Timeline for Using 50 Ω coax cable instead of passive probe
Current License: CC BY-SA 4.0
9 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jul 5 at 15:33 | comment | added | Marco | For future readers and myself, I found some great resources talking about the topic here and here. | |
| Jul 5 at 14:54 | comment | added | Marco | Alright, thank you so much! That really means a lot me. | |
| Jul 5 at 14:53 | comment | added | Dave Tweed | Yes, that's the general idea. | |
| Jul 5 at 14:46 | comment | added | Marco | So assuming the coax has a capacitance of ~30 pF (~30cm) and the input capacitance of the scope is 13pF that would make 43 pF I need to compensate for, right? Multiplying that by 1/9 gives 4.7pF, so about 5pF, would that be an appropriate value in this case? I'm sorry, I'm trying to figure this out by myself but it's not easy. Thank you! | |
| Jul 5 at 14:39 | comment | added | Dave Tweed | You also need to add the capacitance of your length of coax, which, as Spehro said, will be on the order of 100 pF. Your compensating capacitor needs to be 1/9 of whatever that sum is, which is why it's usually an adjustable capacitor. But a fixed value somewhere in the range of 10 to 15 pF probably wouldn't be too far off the mark. | |
| Jul 5 at 14:34 | comment | added | Marco | I see. How would I go about choosing/calculating the capacitor value? My assumption is that I'll probably have to match it with the oscilloscope's input capacitance? The scope I'm intending to make the measurements with has a described input capacitance of 13pF. If you could help me with that I'd be very happy. | |
| Jul 5 at 14:26 | comment | added | Dave Tweed | Well, it's the first point that I made, but in the second and fourth paragraphs. | |
| Jul 5 at 14:20 | comment | added | Marco | Thanks for elaborating. If I understand correctly, the second point is talking about putting a 9 MΩ parallel to a capacitor in series with the test point connection and using the scope with 1 MΩ input impedance? | |
| Jul 5 at 11:40 | history | answered | Dave Tweed | CC BY-SA 4.0 |