Sensoray 118 - Manual (Page 11)

Sensoray Model 118 Smart A/D™ Instruction Manual 9 Connections
you may either connect the excitation source and
excitation sense leads together at the gage or you may
leave the excitation sense leads completely disconnected.
Note that remote sensing of the excitation signals is not
supported.
The gage impedance seen by the Smart A/D™ excitation
source must be at least 120
Ω to prevent automatic current
limiting. If the input impedance is less than 120
Ω
, a
resistor must be inserted in series with the gage excitation
terminals to prevent excessive excitation circuit loading.
Note that this will alter the mV/V rating of the sensor.
Gage output voltage must fall between -500mV and
+500mV under all load conditions, including any offset
caused by bridge imbalance. If the gage output voltage,
at 10V excitation, might exceed these input limits, it may
be necessary to configure the sensor channel for voltage
input and supply external excitation to the gage.
4.8 4-to-20 mA Current Loops
All sensor channels support 4-to-20 mA current loop
inputs, subject to two constraints:
1. A 250
Ω
, 0.01% resistor must be installed as shown in
the following diagram. This resistor, which converts
the loop current to a voltage, is available as an option
for the Smart A/D™; order Sensoray part number
7408R.
2. Only the grounded end of the current loop may be
connected to a sensor channel. This ensures that the
common-mode voltage will not exceed Smart A/D™
input limits.
Figure 11: 4-20 mA current loop connection
It is important to note that the Smart A/D™ does not
provide excitation for current loops. Current loops must
be energized from an external power source, such as the
isolated 12V power supply in Figure 12.
Figure 12: Typical 4-20mA current loop connection
If the external power source is not isolated (i.e., its
negative output terminal is electrically connected to the
backplane power supply return), you should not connect
to the S terminal as shown in the above diagram, as a
potential ground loop would be created. In such a case,
leave the S terminal disconnected and connect to the V+
and V- terminals only.
V +
V –
S
Sensoray P/N 7408R
(+)
(–)
+
-
12V
POWER
SUPPLY
SENSOR
7408R
V+
V-
S