GEN7iB
346
UNDERSTANDING INPUTS AND USAGE OF PROBES
UNDERSTANDING INPUTS AND USAGE OF PROBES
15
Fig. 15.12 Frequency response with 1X probe
Example:Assumingthatthevoltagesourcehasa1MΩresistanceandthe1Xprobe
has a 50 pF capacitance (a 1X probe has no resistance by itself ), the universal ampli-
erinputwouldhavea1MΩresistanceanda100pFcapacitance.
This yields a -3dB point at:
(EQ1)
=1/(6.28x500E+3x150E-12)≈2kHz
The loading due to the input impedance of the instrument and the probe capacitance is
twofold: resistive loading and capacitive loading.
The resistive loading actually reduces the voltage delivered to the instrument:
(EQ2)
The effect of the capacitive loading is more complex and results in an exponential
response in the voltage:
(EQ3)
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