Omega Engineering OMB-DBK-34A, User Manual

Page: 130 / 250

Cable Driving
Operation over long cables is a concern with all types of sensors. Concerns involve cost, frequency
response, noise, ground loops, and distortion caused by insufficient current available to drive the cable
capacitance.
Coupling a short (e.g., 1m) adapter cable from the accelerometer to a long, low-cost cable like RG-58U or
RG-62U with BNC connectors can reduce the cost of long cables. Since most cable failures occur at the
accelerometer connection where vibration is the greatest, only the short adapter cable would need
replacement.
Capacitive loading in long cables acts like a low-pass, second-order filter and can attenuate or amplify
high-frequency signals depending on the output impedance of the accelerometer electronics. Generally this
is not a problem with low-frequency vibration (10 Hz to 2000 Hz). For measurements above 2000 Hz and
cables longer than 100 ft, the possibility of high-frequency amplification or attenuation should be
considered.
The DBK4 constant-current source provides 2 or 4 mA to integral electronics. Use the higher current
setting (4 mA) for long cables, high peak voltages, and high signal frequencies.
The maximum frequency that can be transmitted over a given length of cable is a function of both the cable
capacitance and the ratio of the maximum peak signal voltage to the current available from the constant
current source:
f
K
C
V
Icc Ib
=
−
⎛
⎝
⎜
⎞
⎠
⎟
2
π
Where:
f = Maximum frequency in Hz
K = 3.45 ×10
9
(K is the scale factor to convert Farads to picoFarads and
Amperes to milliAmperes and a factor to allow cable capacitance to
charge to 95% of the final charge.)
C = Cable capacitance in picoFarads
V = Maximum peak measured voltage from sensor in volts
Icc = Constant current from current source in mA
Ib = Current required to bias the internal electronics, typically 1 mA
Drive Current
Cable Length Frequency Response to 5% of
Maximum Output Signal Amplitude
(mA) @30 pF/ft (Ft) ± 1 V ± 5 V
2 10 185 kHz 37 kHz
2 100 18.5 kHz 3.7 kHz
2 1000 1.85 kHz 370 Hz
4 10 550 kHz 110 kHz
4 100 55 kHz 11 kHz
4 1000 5.5 kHz 1.1 kHz
DBK4, pg. 16
958293
DBK Option Cards and Modules