0
\$\begingroup\$

(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?

\$\endgroup\$
1
\$\begingroup\$

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.

\$\endgroup\$
0
\$\begingroup\$

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!

\$\endgroup\$
0
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.