All.
adding any capacitance to the input port of scope would never, repeat
never lower (never half) the total capacitance. Capacitance would be
added - Cap internal + Cap added outside.
What a joke. and resistor of 1 Meg would halve total input resistance,
but would not effect input from capacitance stand point.
Probe is lowering input Cap because it's series Cap small connected with
Cin of scope, and the total of two series Cap's is smaller then the
smallest one in chain.
note: with those kind ideas we going into imaginary world ( like typ.
democrats)
?
Greg
At 04:47 PM 5/29/09, HankC wrote:
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- t is true that adding a 1Meg paralleled with a 20 pF to the scope
input will halve the capacitance, & double the DC resistance. Both of
these results will reduce the loading on the DUT, which is the precise
reason that 10X probes are used !
- The scope's input capacitance is determined by wiring, relays, rotary
attenuator switches, etc.
- All these values are fixed & cannot be made to disappear.
- There might sometimes be a small trim cap to set capacitance to 20 pF
instead of the 18-19 it would otherwise be. This makes all the scopes of
the same model look the same so you don't have to keep recompensating
probes when they are moved around.
