Shock Accelerometer
Page 15
002-096e-01.19
7.2
Driving Long Cables
The voltage mode Piezotron circuit allows for long cable runs with low
noise susceptibility. Most laboratory instruments with built-in
accelerometer power provide current in the range of 2 to 4 mA. Most
Kistler couplers are set at the factory to provide 4mA of source
current. 4mA is a good compromise value for maximum frequency
response and high reliability.
Assuming a cable capacitance of 30pF/foot(98 pF/m), the full
frequency range of shock accelerometers can be realized (±5 Volt
output) up to a length of 430 feet (130 meters) with 4 mA of drive
current. Most Kistler cable types and common RG58 coaxial cable
have a rated capacitance of 30 pF/foot (98 pF/m). For most
laboratory applications this is quite adequate.
As cable length is increased the cable capacitance becomes significant,
thereby loading the Piezotron* impedance converter. If the current is
not sufficient to charge the cable at an adequate rate, high frequency
distortion will be experienced. The solution to this problem is to
increase the drive current
For the user's convenience a chart is provided (See Table 5). The
values given are based on a cable capacitance of 30 pF/foot (98
pF/m). The list of cables available from Kistler can be found in section
9.3. All cables in this section are 30pF/ft (98 pF/m). except for the
1331, 1635 and 1639 which are 20 pF per foot(65 pF/m).
Maximum
Fre/-
5%
Cable Length
Ft (meters)
Current (mA)
Required For
Output Signal
+/- 1 Volt
Current (mA)
Required For
Output Signal
+/- 5 Volts
10 kHz
1000 (300)
2
10
2000 (600)
4
18
9 kHz
1000 (300)
2
9
2000 (600)
4
17
8 kHz
1000 (300)
2
8
2000 (600)
3
15
7 kHz
1000 (300)
2
7
2000 (600)
3
14
6 kHz
1000 (300)
2
6
2000 (600)
3
12
5 kHz
1000 (300)
2
5
2000 (600)
2
10*
* Based on 30 pF/ foot (98 pF/meter)
Table 5:
Current Requirements for Driving Long Cables
