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PDF LMP91000 Data sheet ( Hoja de datos )
| Número de pieza | LMP91000 | |
| Descripción | Sensor AFE System | |
| Fabricantes | National Semiconductor | |
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LMP91000
September 12, 2011
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
AFE Gas Detector
© 2011 National Semiconductor Corporation 301325
30132505
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Symbol
Parameter
Conditions
Min Typ Max
(Note 7) (Note 6) (Note 7)
Units
TIA_ZV Internal zero voltage
3 programmable percentages of VREF
20
50
3 programmable percentages of VDD
67
%
20
50
67
Internal zero voltage Accuracy
±0.04
%
RL Programmable Load
4 programmable resistive loads
10
33
50 Ω
100
Load accuracy
5%
PSRR
Power Supply Rejection Ratio at 2.7 ≤VDD≤5.25V
RE pin
Internal zero 20% VREF
Internal zero 50% VREF
80
110
Internal zero 67% VREF
dB
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 Typ Max Units
(Note 7) (Note 6) (Note 7)
VIH Input High Voltage
VIL Input Low Voltage
VOL Output Low Voltage
Hysteresis (Note 14)
IOUT=3mA
0.7*VDD
0.1*VDD
0.3*VDD
0.4
V
V
V
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
tLOW
tHIGH
tHD;STA
Clock Frequency
Clock Low Time
Clock High Time
Data valid
After this period, the first clock
pulse is generated
10
4.7
4.0
4.0
100 kHz
µs
µs
µs
tSU;STA
tHD;DAT
tSU;DAT
tf
Set-up time for a repeated START condition
Data hold time(Note 13)
Data Setup time
SDA fall time (Note 14)
IL ≤ 3mA;
CL ≤ 400pF
4.7 µs
0 ns
250 ns
250 ns
tSU;STO
Set-up time for STOP condition
4.0 µs
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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.
FIGURE 2. System Block Diagram
30132583
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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| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LMP91000.PDF ] | |
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