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LMP91200
SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016
LMP91200 Configurable AFE for Low-Power Chemical-Sensing Applications
1 Features
2 Applications
•
•
•
1
Active Guarding
Key Specifications
Unless otherwise noted, typical values at
TA = 25°C, VS = (VDD-GND) = 3.3 V
– pH Buffer Input Bias Current (0 < VINP < 3.3 V)
– Maximum at 25°C: ±125 fA
– Maximum at 85°C: ±445 fA
– pH Buffer Input Bias Current
(–500 mV < VINP– VCM < 500 mV),
VS = (VDD – GND) = 0 V
– Maximum at 25°C: ±600 fA
– Maximum at 85°C: ±6.5 pA
– pH Buffer Input Offset Voltage: ±200 µV
– pH Buffer Input Offset Voltage Drift: ±2.5
μV/°C
– Supply Current: 50 μA
– Supply Voltage: 1.8 V to 5.5 V
– Operating Temperature Range: –40°C to
125°C
– Package: 16-Pin TSSOP
pH Sensor Platforms
3 Description
The LMP91200 device is a sensor AFE for use in
low-power, analytical-sensing applications. The
LMP91200 is designed for 2-electrode sensors. This
device provides all of the functionality needed to
detect changes based on a delta voltage at the
sensor. Optimized for low-power applications, the
LMP91200 works over a voltage range of 1.8 V to 5.5
V. With its extremely low input bias current it is
optimized for use with pH sensors. Also, in absence
of supply voltage the very low input bias current
reduces degradation of the pH probe when
connected to the LMP91200. Two guard pins provide
support for high parasitic impedance wiring.
Depending on the configuration, total current
consumption for the device is 50 µA while measuring
pH. Available in a 16-pin TSSOP package, the
LMP91200 operates from –40°C to +125°C.
Device Information(1)
PART NUMBER
LMP91200
PACKAGE
TSSOP (16)
BODY SIZE (NOM)
5.00 mm × 4.40 mm
(1) For all available packages, see the orderable addendum at
the end of the data sheet.
Typical Application
VDD
NC
LMP91200
VDD
MCU
GND
GND
NC
VOUT
ADC
GUARD1
INP
±
pH
BUFFER
+
VOCM
GUARD2
VREF
VCM
GND
VCM
BUFFER
GND
pH Electrode
VCMHI
1
An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications,
intellectual property matters and other important disclaimers. PRODUCTION DATA.
LMP91200
SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016
www.ti.com
Table of Contents
1
2
3
4
5
6
7
Features ..................................................................
Applications ...........................................................
Description .............................................................
Revision History.....................................................
Pin Configuration and Functions .........................
Specifications.........................................................
1
1
1
2
3
4
6.1
6.2
6.3
6.4
6.5
6.6
4
4
4
4
5
8
Absolute Maximum Ratings ......................................
ESD Ratings..............................................................
Recommended Operating Conditions.......................
Thermal Information ..................................................
Electrical Characteristics...........................................
Typical Characteristics ..............................................
Detailed Description ............................................ 13
7.1 Overview ................................................................. 13
7.2 Functional Block Diagram ....................................... 14
7.3 Feature Description................................................. 14
8
Application and Implementation ........................ 15
8.1 Application Information............................................ 15
8.2 Typical Application ................................................. 15
9 Power Supply Recommendations...................... 17
10 Layout................................................................... 17
10.1 Layout Guidelines ................................................. 17
10.2 Layout Example .................................................... 17
11 Device and Documentation Support ................. 18
11.1
11.2
11.3
11.4
Community Resources..........................................
Trademarks ...........................................................
Electrostatic Discharge Caution ............................
Glossary ................................................................
18
18
18
18
12 Mechanical, Packaging, and Orderable
Information ........................................................... 18
4 Revision History
NOTE: Page numbers for previous revisions may differ from page numbers in the current version.
Changes from Revision D (November 2015) to Revision E
•
Page
Deleted SPI Function ............................................................................................................................................................. 1
Changes from Revision C (March 2013) to Revision D
Page
•
Added Pin Configuration and Functions section, ESD Ratings table, Feature Description section, Device Functional
Modes, Application and Implementation section, Power Supply Recommendations section, Layout section, Device
and Documentation Support section, and Mechanical, Packaging, and Orderable Information section .............................. 1
•
Deleted temperature sensor function. ................................................................................................................................... 1
Changes from Revision B (March 2013) to Revision C
•
2
Page
Changed layout of National Data Sheet to TI format ........................................................................................................... 17
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SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016
5 Pin Configuration and Functions
PW Package
16-Pin TSSOP
Top View
VDD
1
16
GND
NC
2
15
GND
NC
3
14
VDD
GUARD1
4
13
GND
INP
5
12
VOUT
GUARD2
6
11
VOCM
VCMHI
7
10
GND
VCM
8
9
VREF
LMP91200
Pin Functions (1)
PIN
NO.
NAME
TYPE
DESCRIPTION
1
VDD
P
Positive Power Supply
2
NC
A
No connect. These pins should be left floating
3
NC
A
No connect. These pins should be left floating
4
GUARD1
A
Active guard pin
5
INP
A
Noninverting analog input of pH buffer
6
GUARD2
A
Active guard pin
7
VCMHI
A
High Impedance Common-Mode output
8
VCM
A
Buffered Common-Mode output
9
VREF
A
Voltage reference input
10
GND
G
Analog ground
11
VOCM
A
Output common-mode voltage
12
VOUT
A
Analog Output
13
GND
G
Connect to GND
14
VDD
P
Connect to VDD
15
GND
G
Connect to GND
16
GND
G
Connect to GND
(1)
D = Digital, A = Analog, P = Power, G = GND
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SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016
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6 Specifications
6.1 Absolute Maximum Ratings
over operating free-air temperature range (unless otherwise noted) (1) (2) (3)
MIN
MAX
UNIT
Supply Voltage (VS = VDD – GND)
–0.3
6
V
Voltage between any two pins
–0.3
VDD + 0.3
V
5
mA
150
°C
150
°C
Current out at any pin
Junction Temperature (4)
Storage Temperature, Tstg
(1)
(2)
(3)
(4)
–65
Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings
only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended
Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
For soldering specifications see product folder at www.ti.com and SNOA549.
If Military/Aerospace specified devices are required, please contact the Texas Instruments Sales Office/ Distributors for availability and
specifications.
The maximum power dissipation is a function of TJ(MAX), RθJA. The maximum allowable power dissipation at any ambient temperature is
PD = (TJ(MAX) – TA)/RθJA. All numbers apply for packages soldered directly onto a PCB.
6.2 ESD Ratings
VALUE
V(ESD)
(1)
(2)
(3)
Electrostatic
discharge (1)
Human body model (HBM), per ANSI/ESDA/JEDEC JS-001 (2)
±2000
Charged-device model (CDM), per JEDEC specification JESD22-C101 (3)
±1000
Machine Model
±150
UNIT
V
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).
JEDEC document JEP155 states that 500-V HBM allows safe manufacturing with a standard ESD control process.
JEDEC document JEP157 states that 250-V CDM allows safe manufacturing with a standard ESD control process.
6.3 Recommended Operating Conditions
over operating free-air temperature range (unless otherwise noted)
MIN
MAX
Supply Voltage (VS = VDD – GND)
1.8
5.5
UNIT
V
Temperature
–40
125
°C
6.4 Thermal Information
LMP91200
THERMAL METRIC (1)
PW (TSSOP)
UNIT
16 PINS
RθJA
(1)
4
Junction-to-ambient thermal resistance
31
°C/W
For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application
report, SPRA953.
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6.5 Electrical Characteristics
Unless otherwise specified, all limits specified for TA = 25°C. VS = (VDD – GND) = 3.3 V. VREF = 3.3 V. (1) (2) (3)
PARAMETER
MIN (4)
TEST CONDITIONS
TYP (5)
MAX (4)
50
54
UNIT
POWER SUPPLY
Supply Current (6) (7)
pH measurement mode
AolpH
Open-loop Gain
INP = 1.65 V, 300 mV =
VOUT = VDD – 300 mV
VospH
Input Voltage Offset (6)
INP = 1/2 VREF
TcVospH
Input offset voltage drift (8) (9)
INP = 1/2 VREF
VOSpH_drift
Long-term VOSpH drift (10)
500 hours OPL
Is
at the temperature
extremes
µA
59
pH BUFFER
IbpH
Input bias current at INP (9)
(9)
120
at the temperature
extremes
at the temperature
extremes
dB
90
–200
200
–350
350
–2.5
µV
2.5
uV/°C
150
µV
0 V < INP < 3.3 V
–125
125
0 V < INP < 3.3 V, 85°C
–445
445
fA
0 V < INP < 3.3 V, 125°C
–1.5
1.5
pA
–500 mV < (INP – VCM) < 500 mV, VS = 0 V.
–600
600
fA
–500 mV < (INP – VCM) < 500 mV,
85°C, VS = 0 V.
–6.5
6.5
pA
–500 mV < (INP – VCM) < 500 mV,
125°C, VS = 0 V.
–100
100
pA
CL = 10 pF, RL = 1 MΩ
fA
GBWPpH
Gain Bandwidth Product
CMRRpH
DC_Common-mode rejection
INP = 1/2 VREF
ratio
PSRRpH
DC_Power supply rejection
ratio
1.8 V < VDD < 5 V
INP = 1/2 VREF
En_RMSpH
Input referred noise (low
frequency) (9)
Integrated 0.1 Hz to 10 Hz
2.6
µVPP
enpH
Input referred noise (high
frequency) (9)
f = 1 kHz
90
nV/√Hz
Sourcing, Vout to GND,
INP = 1.65 V
IscpH
Output short circuit
current (11)
Sinking, Vout to VDD,
INP = 1.65 V
220
KHz
80
dB
80
dB
13
at the temperature
extremes
mA
10
12
at the temperature
extremes
8
mA
(1)
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 specification of parametric performance is indicated in the electrical tables under
conditions of internal self-heating where TJ >TA.
(2) Positive current corresponds to current flowing into the device.
(3) The voltage on any pin should not exceed 6 V relative to any other pins.
(4) Limits are 100% production tested at 25°C. Limits over the operating temperature range are specified through correlations using the
Statistical Quality Control (SQC) method.
(5) 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 specified on shipped
production material.
(6) Boldface limits are production tested at 125°C. Limits are specified through correlations using the Statistical Quality Control (SQC)
method.
(7) Excluding all currents which flows out from the device.
(8) Offset voltage average drift is determined by dividing the change in VOS at the temperature extremes by the total temperature change.
(9) This parameter is specified by design and/or characterization and is not tested in production.
(10) Offset voltage long term drift is determined by dividing the change in VOS at time extremes of OPL procedure by the length of the OPL
procedure. OPL procedure: 500 hours at 150°C are equivalent to about 15 years.
(11) The short circuit test is a momentary open-loop test.
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Electrical Characteristics (continued)
Unless otherwise specified, all limits specified for TA = 25°C. VS = (VDD – GND) = 3.3 V. VREF = 3.3 V.(1)(2)(3)
PARAMETER
MIN (4)
TEST CONDITIONS
TYP (5)
MAX (4)
UNIT
VCM BUFFER
VCMHI_acc
VCMHI accuracy
Tc_VCMHI
VCMHI temperature
coefficient (9) (12)
VCMHI_acc_VREF VCMHI_acc vs. VREF (9) (13)
RoutVCMHI
–1.6
1.6
mV
–40°C < TA < 125°C
–18
–5
8
µV/°C
1.8 V < VREF < 5 V
–500
–100
300
µV/V
VCMHI Output Impedance (9) VCMHI = 1/2 VREF
250
KΩ
120
AolVCM
Open-loop Gain (6)
VCMHI = 1/2 VREF, 300
mV < VCM < VDD – 300
mV
VosVCM
(VCM – VCMHI) (6)
VCMHI = 1/2 VREF
TcVosVCM
Input offset voltage drift
(VCM-VCMHI) (8) (9)
VCMHI = 1/2 VREF
ZoutVCM
Output Impedance (9)
f = 1 KHz
PSRRVCM
DC_Power supply rejection
ratio
1.8 V < VDD < 5 V,
VCMHI = 1/2 VREF
En_RMSVCM
Input referred noise (low
frequency) (9)
Integrated 0.1 Hz to 10 Hz
2.6
µVPP
enVCM
Input referred noise (high
frequency) (9)
f = 1 KHz
90
nV/√Hz
Sourcing, Vout to GND
VCMHI = 1/2 VREF
IscVCM
Output short circuit
current (11)
Sinking, Vout to VDD
VCMHI = 1/2 VREF
at the temperature
extremes
at the temperature
extremes
dB
90
–200
200
–350
350
–2.5
2.5
µV
µV/°C
Ω
4
80
dB
16
at the temperature
extremes
10
12
at the temperature
extremes
mA
8
(12) VCMHI voltage average drift is determined by dividing the change in VCMHI at the temperature extremes by the total temperature
change.
(13) VCMHI_acc vs. VREF is determined by dividing the change in VCMHI_acc at the VREF extremes by the total VREF change.
6
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Electrical Characteristics (continued)
Unless otherwise specified, all limits specified for TA = 25°C. VS = (VDD – GND) = 3.3 V. VREF = 3.3 V.(1)(2)(3)
PARAMETER
MIN (4)
TEST CONDITIONS
TYP (5)
MAX (4)
UNIT
PGA
VosPGA
Input Voltage Offset (6)
+IN_PGA (Internal node) =
at the temperature
500 mV
extremes
TcVosPGA
Input offset voltage drift (9) (8)
+IN_PGA (Internal node) = 500 mV
+IN_PGA (Internal node) =
500 mV
–275
275
–480
480
–2.5
µV
2.5
uV/°C
120
AolPGA
Open loop Gain
AvPGA
Gain
Av_accPGA
Gain accuracy
at the temperature extremes
En_RMSPGA
Input referred noise (low
frequency) (9)
Integrated 0.1 Hz to 10 Hz
2.6
µVPP
enPGA
Input referred noise (high
frequency) (9)
f = 1 kHz
90
nV/√Hz
PSRRPGA
DC_Power supply rejection
ratio
1.8 V < VDD < 5 V,
+IN_PGA (Internal node) = 500 mV
IscPGA
Output short circuit
current (11)
at the temperature
extremes
dB
90
5
Sourcing, Vout to GND
+IN_PGA (Internal node) =
500 mV
Sinking, Vout to VDD
+IN_PGA (Internal node) =
500 mV
-1.3%
V/V
1.3%
80
dB
16
at the temperature
extremes
10
12
at the temperature
extremes
mA
8
REFERENCE INPUT
RinVREF
Input impedance (9)
500
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6.6 Typical Characteristics
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
100
500
Average
Average -31
Average +31
60
300
40
200
20
0
-20
-40
100
0
-100
-200
-60
-300
-80
-400
TA=25°C
-100
0.0
0.5
1.0
1.5 2.0
INP (V)
2.5
3.0
240
3.5
-0.50
5
Average
Average -31
Average +31
INPUT BIAS (pA)
INPUT BIAS (fA)
0
-60
-120
2
1
0
-1
-2
-180
-3
-240
-4
TA=85°C
-300
0.5
1.0
1.5 2.0
INP (V)
2.5
3.0
3.5
-0.50
80
Average
Average -31
Average +31
0
-200
-400
0.50
40
INPUT BIAS (pA)
INPUT BIAS (fA)
200
0.00
0.25
INP-VCM (V)
Average
Average -31
Average +31
60
400
-0.25
Figure 4. pH Buffer Input Bias Current vs
VINP - Device OFF
600
20
0
-20
-40
-600
-60
-800
TA=125°C
-1000
0.5
1.0
1.5 2.0
INP (V)
2.5
3.0
TA=125°C
-80
3.5
Figure 5. pH Buffer Input Bias Current vs
VINP - Device ON
8
TA=85°C
-5
Figure 3. pH Buffer Input Bias Current vs
VINP - Device ON
0.0
0.50
3
60
800
0.00
0.25
INP-VCM (V)
Average
Average -31
Average +31
4
120
1000
-0.25
Figure 2. pH Buffer Input Bias Current vs
VINP - Device OFF
180
0.0
TA=25°C
-500
Figure 1. pH Buffer Input Bias Current vs
VINP - Device ON
300
Average
Average -31
Average +31
400
INPUT BIAS (fA)
INPUT BIAS (fA)
80
-0.50
-0.25
0.00
0.25
INP-VCM (V)
0.50
Figure 6. pH Buffer Input Bias Current vs
VINP - Device OFF
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Typical Characteristics (continued)
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
500
5
Average
Average -31
Average +31
400
3
INPUT BIAS (pA)
INPUT BIAS (fA)
300
200
100
0
-100
-200
2
1
INP-VCM = 100mV
0
-1
INP-VCM = -100mV
-2
-300
-3
-400
-4
INP=1.65V
-500
-5
25
45
65
85
105
TEMPERATURE (°C)
125
25
Figure 7. pH Buffer Input Bias Current
vs Temp - Device ON
50
75
100
TEMPERATURE (°C)
125
Figure 8. pH Buffer Input Bias Current
vs Temp - Device OFF
35
18
UNITS TESTED > 5000
UNITS TESTED > 5000
30
PERCENTAGE (%)
15
PERCENTAGE (%)
Average
Average -31
Average +31
4
12
9
6
25
20
15
10
3
5
0
-200 -150 -100 -50
0
0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
50 100 150 200
VOSPH ( V)
TCVOSPH ( V/°C)
Figure 9. pH Buffer Input Voltage Offset
Figure 10. pH Buffer TCVOS
100
110
VDD=1.8V
VDD=3.3V
VDD=5V
105
PSRR (dB)
105
95
90
100
95
85
90
80
-50 -25
0
25
50
75 100 125
Figure 11. pH Buffer DC CMRR
vs Temperature
-50
-25
0
25 50 75
TEMPERATURE (°C)
100 125
Figure 12. pH Buffer DC PSRR
vs Temperature
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Typical Characteristics (continued)
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
125
INTEGRATED NOISE (500nV/DIV)
100
75
VOSPH ( V)
50
25
0
-25
-50
-75
-100
-125
1
10
100
OPL TIME (h)
TIME (1s/DIV)
Figure 13. pH Buffer Time Domain Voltage Noise
Figure 14. pH Buffer Input Offset Voltage Drift
100
120
90
110
80
100
CMRR (dB)
CMRR (dB)
VDD=VREF=3.3V
70
80
50
70
40
VDD=VREF=3.3V
90
60
0.00
1k
60
0.08
0.16
0.24
INP (V)
0.32
0.40
2.9
Figure 15. pH Buffer CMRR
vs VINP - Lower Rail
3.0
3.1
INP (V)
3.2
3.3
Figure 16. pH Buffer CMRR
vs VINP - upper rail
120
120
VDD=VREF=5V
VDD=VREF=5V
110
CMRR (dB)
CMRR (dB)
110
100
100
90
80
90
70
80
0.00
60
0.12
0.24
0.36
INP (V)
0.48
0.60
Figure 17. pH Buffer CMRR
vs VINP - lower rail
10
4.4
4.5
4.6
4.7
4.8
INP (V)
4.9
5.0
Figure 18. pH Buffer CMRR
vs VINP - upper rail
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Typical Characteristics (continued)
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
90
90
INP=1.65V
80
85
80
60
PSRR (dB)
CMRR (dB)
70
75
50
40
70
30
20
65
10
60
0
10
100
1k
10k
FREQUENCY (Hz)
100k
10
Figure 19. pH Buffer CMRR
vs Frequency
10k
Figure 20. pH Buffer PSRR
vs Frequency
15
40
UNITS TESTED > 5000
PERCENTAGE (%)
9
6
UNITS TESTED > 5000
35
12
PERCENTAGE (%)
100
1k
FREQUENCY (Hz)
30
25
20
15
10
3
5
0
-200 -150 -100 -50
0
0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
50 100 150 200
VOSVCM ( V)
TCVOSVCM ( V/°C)
Figure 21. VCM Buffer Input Voltage Offset
Figure 22. VCM Buffer TCVOS
110
105
VDD=1.8V
VDD=3.3V
VDD=5V
105
PSRR (dB)
CMRR (dB)
100
95
100
90
95
85
90
-50
-25
0
25 50 75
TEMPERATURE (°C)
100 125
Figure 23. VCM Buffer DC CMRR
vs Temperature
-50
-25
0
25 50 75
TEMPERATURE (°C)
100 125
Figure 24. VCM Buffer DC PSRR
vs Temperature
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Typical Characteristics (continued)
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
90
VCMHI=1.65V
INTEGRATED NOISE (500nV/DIV)
80
PSRR (dB)
70
60
50
40
30
20
10
10
TIME (1s/DIV)
Figure 25. VCM Buffer Time Domain Voltage Noise
0.30
VCMHI=1/2VREF
0.25
0.25
0.20
0.20
ERROR (%)
ERROR (%)
VCMHI=1/2VREF
0.15
0.10
0.05
0.15
0.10
0.05
0.00
0.00
-0.05
-0.05
-0.10
-0.10
-25
0
25 50 75
TEMPERATURE (°C)
100 125
1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 5.0
SUPPLY VOLTAGE (V)
Figure 27. VCMHI Error
vs Temp
Figure 28. VCMHI Error
vs Supply Voltage
15
30
UNITS TESTED >5000
+IN_PGA=500mV
27
UNITS TESTED >5000
+IN_PGA=500mV
24
PERCENTAGE (%)
12
PERCENTAGE (%)
10k
Figure 26. VCM Buffer PSRR
vs Frequency
0.30
-50
100
1k
FREQUENCY (Hz)
9
6
3
21
18
15
12
9
6
3
0
0
-275-220-165-110 -55 0 55 110 165 220 275
VOSPGA( V)
Figure 29. PGA Input Voltage Offset
12
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-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5
TCVOSPGA( V/°C)
Figure 30. PGA TCVOS
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Typical Characteristics (continued)
Unless otherwise specified, TA= 25°C, VS= (VDD – GND) = 3.3 V, VREF = 3.3 V.
50
+IN_PGA=500mV
SUPPLY CURRENT ( A)
105.0
PSRR (dB)
102.5
100.0
97.5
95.0
45
40
35
92.5
30
90.0
-50
-25
-50
0
25 50 75 100 125
TEMPERATURE (°C)
-25
0
25
50
75
TEMPERATURE (°C)
100 125
Figure 32. Supply Current
vs Temperature
Figure 31. PGA DC PSRR
vs Temperature
SUPPLY CURRENT ( A)
50
45
40
35
30
1.5
2.0
2.5 3.0 3.5 4.0 4.5
SUPPLY VOLTAGE (V)
5.0
Figure 33. Supply Current
vs Supply Voltage
7 Detailed Description
7.1 Overview
The LMP91200 is a sensor AFE for use in low-power, analytical-sensing applications. The LMP91200 is
designed for 2-electrode sensors. This device provides all of the functionality needed to detect changes based on
a delta voltage at the sensor. Optimized for low-power applications, the LMP91200 works over a voltage range of
1.8 V to 5.5 V. With its extremely low input bias current, it is optimized for use with pH sensors. Also, in the
absence of supply voltage, the very low input bias current reduces degradation of the pH probe when connected
to the LMP91200. Two guard pins provide support for high parasitic impedance wiring.
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7.2 Functional Block Diagram
VDD
LMP91200
VOUT
GUARD1
INP
±
pH
BUFFER
+
VOCM
GUARD2
VREF
VCM
VCM
BUFFER
GND
VCMHI
7.3 Feature Description
7.3.1 pH Buffer
The pH Buffer is a unity gain buffer with a input bias current in the range of tens fA at room temperature. Its very
low bias current introduces a negligible error in the measurement of the pH. The ph buffer is provided with 2
guard pins (GUARD1, GUARD2) in order to minimize the leakage of the input current and to make the design of
a guard ring easy.
7.3.2 VCM Buffer
Both buffered and unbuffered version of the common-mode voltage are available respectively at the VCM pin
and VCMHI pin. A copy of the buffered version is present at VOCM pin in case of differential measurement.
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8 Application and Implementation
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Theory of pH Measurement
The pH electrode measurements are made by comparing the readings in a sample with the readings in
standards whose pH has been defined (buffers). When a pH sensing electrode comes in contact with a sample,
a potential develops across the sensing membrane surface and that membrane potential varies with pH. A
reference electrode provides a second, unvarying potential to quantitatively compare the changes of the sensing
membrane potential. These days, pH electrodes are composed of a sensing electrode with the reference
electrode built into the same electrode body, and they are called combination electrodes. A high input impedance
meter serves as the readout device and calculates the difference between the reference electrode and sensing
electrode potentials in millivolts. The millivolts are then converted to pH units according to the Nernst equation.
Electrode behavior is described by the Nernst equation:
E = Eo + (2.3 RT/nF) log aH+
where
•
•
•
•
E is the measured potential from the sensing electrode,
Eo is related to the potential of the reference electrode,
(2.3 RT/nF) is the Nernst factor,
log aH+ is the pH, (aH+ = activity of Hydrogen ions).
(1)
2.3 RT/nF includes the Gas Law constant (R), Faraday’s constant (F), the temperature in degrees Kelvin (T) and
the stoichiometric number of ions involved in the process (n). For pH, where n = 1, the Nernst factor is 2.3 RT/F.
Because R and F are constants, the factor and therefore electrode behavior is dependent on temperature. The
Nernst Factor is equivalent to the electrode slope which is a measure of the electrode response to the ion being
detected. When the temperature is 25°C, the theoretical Nernst slope is 59.16 mV/pH unit.
8.2 Typical Application
VDD
NC
LMP91200
VDD
MCU
GND
GND
NC
VOUT
ADC
GUARD1
INP
±
pH
BUFFER
+
VOCM
GUARD2
VREF
VCM
GND
VCM
BUFFER
GND
pH Electrode
VCMHI
Figure 34. Typical Application
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Typical Application (continued)
8.2.1 Design Requirements
8.2.1.1 pH Measurement
The output of a pH electrode ranges from 415 mV to −415 mV as the pH changes from 0 to 14 at 25°C. The
output impedance of a pH electrode is extremely high, ranging from 10 MΩ to 1000 MΩ. The low input bias
current of the LMP91200 allows the voltage error produced by the input bias current and electrode resistance to
be minimal. For example, if the output impedance of the pH electrode used is 10 MΩ and an operational amplifier
with 3 nA of Ibias is used, the error caused due to the input bias current of the amplifier and the source
resistance of the pH electrode is 30 mV! This error can be greatly reduced to 1.25 µV by using the LMP91200.
The pH measurement with the LMP91200 is straightforward. The pH electrode must be connected between the
VCM pin and the INP pin. The voltage at the VCM pin represents the internal zero of the system so the potential
of the electrode (voltage at INP pin) will be referred to the VCM voltage.
8.2.2 Detailed Design Procedure
The LMP91200 is configured to execute a pH measurement as described in the pH Measurement section.
8.2.3 Application Curves
0.10
PGA Gain = 5V/V
INTEGRATED NOISE (500nV/DIV)
0.08
GAIN ERROR (%)
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
-0.10
-50
-25
0
25 50 75
TEMPERATURE (°C)
100 125
TIME (1s/DIV)
Figure 35. PGA Gain Error vs Temp
Figure 36. PGA Time Domain Voltage Noise
90
80
+INPGA=100mV
PSRR (dB)
70
60
50
40
30
20
10
0
10
100
1k
FREQUENCY (Hz)
10k
Figure 37. PGA PSRR vs Frequency
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9 Power Supply Recommendations
VDD should be bypassed with 10-µF, 1-µF and 0.1-µF capacitors, placed as close as possible to the LMP91200
VDD pin (pin 1). An LDO is recommended for the supply rail, but a DC-DC switcher may be used if sufficient
filtering is used to attenuate the switching frequency components.
10 Layout
10.1 Layout Guidelines
Due to the high impedance of the ph Electrode in the pH measurement, careful circuit layout and assembly are
required. Guarding techniques are highly recommended to reduce parasitic leakage current by isolating the input
of the LMP91200 from large voltage gradients across the PCB. A guard is a low impedance conductor that
surrounds an input line and its potential is raised to the voltage of the input line. The input pin should be fully
guarded as shown in Figure 38. The guard traces should completely encircle the input connections. In addition,
they should be located on both sides of the PCB and be connected together. The LMP91200 makes the guard
ring easy to be implemented without any other external operational amplifier. The ring needs to be connected to
the guard pins (GUARD1 and GUARD2), which are at the same potential as that of the INP pin. Solder mask
should not cover the input and the guard area, including guard traces on either side of the PCB. Sockets are not
recommended as they can be a significant leakage source. After assembly, a thorough cleaning using
commercial solvent is necessary.
Figure 38 shows a typical guard ring circuit when the LMP912000 is interfaced to a pH probe through a triaxial
cable/connector (usually referred to as triax). The signal conductor and the guard of the triax should be kept at
the same potential. Therefore, the leakage current between them is practically zero. Because the triax has an
extra layer of insulation and a second conducting sheath, it offers greater rejection of interference than coaxial
cable or connector.
10.2 Layout Example
Figure 38. Circuit Board Guard Layout
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11 Device and Documentation Support
11.1 Community Resources
The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective
contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of
Use.
TI E2E™ Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration
among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help
solve problems with fellow engineers.
Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and
contact information for technical support.
11.2 Trademarks
E2E is a trademark of Texas Instruments.
All other trademarks are the property of their respective owners.
11.3 Electrostatic Discharge Caution
These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam
during storage or handling to prevent electrostatic damage to the MOS gates.
11.4 Glossary
SLYZ022 — TI Glossary.
This glossary lists and explains terms, acronyms, and definitions.
12 Mechanical, Packaging, and Orderable Information
The following pages include mechanical, packaging, and orderable information. This information is the most
current data available for the designated devices. This data is subject to change without notice and revision of
this document. For browser-based versions of this data sheet, refer to the left-hand navigation.
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PACKAGE OPTION ADDENDUM
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10-Dec-2020
PACKAGING INFORMATION
Orderable Device
Status
(1)
Package Type Package Pins Package
Drawing
Qty
Eco Plan
(2)
Lead finish/
Ball material
MSL Peak Temp
Op Temp (°C)
Device Marking
(3)
(4/5)
(6)
LMP91200MT/NOPB
ACTIVE
TSSOP
PW
16
92
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
LMP912
00MT
LMP91200MTX/NOPB
ACTIVE
TSSOP
PW
16
2500
RoHS & Green
SN
Level-3-260C-168 HR
-40 to 125
LMP912
00MT
(1)
The marketing status values are defined as follows:
ACTIVE: Product device recommended for new designs.
LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect.
NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design.
PREVIEW: Device has been announced but is not in production. Samples may or may not be available.
OBSOLETE: TI has discontinued the production of the device.
(2)
RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance
do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may
reference these types of products as "Pb-Free".
RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption.
Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of
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