VS
VIN
Frequency
f
3dB
Figure 15.13:
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 amplifier input would have a 1 MΩ resistance and a 100 pF
capacitance.
This yields a
-
3dB point at:
(EQ1)
f
(
-
3
db)
=
1
2
̟
(
R
s
R
IN
) (
C
IN
+
C
probe
)
= 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)
V
IN
=
V
S
R
IN
R
IN
+
R
S
The effect of the capacitive loading is more complex and results in an
exponential response in the voltage:
(EQ3)
V
IN
(
t
) =
V
MAX
1
e
−
t
(
R
S
C
in
+
probe
)
GEN3i
I3763-3.1 en HBM: public
361
