Sensoray 2518 - Manual (Page 16)

Sensoray Model 2518/2519 Ethernet Smart A/D™ 13 Sensor Connections
Small excursions beyond the CMV limit may degrade
measurement accuracy on the offending channel, while
large excursions may cause measurement errors on other
sensor channels, or worse yet, might damage the board.
See “General Specifications” on page 26 for input CMV
limit values.
5.3.2 Excitation Terminals
Passive sensors require connection to the I+ and I-
signals. The I+ and I- signals supply positive and
negative excitation, respectively, to passive sensors.
Different classes of excitation are applied to sensors; the
class of excitation that is applied depends on the type of
sensor to be measured.
Constant current excitation is used for high-accuracy, low
resistance measurements. A current limited, constant
voltage excitation is applied when measuring higher
resistance values. Finally, a 10 Volt, high current
excitation is applied to strain and pressure gages.
5.3.3 Shield Terminal
The fifth lead—named S for shield—connects to the
cable shield, if present. The S signal is internally
connected to the backplane ground. To avoid ground
loops, the cable shield should never be connected to
anything at the sensor end of the cable, nor should it be
connected to another ground point.
A shield conductor is never absolutely required, but is
sometimes an unavoidable noise-abatement measure.
5.4 RTDs, Resistors and Thermistors
RTDs, resistors and thermistors can be connected to the
TB in any of three possible configurations: two-wire,
three-wire, and four-wire. In the following discussion,
sensor is used interchangeably with RTD, resistor and
thermistor.
5.4.1 Two-wire Circuit
The simplest configuration is the two-wire circuit. As the
name implies, this configuration requires only two wires.
The V+ and I+ terminals are shorted together at the TB,
as are the V- and I- terminals.
Figure 11: Two-wire connection.
This circuit conserves wire, but it causes measurement
error due to the voltage drop between the TB and sensor.
There are two potential solutions for this problem: use
short wires, or use more than two wires.
5.4.2 Three-wire Circuit
By using three wires, it is possible to reduce cable loss
errors by 50 percent. Instead of shorting V- and I-
together at the TB, discrete conductors are run from each
of these terminals to the sensor. The wires are then
shorted together at the sensor.
Figure 12: Three-wire connection
In this situation, the high impedance V- terminal detects
the voltage at the sensor before any voltage drop can
occur.
5.4.3 Four-wire Circuit
Unlike the three-wire circuit, which exhibits half the
cable losses of an equivalent two-wire circuit, a four-wire
circuit completely eliminates cable losses.
Figure 13: Four-wire connection
Like the three-wire circuit, separate conductors are run
from the V- and I- terminals to the sensor. In addition,
separate conductors are run from V+ and I+ to the sensor,
V +
I +
V –
I –
S
V +
I +
V –
I –
S
V +
I +
V –
I –
S