Question

The input impedance of a CRO is equivalent to a $1M\Omega$ resistance in parallel with a $45pF$ capacitance. It is used with a compensated 10 to 1 attenuation probe. The effective input capacitance at the probe tip is

• A. 4.5 pF
• B. 5 pF
• C. 45 pF
• D. 450 pF

Answer 1

The correct option is A 4.5 pF



So effective capacitance
$\frac{C_1.C}{C_1+C}=\frac{C}{20}$
$10C_1=C_1+C;9C_1=C$
$C_1=5pF$
But effective input capacitance at probe tip
$=\frac{C_{1}C}{C_1+C}=4.5pF$