounds like it would be a shame.
General purpose 1Mohm//15pF scopes are protected against some high voltages. Typically 50ohm ones aren't, and are fragile: just look at the legend by the input.
I suggest you understand the info at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
Oh, yes. To answer your specific question, you could put 1Mohm in series with the input to make it more "general purpose". But the 1Mohm and 50ohms will be a potential divider that will attenuate the input by a factor of 20000!
ArthurDent:
The previous post is a good answer but have you read the scope manual? It would strike me strange if a general purpose scope didn't have a way to switch the input impedance between 50 and 1M ohm.
Kleinstein:
Some high BW scopes have dedicated 50 Ohms inputs. Beyond some 500 MHz the normal 1 M|| 20 pF input is no longer high impedance, because of the capacitance. In addition cable reflections get more and more important, so that passive probes are about limited to some 300-500 MHz.
A dedicated 50 Ohms input can be lower noise than a high impedance input and is also easier to build for high BW.
So some scopes made for high frequency skip on the high impedance input. If high impedance is needed, one adds an active probe.
Instead of an expensive active probe one may as well by a cheap 200 MHz DSO instead as an alternative instrument. :-DD
tggzzz:
--- Quote from: Kleinstein on July 04, 2019, 03:36:57 pm ---Instead of an expensive active probe one may as well by a cheap 200 MHz DSO instead as an alternative instrument. :-DD
--- End quote ---
That would be preferable for several reasons: cost (especially for >1 probe), robustness, diversity.
As the OP is a beginner, I wouldn't suggest a cheap old analogue scope might be sufficient for their requirements.
If the OP mentions the type of his existing scope, we may be able to offer more insights.
Navigation
[0] Message Index
[#] Next page
Go to full version