Part Number:LMP91000EVM
Part number : LMP91000
Hardware : LMP91000EVM from Texas Instruments, with :
2-wire jumper installed ( short circuited )
Q1 and R12 not populated
C2 and R3 not populated
JMENB jumper with its pins 2 and 3 short circuited ( LMP91K always enabled )
Reference document : LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical-Sensing Applications
SNAS506I – JANUARY 2011–REVISED DECEMBER 2014
Circuit topology : 2-lead Galvanic Cell in Potentiostat Configuration ( Fig. 26 on page 19 of reference document )
Gas type : Oxygen
Sensor part number : S+4OX from DDS Scientific ( Website: www.ddscientific.com; Datasheet : www.ddscientific.com/.../s_4ox_datasheet_iss_5.pdf )
Measurements taken on pin C2 of LPM91000, once Vout is been used to sense temperature.
LMP91K Vref pin : +2.5 VDC
LMP91000 Registers Configuration :
TIA Control Register :
TIACN[ 7:5 ] = 000 ( Reserved )
TIACN[ 4:2 ] = 011 ( RTIA = 7 kΩ )
TIACN[ 1:0 ] = 11 ( RLoad = 100 Ω )
Reference Control Register :
REFCN[ 7 ] = 1 ( External reference source voltage selection )
REFCN[ 6:5 ] = 10 ( 67% of Vref internal ZERO selection )
REFCN[ 4 ] = 0 ( Negative bias polarity )
REFCN[ 3:0 ] = 0000 ( 0% of Vref BIAS selection )
Mode Control Register :
MODCN[ 7 ] = 0 ( Shorting FET feature disabled )
MODCN[ 6:3 ] = 0000 ( Reserved )
MODCN[ 2:0 ] = 111 ( Temperature Measurement - TIA ON )
Problem description :
I've used the LMP91000 in "3-Lead Amperometric Cell" topology, programming the LMP91K in "Temperature measurement (TIA ON)" with success for the Ammonia sensor.
Now, I'm trying to use it with the Oxygen sensor, in "2-Lead Galvanic Cell in Potentiostat Configuration" topology, programming the LMP91K in "Temperature measurement (TIA ON)".
The LMP91000 seems to have accepted the configuration programmed via its I2C interface, once measurements without sensor
in the TIA pins confirm the expected voltage levels.
Taking into account the registers configuration above and the values present on LMP91000EVM, with nothing connected to pins WE, CE and RE,
TIA becomes a voltage follower, and the voltage measured at C2, C1 and WE pins is approximatelly +1.67 VDC ( 67% of 2.5V ), as expected.
When the sensor is put into the circuit by the appropriate pins of LMP91000EVM, in free air, expected voltage is about 0.814 V, for a sensor with 123 µA
current : ( 2.5 V * 0,67 ) - ( 123 µA * 7 KΩ ) = 0.814 V. This voltage is reached, but only after a long time.
It starts at 20 mV, takes about 1 minute to start climbing and another 4+ minutes to reach 99% of its expected value.
I'm sure I'm making a mistake in the configuration of LMP91K and I ask someone to point me the right way, please.