Products > Test Equipment
Probing at high speeds. Will the 10x probe overload the circuit?
(1/5) > >>
ballsystemlord:
Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.

Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M

He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?

Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))

He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?

Thanks!
srb1954:

--- Quote from: ballsystemlord on May 08, 2024, 05:05:38 am ---Hello,
So when I first learned about probing, I learned that you should use the 10x setting of the probe whenever possible so that you don't load down your circuit very much.

Recently, I came upon a video by Mr. Ganssle.
https://youtu.be/aJsJibDNg9M

He shows that a 10pF capacitor goes from over 300ohms of capacitive reactance to just 159ohms of capacitive reactance.
What happened to the 9Mohm resistor in the probe?
Like, it should still be a 10 to 1 input impedance, right?

Formula: sqrt((1/(PI×2×100 000 000×0.000 000 000 01))^2+(9 000 000^2))

--- End quote ---
The capacitance is effectively in parallel with the 10M \$\Omega\$ input resistance so the capacitive reactance will dominate and the input impedance will be ever so slightly less than 159 \$\Omega\$ capacitive reactance.  The formula you have used would apply for the series connection of a capacitor and resistor, which is not what a probe looks like.

--- Quote ---
He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?

--- End quote ---
Such probes are really only useful for higher frequency signals e.g. >100Mhz and the lower impedance circuits typically found with high speed signals. There are commercial probes available, known as Z0 probes, that have a series 450 \$\Omega\$ resistor and have a x10 attenuation when coupled to a scope with a 50 \$\Omega\$ input. Since the cable is terminated at the scope with the cable characteristic impedance it still looks like 50 \$\Omega\$ at the probe end of the cable and doesn't have the excess cable capacitance that is seen with cable terminated by the 1M \$\Omega\$ input of a scope as for a 10M \$\Omega\$ probe. Without the excess capacitance there is no need for compensation.

However such probe present an excessive loading, especially DC loading, on lower frequency and higher impedance circuits so are not generally suitable for most circuit probing needs.
tggzzz:

--- Quote from: ballsystemlord on May 08, 2024, 05:05:38 am ---Like, it should still be a 10 to 1 input impedance, right?

--- End quote ---

It will still be 10:1 division ratio, where the voltage is measured at the probe tip.

But note that a 600ohm output, for example, driving a *10 probe will have a reduced voltage at the probe tip.


--- Quote ---He goes on to discuss using just a piece of coax and 1K resistor.
Wouldn't that need some sort of capacitive compensation?
And if this really is *the answer* to high speed probing, why are we using 10x probes at all? Why not just a resistor hanging off of a piece of coax with maybe an attached pogo-pin and alligator clip on the ground terminal?

--- End quote ---

There is no single answer to probing, since all probes become part of the circuit and all alter the operation/measurement in one way or another. You need to match the class of probe to the circuit and measurement.

Even if using a standard *10 "high" impedance probe, a 6" ground lead makes it unsuitable for high speed circuits. With every probe, a 150nH lead inductance will resonates with the tip impedance; for a 6" lead plus 15pF, that is about 100MHz. For an example, see
https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/

The different classes of probe - and why they need to exist - are discussed in the references at https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/
tggzzz:

--- Quote from: srb1954 on May 08, 2024, 05:37:08 am ---Such probes are really only useful for higher frequency signals e.g. >100Mhz and the lower impedance circuits typically found with high speed signals. There are commercial probes available, known as Z0 probes, that have a series 450 \$\Omega\$ resistor and have a x10 attenuation when coupled to a scope with a 50 \$\Omega\$ input. Since the cable is terminated at the scope with the cable characteristic impedance it still looks like 50 \$\Omega\$ at the probe end of the cable and doesn't have the excess cable capacitance that is seen with cable terminated by the 1M \$\Omega\$ input of a scope as for a 10M \$\Omega\$ probe. Without the excess capacitance there is no need for compensation.

However such probe present an excessive loading, especially DC loading, on lower frequency and higher impedance circuits so are not generally suitable for most circuit probing needs.

--- End quote ---

Whether the loading is "excessive" depends entirely on the circuit. Some resistive divider probes have a 5kohm tip resistance.

Z0 resistive divider probes present a more constant loading, which is often more important that the absolute value.

You need to match the probe class to the circuit and measurement; no surprises there!

alm:
In addition to the signal and the circuit (impedance), which probe to use also depends on what you want to measure. I remember an old Tektronix training video that can probably be found on YouTube saying that for best amplitude accuracy, a FET probe with its high impedance over most of the frequency band is best (less loading that reduces amplitude). But for best rise time accuracy, a Z0 probe with its very flat impedance over frequency performs better.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod