Campbell SAT ARGOS Bedienungsanleitung Seite 250

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Seitenansicht 249
ZENO
®
-3200 USER MANUAL
Coastal Environmental Systems (206) 682-6048
Page 242
DC offset voltages arise due to large currents flowing through the ground return paths that have a
significant resistance. This resistive coupling can be reduced by providing separate ground wires for
each sensor. Use a large enough wire gauge in the sensor leads to prevent voltage drops due to wire
resistance; otherwise, more significant, resistive-coupling errors can arise.
Noise from radio frequency (RF) sources
283
may cause measurement error due to rectification within
the ZENO
®
-3200's sensitive amplifier circuitry. Bonding
284
the ZENO
®
-3200's enclosure to an earth
ground system will prevent most of these errors by providing a return path for RF-induced currents
that will be shunted through input protection circuitry.
The ZENO
®
-3200 provides multiple grounds, to minimize these sources of error and for protection
against lightning. The following figure shows how the different grounds are connected to a central
ground at the A/D converter.
Figure 13-1. Grounds available on ZENO
®
-3200.
The different grounds are appropriate under the following circumstances.
Earth Ground: the casing of the ZENO
®
-3200 is grounded to earth for protection against lightning
and similar contingencies. Coastal Environmental Systems recommends that the ZENO
®
-3200’s
enclosure and earth grounding system should be bonded
285
to a 6 foot (2 meter) grounding rod, driven
at least 5’6” (180 cm) into the ground. A to 2AWG wire, or larger, should be used for the connection.
Digital ground (marked GND): this is suitable only for digital inputs and outputs, and for power
returns – it is too noisy for use with analog sensors.
Analog ground (marked AGND): this is the standard analog ground; it is much less noisy than GND.
Use this ground for your analog sensors, unless you have a reason to use either SGND or
SWEXCRTN.
Switched excitation return (abbreviated SWEXCRTN): this ground is equivalent, in terms of noise,
to the standard analog ground. Use this ground for sensors which are cycled on and off to conserve
power during normal operation – the switch in this circuit is software-configured to cycle excitation
power to the sensor while it is being used.
283
This includes RF noise from handheld radios, motors, motor controllers, relays, welders, broadcast transmitters,
and heavy machinery.
284
Bonding is a bare metal-to-bare metal connection whose electrical resistance does not exceed 1 milli-ohm.
285
Ibid.
A/D
Converter
D
A
Earth
GND
AGND
SWEXCRTN
SGND
Seitenansicht 249
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