Scopes Max input voltage, what does it really mean?

[rolloman]

Ok. this question is only so that I can learn about a scopes Max input specification capability only. I want to understand more about my oscilloscope and what the specs. really mean. Completely for theoretical purposes only not discussions on isolation transformers, earth mains gnd, differential probes, HV probes, measuring practices, etc. etc.

Leader LS8022 20MHz Analog Oscilloscope.
Scope Max input = 400 V (DC + AC peak) Withstands up to 400 V dc + ac Peak Inputs
Sensitivity =  1 mV to 5 V/div,
Display has 8 Divisions.

Questions:
1) Does the Max input 400Vp mean the scope with a X1 coax with alligator clips take 400V pk
    although obviously with DC it would be off the screen and with AC you could only observe a 
    very small portion of the waveform and a large majority of it would be off the screen?
2) Would this damage the scope?
3) Is the Max input only stating what the scope would be capable of displaying using the 5v/div
    setting  along with a x100 probe.     

[Ian.M]

Its rated not to be damaged by any low frequency signal that stays within +/- 400V from ground as measured at the input socket.  This means that you could for example look at the ripple on a 200V DC supply using AC coupling mode.   

Use with an appropriate non-switchable x10 or x100 probe can extend the voltage limits. The lesser of the probe rating or the scope's rating x10 or x100 will apply.  Almost invariably the probe rating is the limit, unless you are using a cheap PC scope pod that is only rated for low voltage signals.

[rolloman]

Thanks. Nice to know it means exactly what is say's and straight into the jack won't damage it. Also the choosing of the probe attenuation ratio is simply to get the signal within a range in order to view it completely and not to prevent any damage to the scope.

[Ian.M]

Its *NOT* a good idea to push the limits of the scope's ratings.  Also high dV/dT can be a problem.   I would suggest avoiding exceeding 50% of its rating or having more than 10% of the total trace amplitude off-screen.

You choose a probe attenuation ratio for a multitude of factors, including reducing capacitive loading, voltage rating and overall Y volts/div scale desired, its NOT simply to get the whole trace on the screen.

[rolloman]

Sounds reasonable. Thanks.

[rolloman]

Another thing I noticed is that the input impedance of my scope is 1MegOhm. Should I select a probe that matches that for the 1X probe setting and a 10 MegOhm with a 10X probe. I see that the capacitance is adjustable to match the 27pf input of my scope.

[Ian.M]

1Meg||~30pF is industry standard for a normal scope input with a BNC connector.  You'll be hard put to find a scope probe with a standard BNC connector that isn't compatible. All attenuating probes must be compensated to match the scope's input capacitance - trim the compensation on a sharp edged squarewave test signal every time you move such a probe between scopes.  If HF signal amplitudes or pulse edges aren't as expected, check the probe compensation!

[rolloman]

Thanks, I understand it and can pick me out a probe now.