Bartington MAG-03PSU, Руководство по эксплуатации и техническому обслуживанию

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OM0065 ISSUE 5 PAGE 5 of 21
5 CABLES
Cables can be supplied to connect any of the
Mag -03 series of sensors to the Mag -03PSU.
Details are given in the operation manual for the sensor. The cable carries the supply voltage to
the magnetic field sensor; 0 V connection is via the screen. Analog signals are returned via three
conductors and the analog return is via a common conductor.
The maximum cable length of 600 metres is derived from the following considerations:
(a)
Above this length the positive supply current of 20 mA will produce a 1.5 V voltage
drop which may restrict the analog output swing especially where a low voltage supply is
used.
(b)
Above this length the maximum capability for capacitance loading of the magnetic field
sensor output will be exceeded.
Cable type
Cables other than those provided by Bartington Instruments should generally conform to the
specifications given and a screened type is essential if low noise performance is to be achieved.
An example of an alternative cable is Belden Type 9536 for use in protected conditions, e.g. ducts
etc.
6 ANALOG VOLTAGE MEASUREMENTS
The low resistance overall cable screen is used as a power 0 V return and, in addition to the three
X, Y and Z analog outputs, a separate signal common line is provided. The provision of a
separate signal common line ensures that the voltage developed in the power return, due to the
power supply currents, does not appear in the analog measurement. Clearly this is only possible
where no electrical connection is made between the power supply and the analog output common
connection at the measurement and power supply end of the cable. Connection of the output
signals to an isolated measurement device such as a battery powered voltmeter will introduce no
errors. Connection of the analog outputs to, for example, a data logger may provide a source of
error if the analog input LOW is connected to the electrical mains supply. Most data loggers have
differential inputs and this eliminates completely any ground-loop problems. It is important,
however, when using a differential input measurement instrument to constrain the common line
voltage to some small value. This is usually accomplished by providing a single ground
connection at the power supply.
To obtain the full precision of the magnetic field sensor the analog outputs should be connected
only to analog inputs which have an impedance greater than 10 k
Ω
.
7 Mag -03PSU WITH Mag -03 SERIES SENSORS
Use of the
Mag -03PSU power supply eliminates any problems which may be encountered in
providing a suitable isolated power source. The unit also provides additional filtering of the
analog signal to improve the signal to noise ratio. The high pass filter section can be selected to
provide either a dc or ac response.