Question on Differential Probe Maximum Input Voltages

[NY2KW]

I would appreciate help understanding correct use of a high voltage differential probe specification (MicSig MDP702) maximum input differential Voltage versus maximum voltage to ground.  I want to use the probe as a single end active probe to trace small signals (~ 3MHz) in a RF tube amplifier with plate voltages ~ 200-300VDC.  I believe my problem is in one of the LC tuned circuits between stages and there is full plate voltage at certain points I need to test.   

My basic question: am I correct that I can the probe at the 20x setting for single input referenced to chassis ground based on the CAT II limits or must I use it at the 200x setting (which has a much higher noise floor)?  My signal levels at some point are in the 100mV range.

Maximum differential input ratings:
@ 20x : 70V (DC+AC PK)
@ 200x : 700V (DC+AC PK)
Max. Voltage to ground rating:
   CAT II 450V

Thanks in advance

Jerry
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[Phil1977]

It´s quite easy: The voltage difference between both probe inputs must not exceed 70V in the x20-setting.

But why do you want to use a differential probe if you measure referenced to chassis ground? A standard passive x10-probe with AC coupling should do the job with low noise to see the signals and with DC coupling you see the potentials.

The differential probe can be extremely handy if you connect both inputs to high potential. If e.g. anode voltage is 350V and you have a signal referenced to anode voltage, then you can precisely and safely see this signal - but then you don't need a GND-connection.

[NY2KW]

My Agilent and Rigol scopes have a maximum of 300V and 100V CAT II, respectively and my amplifier plate voltage is right around 275-300V, so standard passive probes could be risky.  I don't know what other reference point would be useful as either the vacuum tube grids or cathodes are grounded or very near ground potential.  I thought that any differential probe could be used as a simple active probe if one end is grounded.  What does the differential probe specification of maximum CAT II to ground = 450V mean?  Is that only for the highest attenuation setting -- for my MicSig MDP702 = 200x or does it also apply to the 20x attneuation?

Thanks in advance for your advice.

Jerry   

[Phil1977]

AFAIK the 300V / 100V CAT II of the probe inputs are without any attenuation in the probe, so with a x10 probe you can measure higher voltages. Most passive x10 probes I know are specified for up to 300V RMS CAT II, that´s over 420V peak with some tolerance for transients.

The "maximum CAT II to ground = 450V" is the max common mode voltage and that means that none of the inputs should exceed a potential of more than 450V to GND of the scope.

If you put the + input of the differential probe to 380V and the - input to 350V then the scope reports a differential voltage of 30V and everything is in spec. If you connect it to 480V and 450V then the signal is the same, but you are exceeding the common mode voltage.

If you connect - to 0V (GND) and + to 400V then you exceed the max differential voltage in to x20 setting.

[David Hess]

It´s quite easy: The voltage difference between both probe inputs must not exceed 70V in the x20-setting.

The maximum voltage rating applies to both common mode and differential signals, but 70 volts is the maximum that can be measured in x20 mode.  It would be weird if this were not the case because otherwise the probe would be easily damaged.

[Phil1977]

Yes, thanks for clarification. If the differential voltage exceeds the 70V probably nothing gets damaged (else it would be very hard to connect the probe to anything over 70V) but it can not be measured.