Xylem H-5223 - Manual (Page 18)

Figure 2-4 Simple Resistive Sensor
2.2.3.3 Switched +5.00 Volt Reference Excitation
The +5 Volt reference output is used for analog sensors requiring a precision reference voltage.
The output current source maximum level is 10 milliamps. Exceeding this limit will cause the
excitation to possibly sag, and result in possible data errors. The Analog to Digital converter uses
this excitation for its reference to provide a ratio-metric relationship for sensors using the
excitation. What this means is that if a sensor causes loading to the excitation and drags it down
to 4.75 volts for example, then the A/D converter will use the 4.75 volts as its reference, and
maintain a full scale input equal to the reduced excitation. To a point this will reduce errors in
data when the excitation is used. If the excitation is being loaded down and some analog input
channels are not using the excitation, but produce a voltage output on there own, then these
inputs will have a much greater error.
Figure 2-4 shows a simple resistive type of sensor that would be typical of a wind direction type
of sensor. On a wind direction sensor a change in direction caused a variable resistor to move
causing a change in output voltage that is proportional to the direction. In this example the +5Vx
is used to power the sensor.
2-4 Hardware Options and Installation
H-5223