About the probe of a logic probe

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About the probe of a logic probe

Post by sporniket » Fri Jan 08, 2021 9:49 am


Some time ago I've read @rubber_jonnie's explaination of the logic probe : viewtopic.php?f=91&t=3141

So I am considering to get one in the future, maybe self-made. And I've also read about oscilloscope and the fact that beyond a few MHz, 'X10' probes MUST be used instead of 'X1' ones in order to not alter significantly the signal being probed.

And so I was struck by this question : should one consider the same thing for the probe of a logic probe ? I would say yes, and would use a scope probe if I ever build my own logic probe. Am I right ?

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Re: About the probe of a logic probe

Post by exxos » Fri Jan 08, 2021 11:10 am

Logic probes are tricky, you would have to look at the specification of the probe to see whats it loading capacitance is. I would image it be higher than a scope, at least the cheaper ones. Using x10 means you need x10 amplification, which would have to be in your logic probe..

Looking at a random one http://www.farnell.com/datasheets/1955346.pdf says "Input Impedance : 120kΩ" so I guess the higher the better there.
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Re: About the probe of a logic probe

Post by rubber_jonnie » Fri Jan 08, 2021 12:03 pm

Good question.

The thing with a logic probe is that you aren't trying to see the whole signal like you do with an oscilloscope, rather only pick up the change of a TTL state from 0V to 5V and back again, or if it simply high(5V) or low(0V), so it really isn't necessary to have that x10 feature.

5v will always be 5V, 0V will always be 0V, so we only need to know if a signal is high, low or pulsing.

In my search for a logic probe, and I looked at many, from kits to HP models (Really quite expensive, even used), I have never seen any with the facility to change to x10 (Not even the HPs), only the ability to switch between TTL and CMOS logic levels, or inject logic pulses like the model exxos referenced.

I do know that with cheaper probes they are very susceptible to interference, and mine, which was only about £15, will light up like a Christmas tree if it's close to the PSU in one of my STs. For that reason, you may not want to go for a kit/self built device, or even a cheaper model.

I'd probably not use a spare scope probe for a logic probe as I don't believe it necessary, and the ability to change to a x10 value isn't needed and may affect the way it works, or even if it works.
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