Oxford Instruments Mercury iTC, Руководство пользователя

Страница: 71 / 146

Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 71
FUNCTIONAL DESCRIPTION
Original Instructions
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.
The outputs of U13 and either U14 or U16 are also 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 80k
Ω
).
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 current or the reference voltage. The accuracy primarily depends on the
accuracy of the reference resistor plus any errors introduced by operational amplifiers.
4.6.1.3 Description of constant voltage measurement mode
Constant voltage measurement mode is used with sensors that have a negative
temperature coefficient.
Figure 4-5 Block diagram of constant voltage measurement mode