Handbook
November 2011
/
59-UMC0071
/
Issue
01
Mercury
iTC
Page 69
FUNCTIONAL DESCRIPTION
Original Instructions
4.6
Circuit descriptions
4.6.1
Temperature measurement circuit
Depending on the sensor, the circuit can be configured to measure either resistance or
voltage.
4.6.1.1
Description of voltage measurement mode
Voltage measurement mode is used with diode sensors or thermocouples.
Figure 4-3
Block diagram of voltage measurement mode
If an excitation current is required, the sensor and a reference resistor (either R44 or
R45) are connected in series. Digital to analogue converters (DACs) U24 and U25
generate an upper and lower demand voltage. The upper voltage is buffered by U10 and
applied to the upper end of the sensor resistor. The lower voltage is buffered by U15 and
applied to the lower end of the reference resistor. The same current thus passes through
the sensor and the reference resistor.
U13 and either U14 or U16 measure the voltage across the reference resistor and pass
these feedback signals back to U10 and U15. This ensures that the voltage across the
reference resistor is exactly equal to the difference between the output voltages of U24
and U25.
U11 and U7 measure the voltage across the sensor resistor. The outputs of these
devices are passed to the inputs of a 24-bit analogue to digital converter (ADC). The
ADC uses an accurate 4.096V reference for this measurement.
The reference resistors are 2k
Ω
and 80k
Ω
. Depending on the excitation current, the
resistor and gain factor are chosen so that the ADC input voltage is in the optimum
range.
