Sensoray 618 - Manual (Page 8)

Sensoray Model 618/619 Smart A/D 6 Sensor Connections
4.3.2 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.
4.3.3 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.
4.4 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.
4.4.1 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.
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.
4.4.2 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.
Three-wire connection
In this situation, the high impedance V- terminal detects
the voltage at the sensor before any voltage drop can
occur.
4.4.3 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.
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,
where they are tied together. This circuit eliminates cable
loss effects from both the V+ and V- lines.
4.4.4 Recommended Practice
If sensor field wiring is exposed to electrical noise (i.e.,
the cable run is long or is close to noisy conductors) you
should consider using shielded cable. The cable shield
must be connected only to the S terminal on the TB and
left unconnected at the sensor end of the cable.
The four-wire circuit should be used when accuracy is
critical. This is especially important when making
precision low-resistance value measurements, such as
when using RTDs.
V +
I +
V –
I –
S
V +
I +
V –
I –
S
V +
I +
V –
I –
S