BRUEL & KJAER 2663, Техническая документация инструкция

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Chapter 2 — Operation
Accelerometer Input Connection
Preamplifiers Types 2663 and 2663 B
User Manual
BE 1068 – 12
7
2.3 Accelerometer Input Connection
The input connector of Preamplifiers Types 2663 and 2663 B is a two-pole TNC sock-
et, which can be used with three input configurations:
●
Single-ended floating
●
Single-ended grounded
●
Differential
Hum and other forms of noise due to ground loops and radiation pickup in a
vibration-measurement system can introduce considerable measurement errors. To
prevent this, it is essential that the accelerometer and preamplifier, together with
other instruments in the measurement system, are correctly grounded and screened.
The type of input configuration also has a considerable effect on the noise immunity
of the system. The three input configurations are covered in
section 2.3.1 and section
2.3.2 .
2.3.1 Single-ended Connections
Single-ended connections should be used when the preamplifiers are employed with
accelerometers which have one pole of their piezoelectric element connected to their
housing. It is suitable for most general vibration work. It is not suitable for work in
environments where a high level of electromagnetic radiation exists, as radiation
pickup can introduce noise. In such a case, the connection of a differential acceler-
ometer, as described in
section 2.3.2 , is preferred.
Plugs and Cables
Connection of a single-ended accelerometer to the preamplifiers should be made us-
ing one of the single-screened, low-noise accelerometer cables specified in
section
4.1 . These are fitted with microplugs which can be connected to the TNC input of the
preamplifiers using Adaptor Type JJ 0207. This connects the accelerometer housing
via the cable screens to the positive pole of the preamplifier input, thus completing
the measurement system signal-ground Iine.
Grounding
A problem often met in large vibration-measurement installations is the formation of
ground loops which introduce an error voltage that adds to the measured signal. To
avoid ground loops, it is essential that the signal-ground line is grounded at only one
point in the measurement system – preferably at the readout or at the supply instru-
mentation. Consequently, if the vibration test-specimen is grounded, then the trans-
ducer should be isolated from the specimen using an isolating stud and mica washer
when mounting (see
Fig.2.2 ).
Warning: Disconnect the power supply before removing the preamplifier cover
and making any adjustments to the circuit.