RE: Primary Scoping
Subject: RE: Primary Scoping
From: jim.fosse-at-bdt-dot-com (jim fosse)
Date: Wed, 28 Feb 96 21:48 PST
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>>I have thought about using a 1000:1 divider ratio using 4 of the dale
>>100Megohm resistors in series to a 5 parallel combination of 2megohm
>>comp resistors. I'll then put some transorbs to ground before the scope
>>connection. All components will be physically well mounted to prevent
>>BTW, transorbs can be thought of as a 5ps.(thats not a typo: 5*10^-12
>>second) zener with a many thousand Amp ratting. By using 2 of the
>>unidirectional devices in antiparallel I can get any clamp voltage between
>>and 500 volts with only a 200pF capacitance penality.
>You've just built a lowpass filter with a cutoff frequency of 2 Hz! So you
>won't see too much of the signal you are trying to measure (100's KHz).
>Fc = 1/(2*pi*R*C) = 1/(2*3.14*400e6*200e-12) = 2 Hz.
>With such large resistors, any capacitance is going to severely reduce the
>frequency response. This includes the cables and scope input capacitance.
Its bad, just not that bad. You forgot the thevinin equivalent of the 400meg
in parallel with the 400K; call it 400k. Fc= 1/(2*pi*400K*200e-12)=2.0Khz.
Still nothing worth looking at unless I want to watch the 60 cycle go by!
I was thinking out loud. Since then I've found that a 12" wire at 8 feet
works very well. Great isolation too.