SDM-CVO4 4-Channel Current/Voltage Output Module
12
The following shows the connections when the SDM-CVO4 provides the
voltage source to drive the loop:
Remote Current
Input
CVO4
+
–
V
o
I
d
When connecting the SDM-CVO4 to a remote current input, 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. Allow for a
minimum 2.5 V drop between the
I
d
and
⏚
terminal of the SDM-CVO4. 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).
For example, consider the situation where the SDM-CVO4 is powering the
loop. The supply voltage output from
V
o
is 13.5 (absolute minimum). A
remote device with a 250-ohm sense resistor will drop 5 V at 20 mA (using
Ohm’s law), and the SDM-CVO4 will drop 2.5 V in the loop. Therefore, the
maximum allowable voltage drop in the cable of the loop is: 13.5 – 5.0 – 2.5 =
6.0 V. At 20 mA current flow, the loop can 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.
7.2.3 Shield and Ground Connections
Use shielded (screened) cable when connecting the output to any remote
system to reduce noise. The shield is only effective when connected to a good
ground reference point.
Because of the isolation barrier, the channel ground terminals are not good
ground reference points. Therefore, connect the screen to a good ground point
at the remote system rather than on the SDM-CVO4. Do not connect the screen
at both ends of the cable as this may cause a ground loop.
The output circuitry of the SDM-CVO4 includes protective components to
minimize damage caused by transient voltages induced in the signal cable. The
protective components reduce the transient voltages to non-damaging levels
relative to the ground terminals on each output channel.
If the ground point voltage exceeds the data logger ground potential by more
than 2500 VDC, a secondary level protector provides a path for energy to
discharge to the data logger ground. To avoid needing the secondary
breakdown, connect a wire to the SDM-CVO4 output ground terminals to
provide a low impedance path to ground. However, this risks nullifying the
effects of the isolation barrier, such as when the SDM-CVO4 and data logger
are connected to the same ground.
Carefully study the ground connections of the entire system when connecting a
grounding wire to the output ground terminal, to avoid creating a ground
loop/referencing problem or causing a safety issue.
