Author Topic: Oscilloscope Specifications Questions  (Read 336 times)

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Offline Vancouver_KidTopic starter

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Oscilloscope Specifications Questions
« on: December 03, 2024, 05:51:49 pm »
Hello,

I've never paid too much attention to oscilloscope voltage specifications as well as probe voltage vs frequency derating graphs. Yet, I've never toasted any scopes even though I do power-related work... :-//

I attached an image showing the voltage spec of a scope from Siglent (SDS800X HD) I'm looking into, and saw the front end can handle 400Vpk up to 10 kHz (pretty classic), presumably with a sine wave. Now, within the smps world the switch node is usually a square wave (voltage measurement). Square waves have much higher frequency content, so, that spec should derate very quickly not only from the fundamental switching freq POV (because you are most likely switching at 30kHz to 500kHz, or higher) but also with the added harmonics.

Do scopes show "Max Input Voltage" spec with different waveforms, say with square waves?

Or, is that why we just say #$%@ it and use a HV isolated diff probe and forget about it for measuring anything DC-coupling related?

Let's say you don't have access to a HV isolated diff probe but want to measure that switch node with a 10X probe, how would you figure out using datasheets or any information to see if that square wave of "X" amplitude and frequency won't nuke your setup (probe to scope front-end)? Let's assume a PFC feeding some DC-DC input stage, so, 390VDC output with a 20VACrms ripple, and measuring the voltage at the switch node of a hard switched half-bridge.

Cheers
 

Offline 2N3055

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Re: Oscilloscope Specifications Questions
« Reply #1 on: December 03, 2024, 07:02:02 pm »
Hello,

I've never paid too much attention to oscilloscope voltage specifications as well as probe voltage vs frequency derating graphs. Yet, I've never toasted any scopes even though I do power-related work... :-//

I attached an image showing the voltage spec of a scope from Siglent (SDS800X HD) I'm looking into, and saw the front end can handle 400Vpk up to 10 kHz (pretty classic), presumably with a sine wave. Now, within the smps world the switch node is usually a square wave (voltage measurement). Square waves have much higher frequency content, so, that spec should derate very quickly not only from the fundamental switching freq POV (because you are most likely switching at 30kHz to 500kHz, or higher) but also with the added harmonics.

Do scopes show "Max Input Voltage" spec with different waveforms, say with square waves?

Or, is that why we just say #$%@ it and use a HV isolated diff probe and forget about it for measuring anything DC-coupling related?

Let's say you don't have access to a HV isolated diff probe but want to measure that switch node with a 10X probe, how would you figure out using datasheets or any information to see if that square wave of "X" amplitude and frequency won't nuke your setup (probe to scope front-end)? Let's assume a PFC feeding some DC-DC input stage, so, 390VDC output with a 20VACrms ripple, and measuring the voltage at the switch node of a hard switched half-bridge.

Cheers

It is 400V Peak. Means Peak. AC or DC or AC+DC.
At no point voltage applied to input should have more than 400V. DC-10kHz
Over that, I've seen some scopes specify how derating curve looks, like 20dB/oct or something like that.

I don't know what is specification for SDS800xHD.

And I will repeat this once: it is NOT recommended to use classic passive probe for SMPS work.
Probe frequency derating is only minor part of that reason.
Most important reason is that one side of the probe is hard grounded to PE terminal on power cord and will create big badaboom if it touches some energized part of ground referenced circuit.
Which can happen even without intention (probe slips, ground clip unclips and falls where it should not..).

Active differential probe has both probes "safe" to connect anywhere in circuit.

That also makes some measurements possible that otherwise would not be: a voltage across any part of the circuit.

"Just hard work is not enough - it must be applied sensibly."
Dr. Richard W. Hamming
 
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