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APPENDIX B
67
isolation is OFF, the input of the charge amplifier operates in single-ended mode with LO side
connected to digitizer ground.
The accuracy in the pass-band of the charge converter is not dependent on cable capacitance Csh.
This would not be the case if the front-end is a voltage amplifier instead of a charge amplifier since
Csh together with Cin would be an attenuation stage. Csh and Cin affect the high frequency response
of the charge amplifier. Use the equation below to compute the high frequency corner for the charge
amplifier.
F
max
=
1
(𝐶
𝑖𝑛
+C
𝑠ℎ
)∗200π∗K
•
C
sh
: Cable capacitance
•
C
in
: PE transducer source capacitance
•
K: factor depending on attenuation at F
max
: K=3 for -0.5dB; K=6 for -0.1dB; K=1 for -3dB
Figure 4- shows the amplitude frequency response of the front-end charge amplifier stage for various
values of source capacitance (Csh+Cin). This response needs to be combined with the corresponding
charts shown in the Anti-Alias Filter section to get the overall response of the EMX-4380 digitizer
when it is set to Charge input.
Figure 4-8: EMX-4380 Charge Amplifier Front-End Magnitude Frequency Response
The referred to input (RTI) noise of the charge amplifier is dependent on the source impedance at
its input. Use the equation below to compute by how much the noise will increase based on the
source resistance Rs of the transducer:
Noise Multiplier = 1 +
500MΩ
Rs
-21
-18
-15
-12
-9
-6
-3
0
3
1E-1
1E+0
1E+1
1E+2
1E+3
1E+4
1E+5
1E+6
1E+7
N
or
maliz
ed
Amp
lit
u
d
e
(dB)
Frequency (Hz)
1.5nF
3nF
6nF
10nF
