Datasheet

LMP91200
www.ti.com SNAS571C –JANUARY 2012–REVISED MARCH 2013
Table 3.
Bit Name Description
D15 MEAS_MODE 1 Temp measurement
D14 I_MUX 1 RCAL
00 100µA (Pt1000)
[D13:D12] I_VALUE
10 1 mA (Pt100)
D11 PGA 1 10 V/V
Leave these bits as they have been configured for the pH
[D10 :D8] VCM
measurement.
D7 VOCM 1 GND
D6 DIAG_EN 0 DIAGNOSTIC disabled
[D5 :D0] RESERVED RESERVED
Table 4. Configuration register: 2-step measurement
Bit Name Description
D15 MEAS_MODE 1 Temp measurement
D14 I_MUX 0 RTD
00 100µA (Pt1000)
[D13:D12] I_VALUE
10 1 mA (Pt100)
D11 PGA 1 10 V/V
Leave these bits as they have been configured for the pH
[D10 :D8] VCM
measurement.
D7 VOCM 1 GND
D6 DIAG_EN 0 DIAGNOSTIC disabled
[D5 :D0] RESERVED RESERVED
The 2-step temperature measurement has a precision of about ±0.3°C (with R
REF
@ 0.01% of tolerance) which is
good enough in most of pH meter applications.
3-step measurement
This method requires 3 acquisitions and a precision resistor (R
REF
) connected between CAL and GND pin, (the
RTD is always connected between RTD and GND pin). The first two acquisitions measure the voltage across the
precision resistor in 2 different conditions (2 different exciting current and 2 PGA gains) in order to remove the
uncertainty of the current source value and the offset of the path. The third acquisition measures the voltage
across the RTD (similar to the 1-step measure), in this case the formula to calculate the temperature is more
complicate in order to take in account the non-ideality of the system (offset, source current error).
Temp(°C) = (Pt
RES
_calculated – Pt
RES
_nominal) /alpha
where
• alpha is the thermal coefficient of the RTD (it depends on the selected Ptres)
• Ptres_nominal is the value of the Ptres at 0degC (8)
Pt
RES
_calculated=((VOUT_Pt
RES
/PGA_GAIN)-Vos)/I_true
where
• VOUT_Pt
RES
is the amplified voltage across the RTD at VOUT pin (ground referred), when the LMP91200 is
configured according to Table 7
• I_true is the real current which alternatively flows in the external precison resistance R
REF
and in the RTD
• PGA_GAIN is the selected gain of the PGA
• Vos is the offset of the path (9)
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