Xylem XL Series H-522 - Plus - Manual (Page 99)

XL™ Series
Analog Inputs / 5 Volt Excitation. 11-5
11.4.2 Switched +5.00 Volt Reference Excitation
The +5.00 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.
11.5 Analog Input Setup Examples
Example #1: A temperature probe with a 0 to 5 Volt output for a temperature range of !40E to
+60E Celsius would have the following setup. The slope would be calculated as follows:
The Offset value would be calculated using the calculated slope value, a known temperature
value in the measurement range, and the known sensor output voltage for that temperature.
The Offset value was calculated using known values. At a temperature of 60E Celsius, the
temperature probe output will be 5 Volts. Also at a temperature of -40E Celsius, the temperature
probe output will be 0 Volts. Notice here that the offset is the same as the minimum temperature
value. The offset is always equal to the minimum sensor value when the corresponding sensor
output is 0.00 volts.