Datasheet

LMP91200
www.ti.com SNAS571C –JANUARY 2012–REVISED MARCH 2013
The pH measurement with the LMP91200 is straightforward, the pH electrode needs to be connected between
VCM pin and INP pin. The voltage at VCM pin represent the internal zero of the system, so the potential of the
electrode (voltage at INP pin) will be refered to VCM voltage. The common mode voltage can be set to well fit
the input dynamic range of an external ADC connected between VOUT and VOCM when the LMP91200 is
configured with differential output. In Table 1 a typical configuration of the register of the LMP91200 with VCM
set at 1/2 of VREF and differential output.
Table 1. Configuration register: pH measurement
Bit Name Description
D15 MEAS_MODE 0 pH measurement
D14 I_MUX
Leave these bits as they have been configured for the temperature
[D13:D12] I_VALUE
measurement.
D11 PGA
[D10 :D8] VCM 000 1/2 VREEF
D7 VOCM 0 VOCM
D6 DIAG_EN 0 DIAGNOSTIC disabled
[D5 :D0] RESERVED RESERVED
Temperature measurement
The LMP91200 supports temperature measurement with RTD like Pt100 and Pt1000. According to the RTD
connected to the LMP91200 the right amount of exciting current can be programmed: 100µA for Pt1000 and
1mA for Pt100, resulting in a nominal voltage drop of 100mV for both RTD’s at 0°C. This voltage can be
amplified, using an internal amplifier with a factor of 5 or 10 V/V. In case of high precision temperature
measurement it is possible to connect an external high accuracy resistor and implement a calibration procedure.
The exciting current sourced by the LMP91200 can be multiplexed either into the RTD or into the external
precision resistor in order to implement a 2-step or 3-step temperature measurement. The multi step temperature
measurements allows to remove uncertainty of the temperature signal path.
1-step measurement
In the one step measurement the voltage across the RTD (Pt100, Pt1000) due to the exciting current is amplified
and measured. The temperature can be calculated according to the following equation:
Temp(°C) = (Pt
RES
_calculated – Pt
RES
_nominal)/alpha
where
• alpha is the thermal coefficient of the RTD (it depends on the selected Ptres)
• Pt
RES
_nominal is the value of the Ptres at 0degC (1)
Pt
RES
_calculated = (VOUT_Pt
RES
/I_Pt)/PGA_GAIN
where
• VOUT_Pt
RES
is the amplified voltage across the RTD at VOUT pin (ground referred) when the LMP91200 is
configured according to Table 2
• I_Pt is the value of the selected exciting current according to the RTD
• PGA_GAIN is the selected gain of the PGA (2)
Inserting Equation 2 in Equation 1 the temperature is given by the following equation:
Temp(°C) = Temp(°C) = ((VOUT_Pt
RES
/I_Pt)/PGA_GAIN – Pt
RES
_nominal)/alpha (3)
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