Oxford Instruments Mercury iTC, Руководство пользователя

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Handbook
November 2011 / 59-UMC0071 / Issue 01
Mercury iTC
Page 73
FUNCTIONAL DESCRIPTION
Original Instructions
4.6.1.4 Calibrating the temperature measurement circuit
Figure 4-6 shows a block diagram of the temperature measurement circuit in calibration
mode.
Figure 4-6 Temperature measurement circuit calibration
The calibration is performed using a ratiometric circuit configuration. In calibration
mode, DACs U24 and U25 supply a differential calibration voltage to the input of ADC
U12. This same differential voltage is scaled by a reference gain multiplier circuit, and
supplied to the reference voltage inputs of U12.
The ADC input has a programmable gain amplifier (PGA) with five gain settings (1, 8,
16, 64 and 128). An autocalibration is performed for each range.
The reference gain multiplier is also programmable with eight gains (1, 2, 4, 8, 16, 32,
64 and 128). Each gain is calibrated at low and high reference voltages (1.2 V and
2.4 V). The exact gain that is used for calculating sensor resistance is a linear
interpolation between these two values.
It is only necessary to perform this calibration using a positive excitation voltage. The
ADC is operated in chop mode, which eliminates any offsets in the ADC.
Calibration is performed using the highest possible ADC accuracy setting, which is also
the slowest acquisition time. The results are stored in flash memory for use during a
measurement whenever a range is changed. Calibration is not performed every time a
range is changed, as this would cause unacceptable measurement delays.