Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 72
FUNCTIONAL DESCRIPTION
Original Instructions
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.
U7 and U11 measure the voltage across the sensor and feed the inputs of ADC U12.
The outputs of U7 and U11 are also fed back to U10 and U15. This ensures that the
voltage across the sensor is exactly equal to the demand voltage generated by U24 and
U25.
U13 and either U14 or U16 measure the voltage across the reference resistor. This
differential voltage is scaled by a reference gain multiplier, whose gain is selected to
give an output in the range 1 V to 2.5 V. This voltage is then passed to the reference
voltage inputs of the ADC (U12). This circuit configuration produces a ratiometric
measurement technique. The resistance of the sensor is:
Rsensor= Rref x |ADCnorm| x RefGain/ADCgain
Where:
Rref is the reference resistance (either 2k
Ω or 80kΩ).
ADCnorm is the ADC reading, normalised to the range 0 to ±1 V.
RefGain is the interpolated gain from the calibration tables.
ADCgain is the gain setting of the ADC.
The measurement accuracy of this circuit does not depend on the accuracy of the
excitation or reference voltages. The accuracy primarily depends on the accuracy of the
reference resistor plus any errors introduced by operational amplifiers.
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