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scope differential probes

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alm:

--- Quote from: David Hess on October 15, 2023, 09:41:10 am ---The disadvantage of the 7A22 and 7A13 is that they have no built in way to adjust the DC or AC common mode rejection of an attenuating probe.  Attenuating probes with a DC gain trim are required and these have not been made for more than a decade.

--- End quote ---
The 7A22 and 7A13, or 7000 series scopes, have not been made for decades either ;). Lecroy still sells the DA-1855A which is basically a stand-alone version of the 7A13. They also sell attenuating probes to go with it. I don't know if they would work on a 7A13. They require a specific BNC jack spacing, and the input capacitance may or may not be in range.

jonpaul:
Bonjour,

The 7A22/7A13 are NOT designed for use  with probes....

 precision matched pair  diff probes were made at one time.

Jon

adam4521:
For people on the forum learning from this thread, the 'common mode rejection' that members are talking about isn't an esoteric thing that only matters to purists -- having a balanced probe I've found is actually really important when trying to measure across a switching transition on a power supply. That's why floating the scope and using single ended probes is not a great plan even ignoring the safety point: the ground lead and the probe 'pin' have very different impedances from the perspective of the circuit you are measuring and therefore the disturbance you are trying to null out breaks through to your trace and you don't see the truth. So the advice here:
1. Use two identical probes, connect the grounds, use the 'subtract' function or
2. Get a differential probe
are your two practical options.
Option 3, isolate the DUT with an isolating transformer might sometimes be practical but can have similar cost as (2) and will change source parameters that might sometimes be important.
Watch for differential probes on eBay, if you are patient you will eventually find a deal! If you look after them they will hold their value.

David Hess:

--- Quote from: jonpaul on October 15, 2023, 11:13:58 am ---The 7A22/7A13 are NOT designed for use  with probes....
--- End quote ---

They were made to work with either 1x probes, or two of the Tektronix P6055A or later P6135A 10x probes which have DC and compensation adjustments.

There are also some earlier probes which had attenuation adjustments like the 100x P6009, which means they could also have worked with high voltage 1000x probes.

The 7A13 was also intended for comparator applications where there is no requirement for probe matching, and any probe can be used.

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