Using 50 Ω coax cable instead of passive probe

Question

Preface, why am I asking this question?

Instead of using a passive probe with a ground clip, I want a more "permanent" and reliable connection for my measurements. Note: I'm not talking about RF, just regular measurements you would do with a passive probe.

For this reason I'm considering using a UMCC connector on my PCB, something like this:

Also quoting an answer from "Oscilloscope: when to use coaxial instead of probes?" (emphasis mine):

Basically, when it is possible to use coaxial, use it. Coaxial connectors are more reliable than a probe. With a probe, you need to hook it to a test pin or even sometimes hold it in place. You have a grater risk of slipping off of your test point or getting a dodgy connection. With a probe you also get ground from 'somewhere' but not necessarily at the right place. On a coax, you get ground from the connector, where it should be. (this can matter when propagation times become relevant)

My question:

I really have a hard time finding anything on the question I'm having:

So as far as I know there are 50 Ω and 75 Ω coax cables. I know that this isn't the DC resistance but the characteristic impedance. Now, since I don't want to load my signals with a 50 Ω termination load (which the oscilloscope does when set to a 50 Ω input) I want to have a high impedance input like 1 MΩ or 10 MΩ, but with the coax connection.

Basically, I want to replace the passive probe with a coax, so I get a reliable connection — but what about the impedance of the coax cable? Does it matter for signals that are not RF? Because I can't find anything on that topic, really.

Is it okay to use a 50 Ω coax connection for non-RF signals into a scope with 1 MΩ / 10 MΩ input impedance without a load termination resistor?

My assumption is that it'll be okay since the DC resistance of the coax is minimal.

However, I'd like to have this confirmed by someone who has more experience than me.

Answer 1

Yes, you can do it. However.. the capacitive loading is fairly high on small coax cable. Maybe ~100pF per m for 1.13mm⌀ cable. So if you connect it to anything with fast-ish edges it will load the source significantly.

A clip-on test point for a 10:1 scope probe would be around an order of magnitude less loading.

Answer 2

For a good balance of input impedance and capacitive loading, you can use 50 ohm termination and a series resistor at source end.

For example 950 Ω resistor gives 1:20 attenuation and a consistent 1 kΩ impedance up to hundreds of megahertz.

Answer 3

The DC resistance of a coax cable is usually somewhere between 0 and 5 Ohms per 1000 meter.

So yes, when you configure your scope to high input impedance, your proposed setup works out fine.

Answer 4

A passive probe is more than just a 9 MΩ resistor and a piece of coax.

First of all, since both the input of the scope and the coax have significant capacitance, there is a compensating capacitor in parallel with the resistor, usually adjustable. This is why switchable 10:1 / 1:1 probes have such terrible frequency response when in the 1:1 mode. But even in 10:1 mode, there still is a capacitive load on your circuit — but it's a factor of 10 less than 1:1 mode.

Second, in order to deal with the fact that the scope input (when switched to 1 MΩ) is not matched to the coax, the coax itself is actually constructed with a deliberately lossy (resistive) center conductor.

You can't easily deal with the second point, but if you want to do this right, you should at least add a resistor and capacitor to each of your test points in order to create effectively a dedicated 10:1 probe for it.