Questions about differential probes

Question

(Q1) Can an active differential probe such as this Agilent N2752A be used in a single-ended way, i.e., with one of its inputs at the GND of a circuit? Understanding of course, that we have (a) 1/10th the usual input impedance of single-ended probes and (b) that we are limited in our voltage swing to 10Vp-p? Any other 'gotchas' about doing this? The reasons why will be explained in the next question ...

(Q2) I've acquired a scope with a 6GHz bandwidth, and the inputs on the front of this scope have warnings: "All Inputs 50 Ohm, +/- 5V Max Cat I" I am wondering if I can use this scope as general purpose instrument. So in addition to the above-mentioned single-ended measurement, I'd like to extend the voltage range to 50V. Would this mean a 10:1 differential probe is required ( 50V --> 5V for compliance with the warnings on scope )? Would a 50-ohm active differential probe with a 10:1 attenuation translate my single-ended measurement of 50V --> 5V and match this scopes 50 Ohm input impedance?

Answer 1

Here's what our probe experts had to say:

Yes! You can use the N2752A differential probe for single-ended measurement. There are two ways you can make single ended measurement with the N2752A.

1. Connect the – input of the probe to signal ground and + input to the signal, and select the probe input mode to single-eneded.

2. The N2752A offers what we call "InfiniiMode" – it can do all three: differential, single-ended and common mode. Use either a solder-in tip or a socketed tip to use InfiniiMode. Connect the ground input to the signal ground, and either +,- or both signal inputs to two input nodes of differential signals. Select the InfiniiMode input on scope to ‘Single ended A’ or ‘Single ended B’. You then can look at either + side or – side of the differential signal.

For question 2:

You are right. With a 10:1 attenuation ratio probe, 50 V input to the probe is attenuated down to 5V and is fed into the input of the scope. With 5V max input on the scope, you may want to choose a probe with higher than 10:1 attenuation ratio such as 20:1 or 50:1 to ensure the voltage input to the scope stays within the max input range of the scope.

But the problem is that a 50 ohm terminated active probe with >10:1 attenuation ratio is hardly found in the market. Most of the high voltage probe is 1 Mohm terminated. Does your scope have only 50 ohm input? If so, one way to get around is to use a high impedance adapter to the input of the scope, turning input impedance of the scope to 1 Mohm from 50 ohm. Then you may choose a variety of high voltage probe terminated into 1 Mohm.

Answer 2

I am personally not aware of any specific reason why not to use a differential probe as single-ended. I have, from times to times, also done something similar.

But you definitely have to consider the restrictions of a differential probe. Meaning respect the usually more restricted DC offset in comparison with a single ended probe and the dynamic range.

Regarding the second question: Be careful that in the webpage you linked us, it is stated that in 10:1 configuration your range can be only up to 10Vpp! I would be very careful in keeping these specifications and wouldn't use it to measure a 50V signal, or you could end up crying over a dead probe!

Answer 3

A differential probe doesn't care what the inputs connect to within reason. If one connection is to ground then you are making a differential measurement between ground and your signal.

Regarding the 50 ohm input and extending your probe range up to 50 volts, I would take advice from oscilloscope supplier in preference to anything else.