Campbell Scientific SDM-CVO4 4-Channel Current/Voltage Output Module - Manual (Page 11)

SDM-CVO4 4-Channel Current/Voltage Output Module
or
V+
CVO4
Remote Current
Input
V-
+
I
d
b) The SDM-CVO4 provides the voltage source to drive the loop. In this case
the only real option is:
Remote Current
Input
CVO4
+
–
V
o
I
d
In both cases you need to consider whether the voltage supply is high enough
to allow the maximum current to flow (20 mA) through all of the series
resistances in the current loop. When considering this factor you need to allow
for a minimum 2.5 V drop between the I
d
and terminal of the
SDM-CVO4. You need to add this value to the estimated drop of 20 mA
flowing through the total resistance of the cable in the ‘loop’ plus any voltage
drop in the remote device (normally the voltage drop across a fixed sense
resistor).
As a simple example, consider the situation where the SDM-CVO4 is
powering the loop (as in (b) above), then the supply voltage output from V
o
is
13.5 (absolute minimum). If the remote device has a 250 ohms sense resistor
this will drop 5V at 20mA (using Ohm’s law), which in addition to the
SDM-CVO4's 2.5 V drop in the loop means the maximum allowable voltage
drop in the cable of the loop should be 13.5 - 5.0 - 2.5 = 6.0 V. At 20 mA
current flow, the loop could therefore have a maximum resistance of 300
ohms. Standard 24 AWG (7/0.2 mm) cable has a typical resistance of 85 ohms
per 1000 m. Allowing double this resistance to form a loop (there and back),
will mean the total cable length could be as long as 1700 m before the supply
voltage started to limit the current flowing.
4.2.3 Shield and Ground Connections
It is generally advisable to used shielded (screened) cable when connecting the
output to any remote system to reduce the chances of noise pickup. The shield
will only be effective if it is connected to a good ground reference point.
As a result of the isolation barrier in the SDM-CVO4, the ground terminals
associated with the output connector of each channel cannot be considered as
good ground reference points. Therefore it is advisable to make the connection
of the screen to a good ground point at the remote system rather than the
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