LMP91000
February 6, 2012
Sensor AFE System: Configurable AFE Potentiostat for
Low-Power Chemical Sensing Applications
General Description
The LMP91000 is a programmable Analog Front End (AFE)
for use in micro-power electrochemical sensing applications.
It provides a complete signal path solution between a sensor
and a microcontroller that generates an output voltage pro-
portional to the cell current. The LMP91000’s programmability
enables it to support multiple electrochemical sensors such
as 3-lead toxic gas sensors and 2-lead galvanic cell sensors
with a single design as opposed to the multiple discrete so-
lutions. The LMP91000 supports gas sensitivities over a
range of 0.5 nA/ppm to 9500 nA/ppm. It also allows for an
easy conversion of current ranges from 5µA to 750µA full
scale.
The LMP91000’s adjustable cell bias and transimpedance
amplifier (TIA) gain are programmable through the the I2C in-
terface. The I2C interface can also be used for sensor diag-
nostics. An integrated temperature sensor can be read by the
user through the VOUT pin and used to provide additional
signal correction in the µC or monitored to verify temperature
conditions at the sensor.
The LMP91000 is optimized for micro-power applications and
operates over a voltage range of 2.7V to 5.25V. The total cur-
rent consumption can be less than 10μA. Further power sav-
ings are possible by switching off the TIA amplifier and
shorting the reference electrode to the working electrode with
an internal switch.
Features
Typical Values, TA = 25°C
■Supply voltage 2.7 V to 5.25 V
■Supply current (average over time) <10 µA
■Cell conditioning current up to 10 mA
■Reference electrode bias current (85°C) 900pA (max)
■Output drive current 750µA
■Complete potentiostat circuit to interface to most chemical
cells
■Programmable cell bias voltage
■Low bias voltage drift
■Programmable TIA gain 2.75kΩ to 350kΩ
■Sink and source capability
■I2C compatible digital interface
■Ambient operating temperature -40°C to 85°C
■Package 14 pin LLP
■Supported by Webench Sensor AFE Designer
Applications
■Chemical species identification
■Amperometric applications
■Electrochemical blood glucose meter
Typical Application
30132505
AFE Gas Detector
© 2012 Texas Instruments Incorporated 301325 SNAS506G www.ti.com
LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing
Applications
Ordering Information
Package Part Number Package
Marking Transport Media NSC Drawing
14-Pin LLP
LMP91000SD
L91000
1k Units Tape and Reel
SDA14BLMP91000SDE 250 Units Tape and Reel
LMP91000SDX 4.5k Units Tape and Reel
Connection Diagram
14–Pin LLP
30132502
Top View
Pin Descriptions
Pin Name Description
1 DGND Connect to ground
2 MENB Module Enable, Active Low
3 SCL Clock signal for I2C compatible interface
4 SDA Data for I2C compatible interface
5 NC Not Internally Connected
6 VDD Supply Voltage
7 AGND Ground
8 VOUT Analog Output
9 C2 External filter connector (Filter between C1 and C2)
10 C1 External filter connector (Filter between C1 and C2)
11 VREF Voltage Reference input
12 WE Working Electrode. Output to drive the Working Electrode of the chemical
sensor
13 RE Reference Electrode. Input to drive Counter Electrode of the chemical sensor
14 CE Counter Electrode. Output to drive Counter Electrode of the chemical sensor
DAP Connect to AGND
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LMP91000
Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required,
please contact the Texas Instruments Sales Office/
Distributors for availability and specifications.
ESD Tolerance (Note 2)
Human Body Model 2kV
Charge-Device Model 1kV
Machine Model 200V
Voltage between any two pins 6.0V
Current through VDD or VSS 50mA
Current sunk and sourced by CE pin 10mA
Current out of other pins(Note 3) 5mA
Storage Temperature Range -65°C to 150°C
Junction Temperature (Note 4) 150°C
For soldering specifications:
see product folder at www.national.com and
www.national.com/ms/MS/MS-SOLDERING.pdf
Operating Ratings (Note 1)
Supply Voltage VS=(VDD - AGND) 2.7V to 5.25V
Temperature Range (Note 4) -40°C to 85°C
Package Thermal Resistance (Note 4)
14-Pin LLP (θJA)44 °C/W
Electrical Characteristics (Note 5)
Unless otherwise specified, all limits guaranteed for TA = 25°C, VS=(VDD – AGND), VS=3.3V and AGND = DGND =0V,
VREF= 2.5V, Internal Zero= 20% VREF. Boldface limits apply at the temperature extremes.
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
Power Supply Specification
ISSupply Current 3-lead amperometric cell mode
MODECN = 0x03 10 15
13.5
µA
Standby mode
MODECN = 0x02 6.5 10
8
Temperature Measurement mode with TIA OFF
MODECN = 0x06 11.4 15
13.5
Temperature Measurement mode with TIA ON
MODECN = 0x07 14.9 20
18
2-lead ground referred galvanic cell mode
VREF=1.5V
MODECN = 0x01
6.2 9
8
Deep Sleep mode
MODECN = 0x00 0.6 1
0.85
Potentiostat
Bias_RW Bias Programming range
(differential voltage between RE
pin and WE pin)
Percentage of voltage referred to VREF or VDD
±24 %
Bias Programming Resolution First two smallest step ±1 %
All other steps ±2
IRE Input bias current at RE pin
VDD=2.7V;
Internal Zero 50% VDD
-90
-800 90
800 pA
VDD=5.25V;
Internal Zero 50% VDD
-90
-900 90
900
ICE Minimum operating current
capability
sink 750 µA
source 750
Minimum charging capability
(Note 9)
sink 10 mA
source 10
AOL_A1 Open loop voltage gain of
control loop op amp (A1)
300mV≤VCE≤Vs-300mV;
-750µA≤ICE≤750µA 104 120 dB
en_RW Low Frequency integrated noise
between RE pin and WE pin
0.1Hz to 10Hz, Zero Bias
(Note 10) 3.4
µVpp
0.1Hz to 10Hz, with Bias
(Note 10, Note 11) 5.1
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LMP91000
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
VOS_RW
WE Voltage Offset referred to
RE
BIAS polarity
(Note 12)
0% VREF
Internal Zero=20% VREF
-550 550
µV
0% VREF
Internal Zero=50% VREF
0% VREF
Internal Zero=67% VREF
±1% VREF -575 575
±2% VREF -610 610
±4% VREF -750 750
±6% VREF -840 840
±8% VREF -930 930
±10% VREF -1090 1090
±12% VREF -1235 1235
±14% VREF -1430 1430
±16% VREF -1510 1510
±18% VREF -1575 1575
±20% VREF -1650 1650
±22% VREF -1700 1700
±24% VREF -1750 1750
TcVOS_RW
WE Voltage Offset Drift referred
to RE from -40°C to 85°C
(Note 8)
BIAS polarity
(Note 12)
0% VREF
Internal Zero=20% VREF
-4 4
µV/°C
0% VREF
Internal Zero=50% VREF
0% VREF
Internal Zero=67% VREF
±1% VREF -4 4
±2% VREF -4 4
±4% VREF -5 5
±6% VREF -5 5
±8% VREF -5 5
±10% VREF -6 6
±12% VREF -6 6
±14% VREF -7 7
±16% VREF -7 7
±18% VREF -8 8
±20% VREF -8 8
±22% VREF -8 8
±24% VREF -8 8
TIA_GAIN Transimpedance gain accuracy 5 %
Linearity ±0.05 %
Programmable TIA Gains 7 programmable gain resistors
2.75
3.5
7
14
35
120
350
kΩ
Maximum external gain resistor 350
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LMP91000
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)Units
TIA_ZV Internal zero voltage 3 programmable percentages of VREF
20
50
67
%
3 programmable percentages of VDD
20
50
67
Internal zero voltage Accuracy ±0.04 %
RL Programmable Load 4 programmable resistive loads
10
33
50
100
Ω
Load accuracy 5 %
PSRR Power Supply Rejection Ratio at
RE pin
2.7 ≤VDD≤5.25V Internal zero 20% VREF
80 110 dB
Internal zero 50% VREF
Internal zero 67% VREF
Temperature Sensor Specification (Refer to Temperature Sensor Transfer Table in the Function Description section for details)
Temperature Error TA=-40˚C to 85˚C -3 3°C
Sensitivity TA=-40˚C to 85˚C -8.2 mV/°C
Power on time 1.9 ms
External reference specification
VREF External Voltage reference
range
1.5 VDD V
Input impedance 10 MΩ
I2C Interface (Note 5)
Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS=(VDD – AGND), 2.7V <VS< 5.25V and AGND = DGND =0V,
VREF= 2.5V. Boldface limits apply at the temperature extremes
Symbol Parameter Conditions Min
(Note 7)
Typ
(Note 6)
Max
(Note 7)
Units
VIH Input High Voltage 0.7*VDD V
VIL Input Low Voltage 0.3*VDD V
VOL Output Low Voltage IOUT=3mA 0.4 V
Hysteresis (Note 14) 0.1*VDD V
CIN Input Capacitance on all digital pins 0.5 pF
Timing Characteristics (Note 5)
Unless otherwise specified, all limits guaranteed for TA = 25°C, VS=(VDD – AGND), VS=3.3V and AGND = DGND =0V, VREF=
2.5V, Internal Zero= 20% VREF. Boldface limits apply at the temperature extremes. Refer to timing diagram in Figure 1.
Symbol Parameter Conditions Min Typ Max Units
fSCL Clock Frequency 10 100 kHz
tLOW Clock Low Time 4.7 µs
tHIGH Clock High Time 4.0 µs
tHD;STA Data valid After this period, the first clock
pulse is generated 4.0 µs
tSU;STA Set-up time for a repeated START condition 4.7 µs
tHD;DAT Data hold time(Note 13) 0 ns
tSU;DAT Data Setup time 250 ns
tfSDA fall time (Note 14)IL ≤ 3mA;
CL ≤ 400pF 250 ns
tSU;STO Set-up time for STOP condition 4.0 µs
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LMP91000
Symbol Parameter Conditions Min Typ Max Units
tBUF
Bus free time between a STOP and START
condition
4.7 µs
tVD;DAT Data valid time 3.45 µs
tVD;ACK Data valid acknowledge time 3.45 µs
tSP Pulse width of spikes that must be
suppressed by the input filter(Note 14)
50 ns
t_timeout SCL and SDA Timeout 25 100 ms
tEN;START I2C Interface Enabling 600 ns
tEN;STOP I2C Interface Disabling 600 ns
tEN;HIGH time between consecutive I2C interface
enabling and disabling
600 ns
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur, including inoperability and degradation of device reliability
and/or performance. Functional operation of the device and/or non-degradation at the Absolute Maximum Ratings or other conditions beyond those indicated in
the Operating Ratings is not implied. Operating Ratings indicate conditions at which the device is functional and the device should not be operated beyond such
conditions.
Note 2: Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Machine Model, applicable std. JESD22-A115-A (ESD MM std. of JEDEC) Field-
Induced Charge-Device Model, applicable std. JESD22-C101-C (ESD FICDM std. of JEDEC).
Note 3: All non-power pins of this device are protected against ESD by snapback devices. Voltage at such pins will rise beyond absmax if current is forced into
pin.
Note 4: The maximum power dissipation is a function of TJ(MAX), θJA, and the ambient temperature, TA. The maximum allowable power dissipation at any ambient
temperature is PDMAX = (TJ(MAX) - TA)/ θJA All numbers apply for packages soldered directly onto a PC board.
Note 5: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating
of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self-heating where TJ >
TA. Absolute Maximum Ratings indicate junction temperature limits beyond which the device may be permanently degraded, either mechanically or electrically.
Note 6: Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will
also depend on the application and configuration. The typical values are not tested and are not guaranteed on shipped production material.
Note 7: Limits are 100% production tested at 25°C. Limits over the operating temperature range are guaranteed through correlations using statistical quality
control (SQC) method.
Note 8: Offset voltage temperature drift is determined by dividing the change in VOS at the temperature extremes by the total temperature change.
Starting from the measured voltage offset at temperature T1 (VOS_RW(T1)), the voltage offset at temperature T2 (VOS_RW(T2)) is calculated according the following
formula: VOS_RW(T2)=VOS_RW(T1)+ABS(T2–T1)* TcVOS_RW.
Note 9: At such currents no accuracy of the output voltage can be expected.
Note 10: This parameter includes both A1 and TIA's noise contribution.
Note 11: In case of external reference connected, the noise of the reference has to be added.
Note 12: For negative bias polarity the Internal Zero is set at 67% VREF.
Note 13: LMP91000 provides an internal 300ns minimum hold time to bridge the undefined region of the falling edge of SCL.
Note 14: This parameter is guaranteed by design or characterization.
Timing Diagram
30132541
FIGURE 1. I2C Interface Timing Diagram
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LMP91000
Typical Performance Characteristics Unless otherwise specified, TA = 25°C, VS=(VDD – AGND),
2.7V <VS< 5.25V and AGND = DGND =0V, VREF= 2.5V.
Input VOS_RW vs. temperature (Vbias 0mV)
-50 -25 0 25 50 75 100
-300
-280
-260
-240
-220
-200
-180
-160
-140
-120
-100
VOS (μV)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132563
Input VOS_RW vs. VDD (Vbias 0mV)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
-300
-280
-260
-240
-220
-200
-180
-160
-140
-120
-100
VOS (μV)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132562
IWE Step current response (rise)
NORMALIZED OUTPUT (200mV/DIV)
TIME (200μs/DIV)
IWE (50μA/DIV)
IWE
2.75kΩ
3.5kΩ
7kΩ
14kΩ
35kΩ
120kΩ
350kΩ
30132564
IWE Step current response (fall)
NORMALIZED OUTPUT TIA (200mV/DIV)
TIME (200μs/DIV)
IWE (50μA/DIV)
IWE
2.75kΩ
3.5kΩ
7kΩ
14kΩ
35kΩ
120kΩ
350kΩ
30132566
AC PSRR vs. Frequency
10 100 1k 10k 100k
80
90
100
110
120
130
140
PSRR (dB)
FREQUENCY (Hz)
30132560
Temperature sensor output vs. VDD (Temperature = 30°C)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
1310
1312
1314
1316
1318
1320
VOUT (mV)
SUPPLY VOLTAGE (V)
30132569
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LMP91000
Supply current vs. temperature (Deep Sleep Mode)
-50 -25 0 25 50 75 100
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132591
Supply current vs. VDD (Deep Sleep Mode)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132597
Supply current vs. temperature (Standby Mode)
-50 -25 0 25 50 75 100
5.50
5.75
6.00
6.25
6.50
6.75
7.00
7.25
7.50
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132587
Supply current vs. VDD (Standby Mode)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
5.50
5.75
6.00
6.25
6.50
6.75
7.00
7.25
7.50
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132592
Supply current vs. temperature (3-lead amperometric Mode)
-50 -25 0 25 50 75 100
9.0
9.2
9.4
9.6
9.8
10.0
10.2
10.4
10.6
10.8
11.0
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132586
Supply current vs. VDD (3-lead amperometric Mode)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
9.0
9.2
9.4
9.6
9.8
10.0
10.2
10.4
10.6
10.8
11.0
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132593
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LMP91000
Supply current vs. temperature (Temp Measurement TIA ON)
-50 -25 0 25 50 75 100
13.0
13.5
14.0
14.5
15.0
15.5
16.0
16.5
17.0
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132588
Supply current vs. VDD (Temp Measurement TIA ON)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
14.0
14.2
14.4
14.6
14.8
15.0
15.2
15.4
15.6
15.8
16.0
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132594
Supply current vs. temperature (Temp Measurement TIA
OFF)
-50 -25 0 25 50 75 100
9.0
9.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132589
Supply current vs. VDD (Temp Measurement TIA OFF)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
10.0
10.5
11.0
11.5
12.0
12.5
13.0
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132595
Supply current vs. temperature (2-lead ground referred
amperometric Mode)
-50 -25 0 25 50 75 100
5.00
5.25
5.50
5.75
6.00
6.25
6.50
6.75
7.00
7.25
7.50
SUPPLY CURRENT (μA)
TEMPERATURE (°C)
VDD = 2.7V
VDD = 3.3V
VDD = 5V
30132590
Supply current vs. VDD (2-lead ground referred
amperometric Mode)
2.5 3.0 3.5 4.0 4.5 5.0 5.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
SUPPLY CURRENT (μA)
SUPPLY VOLTAGE (V)
85°C
25°C
-40°C
30132596
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LMP91000
0.1Hz to 10Hz noise, 0V bias
012345678910
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
EN_RW (μV)
TIME (s)
30132598
0.1Hz to 10Hz noise, 300mV bias
012345678910
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
EN_RW (μV)
TIME (s)
30132599
0.1Hz to 10Hz noise, 600mV bias
012345678910
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
EN_RW (μV)
TIME (s)
301325100
A VOUT step response 100 ppm to 400 ppm CO
(CO gas sensor connected to LMP91000)
0 25 50 75 100 125 150
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
VOUT (V)
TIME (s)
RTIA=35kΩ,
Rload=10Ω,
VREF=5V
LMP91000
30132568
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LMP91000
Function Description
GENERAL
The LMP91000 is a programmable AFE for use in micropower
chemical sensing applications. The LMP91000 is designed
for 3-lead single gas sensors and for 2-lead galvanic cell sen-
sors. This device provides all of the functionality for detecting
changes in gas concentration based on a delta current at the
working electrode. The LMP91000 generates an output volt-
age proportional to the cell current. Transimpedance gain is
user programmable through an I2C compatible interface from
2.75kΩ to 350kΩ making it easy to convert current ranges
from 5µA to 750µA full scale. Optimized for micro-power ap-
plications, the LMP91000 AFE works over a voltage range of
2.7V to 5.25 V. The cell voltage is user selectable using the
on board programmability. In addition, it is possible to connect
an external transimpedance gain resistor. A temperature sen-
sor is embedded and it can be power cycled through the
interface. The output of this temperature sensor can be read
by the user through the VOUT pin. It is also possible to have
both temperature output and output of the TIA at the same
time; the pin C2 is internally connected to the output of the
transimpedance (TIA), while the temperature is available at
the VOUT pin. Depending on the configuration, total current
consumption for the device can be less than 10µA. For power
savings, the transimpedance amplifier can be turned off and
instead a load impedance equivalent to the TIA’s inputs
impedance is switched in.
30132583
FIGURE 2. System Block Diagram
POTENTIOSTAT CIRCUITRY
The core of the LMP91000 is a potentiostat circuit. It consists
of a differential input amplifier used to compare the potential
between the working and reference electrodes to a required
working bias potential (set by the Variable Bias circuitry).
The error signal is amplified and applied to the counter elec-
trode (through the Control Amplifier - A1). Any changes in
the impedance between the working and reference elec-
trodes will cause a change in the voltage applied to the
counter electrode, in order to maintain the constant voltage
between working and reference electrodes. A Tran-
simpedance Amplifier connected to the working electrode,
is used to provide an output voltage that is proportional to the
cell current. The working electrode is held at virtual ground
(Internal ground) by the transimpedance amplifier. The po-
tentiostat will compare the reference voltage to the desired
bias potential and adjust the voltage at the counter electrode
to maintain the proper working-to-reference voltage.
Transimpedance amplifier
The transimpedance amplifier (TIA in Figure 2) has 7 pro-
grammable internal gain resistors. This accommodates the
full scale ranges of most existing sensors. Moreover an ex-
ternal gain resistor can be connected to the LMP91000 be-
tween C1 and C2 pins. The gain is set through the I2C
interface.
Control amplifier
The control amplifier (A1 op amp in Figure 2) has two tasks:
a) providing initial charge to the sensor, b) providing a bias
voltage to the sensor. A1 has the capability to drive up to 10-
mA into the sensor in order to to provide a fast initial condi-
tioning. A1 is able to sink and source current according to the
connected gas sensor (reducing or oxidizing gas sensor). It
can be powered down to reduce system power consumption.
However powering down A1 is not recommended, as it may
take a long time for the sensor to recover from this situation.
Variable Bias
The Variable Bias block circuitry (Figure 2) provides the
amount of bias voltage required by a biased gas sensor be-
tween its reference and working electrodes. The bias voltage
can be programmed to be 1% to 24% (14 steps in total) of the
supply, or of the external reference voltage. The 14 steps can
be programmed through the I2C interface. The polarity of the
bias can be also programmed.
Internal zero
The internal Zero is the voltage at the non-inverting pin of the
TIA. The internal zero can be programmed to be either 67%,
50% or 20%, of the supply, or the external reference voltage.
This provides both sufficient headroom for the counter elec-
trode of the sensor to swing, in case of sudden changes in the
gas concentration, and best use of the ADC’s full scale input
range.
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LMP91000
The Internal zero is provided through an internal voltage di-
vider (Vref divider box in Figure 2). The divider is programmed
through the I2C interface.
Temperature sensor
The embedded temperature sensor can be switched off dur-
ing gas concentration measurement to save power. The tem-
perature measurement is triggered through the I2C interface.
The temperature output is available at the VOUT pin until the
configuration bit is reset. The output signal of the temperature
sensor is a voltage, referred to the ground of the LMP91000
(AGND).
Temperature Sensor Transfer Table
Temperature
(°C)
Output
Voltage
(mV)
Temperature
(°C)
Output
Voltage
(mV)
-40 1875 23 1375
-39 1867 24 1367
-38 1860 25 1359
-37 1852 26 1351
-36 1844 27 1342
-35 1836 28 1334
-34 1828 29 1326
-33 1821 30 1318
-32 1813 31 1310
-31 1805 32 1302
-30 1797 33 1293
-29 1789 34 1285
-28 1782 35 1277
-27 1774 36 1269
-26 1766 37 1261
-25 1758 38 1253
-24 1750 39 1244
-23 1742 40 1236
-22 1734 41 1228
-21 1727 42 1220
-20 1719 43 1212
-19 1711 44 1203
-18 1703 45 1195
-17 1695 46 1187
-16 1687 47 1179
-15 1679 48 1170
-14 1671 49 1162
-13 1663 50 1154
-12 1656 51 1146
-11 1648 52 1137
-10 1640 53 1129
-9 1632 54 1121
-8 1624 55 1112
-7 1616 56 1104
-6 1608 57 1096
-5 1600 58 1087
-4 1592 59 1079
-3 1584 60 1071
-2 1576 61 1063
-1 1568 62 1054
0 1560 63 1046
1 1552 64 1038
2 1544 65 1029
3 1536 66 1021
4 1528 67 1012
5 1520 68 1004
6 1512 69 996
7 1504 70 987
8 1496 71 979
9 1488 72 971
10 1480 73 962
11 1472 74 954
12 1464 75 945
13 1456 76 937
14 1448 77 929
15 1440 78 920
16 1432 79 912
17 1424 80 903
18 1415 81 895
19 1407 82 886
20 1399 83 878
21 1391 84 870
22 1383 85 861
Although the temperature sensor is very linear, its response
does have a slight downward parabolic shape. This shape is
very accurately reflected in the temperature sensor Transfer
Table. For a linear approximation, a line can easily be calcu-
lated over the desired temperature range from the Table using
the two-point equation:
V-V1=((V2–V1)/(T2–T1))*(T-T1)
Where V is in mV, T is in °C, T1 and V1 are the coordinates of
the lowest temperature, T2 and V2 are the coordinates of the
highest temperature.
For example, if we want to determine the equation of a line
over a temperature range of 20°C to 50°C, we would proceed
as follows:
V-1399mV=((1154mV - 1399mV)/(50°C -20°C))*(T-20°C)
V-1399mV= -8.16mV/°C*(T-20°C)
V=(-8.16mV/°C)*T+1562.2mV
Using this method of linear approximation, the transfer func-
tion can be approximated for one or more temperature ranges
of interest.
I2C INTERFACE
The I2C compatible interface operates in Standard mode
(100kHz). Pull-up resistors or current sources are required on
the SCL and SDA pins to pull them high when they are not
being driven low. A logic zero is transmitted by driving the
output low. A logic high is transmitted by releasing the output
and allowing it to be pulled-up externally. The appropriate
pull-up resistor values will depend upon the total bus capac-
itance and operating speed. The LMP91000 comes with a 7
bit bus fixed address: 1001 000.
www.ti.com 12
LMP91000
WRITE AND READ OPERATION
In order to start any read or write operation with the
LMP91000, MENB needs to be set low during the whole com-
munication. Then the master generates a start condition by
driving SDA from high to low while SCL is high. The start con-
dition is always followed by a 7-bit slave address and a Read/
Write bit. After these 8 bits have been transmitted by the mas-
ter, SDA is released by the master and the LMP91000 either
ACKs or NACKs the address. If the slave address matches,
the LMP91000 ACKs the master. If the address doesn't
match, the LMP91000 NACKs the master. For a write opera-
tion, the master follows the ACK by sending the 8-bit register
address pointer. Then the LMP91000 ACKs the transfer by
driving SDA low. Next, the master sends the 8-bit data to the
LMP91000. Then the LMP91000 ACKs the transfer by driving
SDA low. At this point the master should generate a stop con-
dition and optionally set the MENB at logic high level (refer
to Figure 3).
A read operation requires the LMP91000 address pointer to
be set first, also in this case the master needs setting at low
logic level the MENB, then the master needs to write to the
device and set the address pointer before reading from the
desired register. This type of read requires a start, the slave
address, a write bit, the address pointer, a Repeated Start (if
appropriate), the slave address, and a read bit (refer to
Figure 3). Following this sequence, the LMP91000 sends out
the 8-bit data of the register.
When just one LMP91000 is present on the I2C bus the MENB
can be tied to ground (low logic level).
30132572
(a) Register write transaction
30132571
(b) Pointer set transaction
30132570
(c) Register read transaction
FIGURE 3. READ and WRITE transaction
13 www.ti.com
LMP91000
TIMEOUT FEATURE
The timeout is a safety feature to avoid bus lockup situation.
If SCL is stuck low for a time exceeding t_timeout, the
LMP91000 will automatically reset its I2C interface. Also, in
the case the LMP91000 hangs the SDA for a time exceeding
t_timeout, the LMP91000’s I2C interface will be reset so that
the SDA line will be released. Since the SDA is an open-drain
with an external resistor pull-up, this also avoids high power
consumption when LMP91000 is driving the bus and the SCL
is stopped.
REGISTERS
The registers are used to configure the LMP91000.
If writing to a reserved bit, user must write only 0. Readback value is unspecified and should be discarded.
Register map
Address Name Power on default Access Lockable?
0x00 STATUS 0x00 Read only N
0x01 LOCK 0x01 R/W N
0x02 through 0x09 RESERVED
0x10 TIACN 0x03 R/W Y
0x11 REFCN 0x20 R/W Y
0x12 MODECN 0x00 R/W N
0x13 through 0xFF RESERVED
STATUS -- Status Register (address 0x00)
The status bit is an indication of the LMP91000's power-on status. If its readback is “0”, the LMP91000 is not ready to accept other
I2C commands.
Bit Name Function
[7:1] RESERVED
0 STATUS
Status of Device
0 Not Ready (default)
1 Ready
LOCK -- Protection Register (address 0x01)
The lock bit enables and disables the writing of the TIACN and the REFCN registers. In order to change the content of the TIACN
and the REFCN registers the lock bit needs to be set to “0”.
Bit Name Function
[7:1] RESERVED
0 LOCK
Write protection
0 Registers 0x10, 0x11 in write mode
1 Registers 0x10, 0x11 in read only mode (default)
www.ti.com 14
LMP91000
TIACN -- TIA Control Register (address 0x10)
The parameters in the TIA control register allow the configuration of the transimpedance gain (RTIA) and the load resistance
(RLoad).
Bit Name Function
[7:5] RESERVED RESERVED
[4:2] TIA_GAIN
TIA feedback resistance selection
000 External resistance (default)
001 2.75kΩ
010 3.5kΩ
011 7kΩ
100 14kΩ
101 35kΩ
110 120kΩ
111 350kΩ
[1:0] RLOAD
RLoad selection
00 10Ω
01 33Ω
10 50Ω
11 100Ω (default)
REFCN -- Reference Control Register (address 0x11)
The parameters in the Reference control register allow the configuration of the Internal zero, Bias and Reference source. When
the Reference source is external, the reference is provided by a reference voltage connected to the VREF pin. In this condition the
Internal Zero and the Bias voltage are defined as a percentage of VREF voltage instead of the supply voltage.
Bit Name Function
7 REF_SOURCE
Reference voltage source selection
0 Internal (default)
1 external
[6:5] INT_Z
Internal zero selection (Percentage of the source reference)
00 20%
01 50% (default)
10 67%
11 Internal zero circuitry bypassed (only in O2 ground referred measurement)
4 BIAS_SIGN
Selection of the Bias polarity
0 Negative (VWE – VRE)<0V (default)
1 Positive (VWE –VRE)>0V
[3:0] BIAS
BIAS selection (Percentage of the source reference)
0000 0% (default)
0001 1%
0010 2%
0011 4%
0100 6%
0101 8%
0110 10%
0111 12%
1000 14%
1001 16%
1010 18%
1011 20%
1100 22%
1101 24%
15 www.ti.com
LMP91000
MODECN -- Mode Control Register (address 0x12)
The Parameters in the Mode register allow the configuration of the Operation Mode of the LMP91000.
Bit Name Function
7 FET_SHORT
Shorting FET feature
0 Disabled (default)
1 Enabled
[6:3] RESERVED
[2:0] OP_MODE
Mode of Operation selection
000 Deep Sleep (default)
001 2-lead ground referred galvanic cell
010 Standby
011 3-lead amperometric cell
110 Temperature measurement (TIA OFF)
111 Temperature measurement (TIA ON)
When the LMP91000 is in Temperature measurement (TIA ON) mode, the output of the temperature sensor is present at the VOUT
pin, while the output of the potentiostat circuit is available at pin C2.
GAS SENSOR INTERFACE
The LMP91000 supports both 3-lead and 2-lead gas sensors.
Most of the toxic gas sensors are amperometric cells with 3
leads (Counter, Worker and Reference). These leads should
be connected to the LMP91000 in the potentiostat topology.
The 2-lead gas sensor (known as galvanic cell) should be
connected as simple buffer either referred to the ground of the
system or referred to a reference voltage. The LMP91000
support both connections for 2-lead gas sensor.
3-lead Amperometric Cell In Potentiostat Configuration
Most of the amperometric cell have 3 leads (Counter, Refer-
ence and Working electrodes). The interface of the 3-lead gas
sensor to the LMP91000 is straightforward, the leads of the
gas sensor need to be connected to the namesake pins of the
LMP91000.
The LMP91000 is then configured in 3-lead amperometric cell
mode; in this configuration the Control Amplifier (A1) is ON
and provides the internal zero voltage and bias in case of bi-
ased gas sensor. The transimpedance amplifier (TIA) is ON,
it converts the current generated by the gas sensor in a volt-
age, according to the transimpedance gain:
Gain=RTIA
If different gains are required, an external resistor can be
connected between the pins C1 and C2. In this case the in-
ternal feedback resistor should be programmed to “external”.
The RLoad together with the output capacitance of the gas
sensor acts as a low pass filter.
www.ti.com 16
LMP91000
30132583
FIGURE 4. 3-Lead Amperometric Cell
2-lead Galvanic Cell In Ground Referred Configuration
When the LMP91000 is interfaced to a galvanic cell (for in-
stance to an Oxygen gas sensor) referred to the ground of the
system, an external resistor needs to be placed in parallel to
the gas sensor; the negative electrode of the gas sensor is
connected to the ground of the system and the positive elec-
trode to the Vref pin of the LMP91000, the working pin of the
LMP91000 is connected to the ground.
The LMP91000 is then configured in 2-lead galvanic cell
mode and the Vref bypass feature needs to be enabled. In
this configuration the Control Amplifier (A1) is turned off, and
the output of the gas sensor is amplified by the Tran-
simpedance Amplifier (TIA) which is configured as a simple
non-inverting amplifier.
The gain of this non inverting amplifier is set according the
following formula
Gain= 1+(RTIA/RLoad)
If different gains are required, an external resistor can be
connected between the pins C1 and C2. In this case the in-
ternal feedback resistor should be programmed to “external”.
17 www.ti.com
LMP91000
30132575
FIGURE 5. 2-Lead Galvanic Cell Ground Referred
2-lead Galvanic Cell In Potentiostat Configuration
When the LMP91000 is interfaced to a galvanic cell (for in-
stance to an Oxygen gas sensor) referred to a reference, the
Counter and the Reference pin of the LMP91000 are shorted
together and connected to negative electrode of the galvanic
cell. The positive electrode of the galvanic cell is then con-
nected to the Working pin of the LMP91000.
The LMP91000 is then configured in 3-lead amperometric cell
mode (as for amperometric cell). In this configuration the
Control Amplifier (A1) is ON and provides the internal zero
voltage. The transimpedance amplifier (TIA) is also ON, it
converts the current generated by the gas sensor in a voltage,
according to the transimpedance gain:
Gain= RTIA
If different gains are required, an external resistor can be
connected between the pins C1 and C2. In this case the in-
ternal feedback resistor should be programmed to “external”.
www.ti.com 18
LMP91000
30132584
FIGURE 6. 2-Lead Galvanic Cell In Potentiostat Configuration
Application Information
CONNECTION OF MORE THAN ONE LMP91000 TO THE
I2C BUS
The LMP91000 comes out with a unique and fixed I2C slave
address. It is still possible to connect more than one
LMP91000 to an I2C bus and select each device using the
MENB pin. The MENB simply enables/disables the I2C com-
munication of the LMP91000. When the MENB is at logic level
low all the I2C communication is enabled, it is disabled when
MENB is at high logic level.
In a system based on a μcontroller and more than one
LMP91000 connected to the I2C bus, the I2C lines (SDA and
SCL) are shared, while the MENB of each LMP91000 is con-
nected to a dedicate GPIO port of the μcontroller.
The μcontroller starts communication asserting one out of N
MENB signals where N is the total number of LMP91000s
connected to the I2C bus. Only the enabled device will ac-
knowledge the I2C commands. After finishing communicating
with this particular LMP91000, the microcontroller de-asserts
the corresponding MENB and repeats the procedure for other
LMP91000s. Figure 7 shows the typical connection when
more than one LMP91000 is connected to the I2C bus.
30132581
FIGURE 7. More than one LMP91000 on I2C bus
19 www.ti.com
LMP91000
SMART GAS SENSOR ANALOG FRONT END
The LMP91000 together with an external EEPROM repre-
sents the core of a SMART GAS SENSOR AFE. In the
EEPROM it is possible to store the information related to the
GAS sensor type, calibration and LMP91000's configuration
(content of registers 10h, 11h, 12h). At startup the microcon-
troller reads the EEPROM's content and configures the
LMP91000. A typical smart gas sensor AFE is shown in Fig-
ure 8. The connection of MENB to the hardware address pin
A0 of the EEPROM allows the microcontroller to select the
LMP91000 and its corresponding EEPROM when more than
one smart gas sensor AFE is present on the I2C bus. Note:
only EEPROM I2C addresses with A0=0 should be used in
this configuration.
30132580
FIGURE 8. SMART GAS SENSOR AFE
SMART GAS SENSOR AFES ON I2C BUS
The connection of Smart gas sensor AFEs on the I2C bus is
the natural extension of the previous concepts. Also in this
case the microcontroller starts communication asserting 1 out
of N MENB signals where N is the total number of smart gas
sensor AFE connected to the I2C bus. Only one of the devices
(either LMP91000 or its corresponding EEPROM) in the
smart gas sensor AFE enabled will acknowledge the I2C com-
mands. When the communication with this particular module
ends, the microcontroller de-asserts the corresponding
MENB and repeats the procedure for other modules.
Figure 9 shows the typical connection when several smart gas
sensor AFEs are connected to the I2C bus.
30132582
FIGURE 9. SMART GAS SENSOR AFEs on I2C bus
www.ti.com 20
LMP91000
POWER CONSUMPTION
The LMP91000 is intended for use in portable devices, so the
power consumption is as low as possible in order to guarantee
a long battery life. The total power consumption for the
LMP91000 is below 10µA @ 3.3v average over time, (this
excludes any current drawn from any pin). A typical usage of
the LMP91000 is in a portable gas detector and its power
consumption is summarized in the Power Consumption Sce-
nario table. This has the following assumptions:
-Power On only happens a few times over life, so its power
consumption can be ignored
-Deep Sleep mode is not used
-The system is used about 8 hours a day, and 16 hours a day
it is in Standby mode.
-Temperature Measurement is done about once per minute
This results in an average power consumption of approxi-
mately 7.95 µA. This can potentially be further reduced, by
using the Standby mode between gas measurements. It may
even be possible, depending on the sensor used, to go into
deep sleep for some time between measurements, further
reducing the average power consumption.
Power Consumption Scenario
Deep Sleep StandBy
3-Lead
Amperometric
Cell
Temperature
Measurement
TIA OFF
Temperature
Measurement
TIA ON
Total
Current consumption
(µA)
typical value 0.6 6.5 10 11.4 14.9
Time ON
(%) 0 60 39 0 1
Average
(µA) 0 3.9 3.9 0 0.15 7.95
Notes
A1 OFF ON ON ON ON
TIA OFF OFF ON OFF ON
TEMP SENSOR OFF OFF OFF ON ON
I2C interface ON ON ON ON ON
SENSOR TEST PROCEDURE
The LMP91000 has all the hardware and programmability
features to implement some test procedures. The purpose of
the test procedure is to:
a) test proper function of the sensor (status of health)
b) test proper connection of the sensor to the LMP91000
The test procedure is very easy. The variable bias block is
user programmable through the digital interface. A step volt-
age can be applied by the end user to the positive input of A1.
As a consequence a transient current will start flowing into the
sensor (to charge its internal capacitance) and it will be de-
tected by the TIA. If the current transient is not detected, either
a sensor fault or a connection problem is present. The slope
and the aspect of the transient response can also be used to
detect sensor aging (for example, a cell that is drying and no
longer efficiently conducts the current). After it is verified that
the sensor is working properly, the LMP91000 needs to be
reset to its original configuration. It is not required to observe
the full transient in order to contain the testing time. All the
needed information are included in the transient slopes (both
edges). Figure 10 shows an example of the test procedure, a
Carbon Monoxide sensor is connected to the LMP91000, two
pulses are then sequentially applied to the bias voltage:
first step: from 0mV to 40mV
second step : from 40mV to -40mV
and finally the bias is set again at 0mV since this is the normal
operation condition for this sensor.
OUTPUTT VOLTTAGE (1V/DIV)
TIME (25ms/DIV)
INPUT PULSE (100mV/DIV)
LMP91000 OUTPUT
TEST PULSE
30132561
FIGURE 10. TEST PROCEDURE EXAMPLE
21 www.ti.com
LMP91000
Physical Dimensions inches (millimeters) unless otherwise noted
NS Package Number SDA14B
www.ti.com 22
LMP91000
Notes
23 www.ti.com
LMP91000
Notes
LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing
Applications
www.ti.com
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