LMP91002 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications General Description Features The LMP91002 is a programmable Analog Front End (AFE) for use in micro-power electrochemical sensing applications. It provides a complete signal path solution between a not biased gas sensor and a microcontroller generating an output voltage proportional to the cell current. The LMP91002’s programmability enables it support not biased electro-chemical gas sensor with a single design. The LMP91002 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 LMP91002’s transimpedance amplifier (TIA) gain is programmable through the I2C interface. The I2C interface can also be used for sensor diagnostics. The LMP91002 is optimized for micro-power applications and operates over a voltage range of 2.7V to 3.6V. The total current consumption can be less than 10μA. Further power savings are possible by switching off the TIA amplifier and shorting the reference electrode to the working electrode with an internal switch. Typical Values, TA = 25°C 2.7 V to 3.6 V ■ Supply voltage <10 µA ■ Supply current (average over time) 10 mA ■ Cell conditioning current up to 900pA (max) ■ Reference electrode bias current (85°C) 750µA ■ Output drive current ■ Complete potentiostat circuit to interface to most not biased gas sensors ■ Low bias voltage drift 2.75kΩ to 350kΩ ■ Programmable TIA gain ■ I2C compatible digital interface -40°C to 85°C ■ Ambient operating temperature 14 pin LLP ■ Package ■ Supported by Webench Sensor AFE Designer Applications ■ Gas detector ■ Amperometric applications ■ Electrochemical blood glucose meter Typical Application 30182505 AFE Gas Detector © 2012 Texas Instruments Incorporated 301825 SNIS163 www.ti.com LMP91002 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications April 24, 2012 LMP91002 Ordering Information Package Package Marking Part Number Transport Media LMP91002SD 14-Pin LLP NSC Drawing 1k Units Tape and Reel LMP91002SDE 250 Units Tape and Reel L91002 LMP91002SDX SDA14B 4.5k Units Tape and Reel Connection Diagram 14–Pin LLP 30182502 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 6 VDD 7 AGND Ground 8 VOUT Analog Output Supply Voltage 9 C2 External filter connector (Filter between C1 and C2) 10 C1 External filter connector (Filter between C1 and C2) 11 VREF 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 www.ti.com Not Internally Connected Voltage Reference input Connect to AGND 2 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 Charge-Device Model Machine Model Voltage between any two pins Current through VDD or VSS Current sunk and sourced by CE pin Current out of other pins(Note 3) Storage Temperature Range Junction Temperature (Note 4) Electrical Characteristics LMP91002 For soldering specifications: see product folder at www.national.com and www.national.com/ms/MS/MS-SOLDERING.pdf Absolute Maximum Ratings (Note 1) Operating Ratings (Note 1) Supply Voltage VS=(VDD - AGND) Temperature Range (Note 4) Package Thermal Resistance (Note 4) 2kV 1kV 200V 6.0V 50mA 10mA 5mA -65°C to 150°C 150°C 2.7V to 3.6V -40°C to 85°C 14-Pin LLP (θJA) 44 °C/W (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 Typ Max (Note 7) (Note 6) (Note 7) Units Power Supply Specification IS Supply Current 3-lead amperometric cell mode MODECN = 0x03 10 15 13.5 Standby mode MODECN = 0x02 6.5 10 8 Deep Sleep mode MODECN = 0x00 0.6 1 0.85 µA Potentiostat VDD=2.7V; Internal Zero 50% VDD -90 -800 90 800 VDD=3.6V; Internal Zero 50% VDD -90 -900 90 900 IRE Input bias current at RE pin ICE Minimum operating current capability sink 750 source 750 Minimum charging capability (Note 9) sink 10 source 10 Open loop voltage gain of control loop op amp (A1) 300mV≤VCE≤Vs-300mV; AOL_A1 en_RW -750µA≤ICE≤750µA 104 Low Frequency integrated noise 0.1Hz to 10Hz between RE pin and WE pin (Note 10) pA µA mA 120 dB 3.4 µVpp 0% VREF Internal Zero=20% VREF VOS_RW WE Voltage Offset referred to RE 0% VREF Internal Zero=50% VREF -550 550 µV -4 4 µV/°C 0% VREF Internal Zero=67% VREF 0% VREF Internal Zero=20% VREF TcVOS_RW WE Voltage Offset Drift referred 0% VREF to RE from -40°C to 85°C Internal Zero=50% VREF (Note 8) 0% VREF Internal Zero=67% VREF 3 www.ti.com LMP91002 Symbol TIA_GAIN Parameter Conditions Min Typ Max (Note 7) (Note 6) (Note 7) Units 5 % ±0.05 % Transimpedance gain accuracy Linearity Programmable TIA Gains TIA_ZV 7 programmable gain resistors Internal zero voltage 2.75 3.5 7 14 35 120 350 Maximum external gain resistor 350 3 programmable percentages of VREF 20 50 67 3 programmable percentages of VDD 20 50 67 Internal zero voltage Accuracy RL PSRR kΩ % ±0.04 % Load Resistor 10 Load accuracy 5 Ω % 110 dB Power Supply Rejection Ratio at RE pin 2.7 ≤VDD≤5.25V Internal zero 20% VREF Internal zero 50% VREF 80 Internal zero 67% VREF External reference specification VREF External Voltage reference range 1.5 Input impedance I2C Interface VDD 10 V MΩ (Note 5) Unless otherwise specified, all limits guaranteed for at TA = 25°C, VS=(VDD – AGND), 2.7V <VS< 3.6V and AGND = DGND =0V, VREF= 2.5V. Boldface limits apply at the temperature extremes Symbol Parameter VIH Input High Voltage VIL Input Low Voltage VOL Output Low Voltage Conditions Max (Note 7) Units V IOUT=3mA 0.3*VDD V 0.4 V V 0.1*VDD Input Capacitance on all digital pins Timing Characteristics Typ (Note 6) 0.7*VDD Hysteresis (Note 13) CIN Min (Note 7) pF 0.5 (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 fSCL Clock Frequency 10 tLOW Clock Low Time 4.7 µs tHIGH Clock High Time 4.0 µs 4.0 µs 4.7 µs 0 ns 250 ns tHD;STA Data valid tSU;STA Set-up time for a repeated START condition tHD;DAT Data hold time(Note 12) tSU;DAT Data Setup time www.ti.com Conditions After this period, the first clock pulse is generated 4 Min Typ Max Units 100 kHz Parameter tf SDA fall time (Note 13) tSU;STO Set-up time for STOP condition 4.0 µs 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 13) 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 600 ns tEN;HIGH time between consecutive enabling and disabling Conditions Min IL ≤ 3mA; CL ≤ 400pF I 2C interface Typ Max Units 250 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) FieldInduced 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: LMP91002 provides an internal 300ns minimum hold time to bridge the undefined region of the falling edge of SCL. Note 13: This parameter is guaranteed by design or characterization. 5 www.ti.com LMP91002 Symbol LMP91002 Timing Diagram 30182541 FIGURE 1. I2C Interface Timing Diagram www.ti.com 6 Unless otherwise specified, TA = 25°C, VS=(VDD – AGND), 2.7V <VS< 3.6V and AGND = DGND =0V, VREF= 2.5V. Input VOS_RW vs. temperature -100 -100 VDD = 2.7V VDD = 3.3V -120 -120 -140 -140 -160 -160 -180 -180 VOS (μV) -200 -220 -200 -220 -240 -240 -260 -260 -280 -280 -300 85°C 25°C -40°C -300 -50 -25 0 25 50 75 TEMPERATURE (°C) 100 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 SUPPLY VOLTAGE (V) 30182563 30182562 IWE Step current response (fall) NORMALIZED OUTPUT TIA (200mV/DIV) NORMALIZED OUTPUT (200mV/DIV) IWE Step current response (rise) IWE (50μA/DIV) IWE 2.75kΩ 3.5kΩ 7kΩ 14kΩ 35kΩ 120kΩ 350kΩ IWE IWE (50μA/DIV) VOS (μV) Input VOS_RW vs. VDD 2.75kΩ 3.5kΩ 7kΩ 14kΩ 35kΩ 120kΩ 350kΩ TIME (200μs/DIV) TIME (200μs/DIV) 30182564 30182566 AC PSRR vs. Frequency Supply current vs. temperature (Deep Sleep Mode) 1.0 140 0.9 SUPPLY CURRENT (μA) PSRR (dB) 130 120 110 100 90 VDD = 2.7V VDD = 3.3V 0.8 0.7 0.6 0.5 0.4 0.3 0.2 80 0.1 10 100 1k 10k FREQUENCY (Hz) 100k -50 30182560 -25 0 25 50 75 TEMPERATURE (°C) 100 30182591 7 www.ti.com LMP91002 Typical Performance Characteristics LMP91002 Supply current vs. VDD (Deep Sleep Mode) SUPPLY CURRENT (μA) 0.9 7.50 85°C 25°C -40°C VDD = 2.7V VDD = 3.3V 7.25 SUPPLY CURRENT (μA) 1.0 Supply current vs. temperature (Standby Mode) 0.8 0.7 0.6 0.5 0.4 0.3 7.00 6.75 6.50 6.25 6.00 0.2 5.75 0.1 5.50 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 SUPPLY VOLTAGE (V) -50 -25 0 25 50 75 TEMPERATURE (°C) 30182597 Supply current vs. VDD (Standby Mode) SUPPLY CURRENT (μA) 7.25 30182587 Supply current vs. temperature (3-lead amperometric Mode) 11.0 85°C 25°C -40°C VDD = 2.7V VDD = 3.3V VDD = 5V 10.8 SUPPLY CURRENT (μA) 7.50 7.00 6.75 6.50 6.25 6.00 5.75 10.6 10.4 10.2 10.0 9.8 9.6 9.4 9.2 5.50 9.0 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 SUPPLY VOLTAGE (V) -50 -25 0 25 50 75 TEMPERATURE (°C) 30182592 10.6 0.1Hz to 10Hz noise 1.5 85°C 25°C -40°C 1.0 10.4 EN_RW (μV) SUPPLY CURRENT (μA) 10.8 10.2 10.0 9.8 9.6 9.4 0.5 0.0 -0.5 -1.0 9.2 9.0 -1.5 2.7 2.8 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 SUPPLY VOLTAGE (V) 0 30182593 www.ti.com 100 30182586 Supply current vs. VDD (3-lead amperometric Mode) 11.0 100 1 2 3 4 5 6 TIME (s) 7 8 9 10 30182598 8 LMP91002 A VOUT step response 100 ppm to 400 ppm CO (CO gas sensor connected to LMP91002) 2.0 LMP91000 1.9 1.8 VOUT (V) 1.7 1.6 1.5 1.4 1.3 1.2 RTIA=35kΩ, Rload=10Ω, 1.1 1.0 0 25 50 75 100 TIME (s) 125 150 30182568 9 www.ti.com LMP91002 2.75kΩ to 350kΩ making it easy to convert current ranges from 5µA to 750µA full scale. Optimized for micro-power applications, the LMP91002 AFE works over a voltage range of 2.7V to 3.6 V. The cell voltage is user selectable using the on board programmability. In addition, it is possible to connect an external transimpedance gain resistor. 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. Function Description GENERAL The LMP91002 is a programmable AFE for use in micropower chemical sensing applications. The LMP91002 is designed for 3-lead not biased gas sensors and for 2 leads galvanic cell. This device provides all of the functionality for detecting changes in gas concentration based on a delta current at the working electrode. The LMP91002 generates an output voltage proportional to the cell current. Transimpedance gain is user programmable through an I2C compatible interface from 30182583 FIGURE 2. System Block Diagram POTENTIOSTAT CIRCUITRY The core of the LMP91002 is a potentiostat circuit. It consists of a differential input amplifier used to compare the potential between the working and reference electrodes to a zero bias potential.. The error signal is amplified and applied to the counter electrode (through the Control Amplifier - A1). Any changes in the impedance between the working and reference electrodes 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 Transimpedance 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 potentiostat 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 programmable internal gain resistors. This accommodates the full scale ranges of most existing sensors. Moreover an external gain resistor can be connected to the LMP91002 between C1 and C2 pins. The gain is set through the I2C interface. www.ti.com Control amplifier The control amplifier (A1 op amp in Figure 2) provides initial charge to the sensor. A1 has the capability to drive up to 10mA into the sensor in order to to provide a fast initial conditioning. 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. 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 electrode 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. The Internal zero is provided through an internal voltage divider (Vref divider box in Figure 2). The divider is programmed through the I2C interface. 10 WRITE AND READ OPERATION In order to start any read or write operation with the LMP91002, MENB needs to be set low during the whole communication. Then the master generates a start condition by driving SDA from high to low while SCL is high. The start condition is always followed by a 7-bit slave address and a Read/ Write bit. After these 8 bits have been transmitted by the master, SDA is released by the master and the LMP91002 either ACKs or NACKs the address. If the slave address matches, the LMP91002 ACKs the master. If the address doesn't 30182572 (a) Register write transaction 30182571 (b) Pointer set transaction 11 www.ti.com LMP91002 match, the LMP91002 NACKs the master. For a write operation, the master follows the ACK by sending the 8-bit register address pointer. Then the LMP91002 ACKs the transfer by driving SDA low. Next, the master sends the 8-bit data to the LMP91002. Then the LMP91002 ACKs the transfer by driving SDA low. At this point the master should generate a stop condition and optionally set the MENB at logic high level (refer to Figure 3). A read operation requires the LMP91002 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 LMP91002 sends out the 8-bit data of the register. When just one LMP91002 is present on the I2C bus the MENB can be tied to ground (low logic level). 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 capacitance and operating speed. The LMP91002 comes with a 7 bit bus fixed address: 1001 000. LMP91002 30182570 (c) Register read transaction FIGURE 3. READ and WRITE transaction t_timeout, the LMP91002’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 LMP91002 is driving the bus and the SCL is stopped. 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 LMP91002 will automatically reset its I2C interface. Also, in the case the LMP91002 hangs the SDA for a time exceeding REGISTERS The registers are used to configure the LMP91002. 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 LMP91002's power-on status. If its readback is “0”, the LMP91002 is not ready to accept other I2C commands. Bit Name [7:1] RESERVED 0 STATUS Function 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 [7:1] RESERVED 0 LOCK www.ti.com Function Write protection 0 Registers 0x10, 0x11 in write mode 1 Registers 0x10, 0x11 in read only mode (default) 12 LMP91002 TIACN -- TIA Control Register (address 0x10) The parameters in the TIA control register allow the configuration of the transimpedance gain (RTIA). Bit Name [7:5] RESERVED [4:2] TIA_GAIN [1:0] RESERVED Function RESERVED TIA feedback resistance selection 000 External resistance (default) 001 2.75kΩ 010 3.5kΩ 011 7kΩ 100 14kΩ 101 35kΩ 110 120kΩ 111 350kΩ RESERVED REFCN -- Reference Control Register (address 0x11) The parameters in the Reference control register allow the configuration of the Internal zero, 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 is defined as a percentage of VREF voltage instead of the supply voltage. Bit Name 7 REF_SOURCE [6:5] INT_Z [4] RESERVED [3:0] DIAGNOSTIC Function Reference voltage source selection 0 Internal (default) 1 external Internal zero selection (Percentage of the source reference) 00 20% 01 50% (default) 10 67% RESERVED Diagnostic step (Percentage of the source reference) 0000 0% (default) 0001 1% 13 www.ti.com LMP91002 MODECN -- Mode Control Register (address 0x12) The Parameters in the Mode register allow the configuration of the Operation Mode of the LMP91002. Bit Name 7 FET_SHORT Shorting FET feature 0 Disabled (default) 1 Enabled [6:3] RESERVED RESERVED OP_MODE Mode of Operation selection 000 Deep Sleep (default) 010 Standby 011 3-lead amperometric cell [2:0] Function and provides the internal zero voltage and bias in case of biased gas sensor. The transimpedance amplifier (TIA) is 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 internal feedback resistor should be programmed to “external”. The RLoad together with the output capacitance of the gas sensor acts as a low pass filter. GAS SENSOR INTERFACE The LMP91002 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 LMP91002 in the potentiostat topology. 3-lead Amperometric Cell In Potentiostat Configuration Most of the amperometric cell have 3 leads (Counter, Reference and Working electrodes). The interface of the 3-lead gas sensor to the LMP91002 is straightforward, the leads of the gas sensor need to be connected to the namesake pins of the LMP91002. The LMP91002 is then configured in 3-lead amperometric cell mode; in this configuration the Control Amplifier (A1) is ON 30182583 FIGURE 4. 3-Lead Amperometric Cell www.ti.com 14 30182584 FIGURE 5. SCL) are shared, while the MENB of each LMP91002 is connected 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 LMP91002s connected to the I2C bus. Only the enabled device will acknowledge the I2C commands. After finishing communicating with this particular LMP91002, the microcontroller de-asserts the corresponding MENB and repeats the procedure for other LMP91002s. Figure 6 shows the typical connection when more than one LMP91002 is connected to the I2C bus. Application Information CONNECTION OF MORE THAN ONE LMP91002 TO THE I2C BUS The LMP91002 comes out with a unique and fixed I2C slave address. It is still possible to connect more than one LMP91002 to an I2C bus and select each device using the MENB pin. The MENB simply enables/disables the I2C communication of the LMP91002. 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 LMP91002 connected to the I2C bus, the I2C lines (SDA and 15 www.ti.com LMP91002 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 internal feedback resistor should be programmed to “external”. 2-lead Galvanic Cell in Potentiostat Configuration When the LMP91002 is interfaced to a galvanic cell (for instance to an Oxygen gas sensor) referred to a reference, the Counter and the Reference pin of the LMP91002 are shorted together and connected to negative electrode of the galvanic cell. The positive electrode of the galvanic cell is then connected to the Working pin of the LMP91002. The LMP91002 is then configured in 3-lead amperometric cell mode (as for amperometric cell). In this configuration the LMP91002 30182581 FIGURE 6. More than one LMP91002 on I2C bus LMP91002. A typical smart gas sensor AFE is shown in Figure 7. The connection of MENB to the hardware address pin A0 of the EEPROM allows the microcontroller to select the LMP91002 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. SMART GAS SENSOR ANALOG FRONT END The LMP91002 together with an external EEPROM represents 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 LMP91002's configuration (content of registers 10h, 11h, 12h). At startup the microcontroller reads the EEPROM's content and configures the 30182580 FIGURE 7. SMART GAS SENSOR AFE smart gas sensor AFE enabled will acknowledge the I2C commands. When the communication with this particular module ends, the microcontroller de-asserts the corresponding MENB and repeats the procedure for other modules. Figure 8 shows the typical connection when several smart gas sensor AFEs are connected to the I2C bus. 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 LMP91002 or its corresponding EEPROM) in the www.ti.com 16 LMP91002 30182582 FIGURE 8. SMART GAS SENSOR AFEs on I2C bus 17 www.ti.com Power Consumption Scenario Deep Sleep StandBy 3-Lead Amperometric Cell Current consumption (µA) typical value 0.6 6.5 10 Time ON (%) 0 60 39 Average (µA) 0 3.9 3.9 A1 OFF ON ON TIA OFF OFF ON I2C interface ON ON ON Total 7.8 Notes This operation will apply a potential (VRW) between RE and WE pin (VRE>VWE), VRW=1% Source reference 2. Put in the [3:0] bit of the register REFCN (0x11) the 0000b value, leaving the other bit unchanged. This operation will remove the potential (VRW) between RE and WE pin (VRE>VWE), VRW=0V. The width of the pulse is simply the time between the two writing operation. www.ti.com LMP91000 OUTPUT TEST PULSE INPUT PULSE (50mV/DIV) SENSOR TEST PROCEDURE The LMP91002 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 LMP91002 The test procedure is very easy. The diagnostic block is user programmable through the digital interface. A step voltage 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 detected 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 LMP91002 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 9 shows an example of the test procedure, a Carbon Monoxide sensor is connected to the LMP91002, a 25mVpp pulse is applied between Reference and Working pin. The following procedure shows how to implement the sensor test: Preliminary conditions: The LMP91002 is unlocked and it is in 3-Lead Amperometric Cell Mode 1. Put in the [3:0] bit of the register REFCN (0x11) the 0001b value, leaving the other bit unchanged. OUTPUT VOLTTAGE (500mV/DIV) LMP91002 -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. This results in an average power consumption of approximately 7.8 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 The LMP91002 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 LMP91002 is below 10µA @ 3.3v average over time, (this excludes any current drawn from any pin). A typical usage of the LMP91002 is in a portable gas detector and its power consumption is summarized in the Power Consumption Scenario table. This has the following assumptions: -Power On only happens a few times over life, so its power consumption can be ignored TIME (25ms/DIV) 30182561 FIGURE 9. TEST PROCEDURE EXAMPLE 18 LMP91002 Physical Dimensions inches (millimeters) unless otherwise noted NS Package Number SDA14B 19 www.ti.com LMP91002 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications Notes www.ti.com IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI’s terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI’s standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily performed. TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. To minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of TI information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Information of third parties may be subject to additional restrictions. Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. TI products are not authorized for use in safety-critical applications (such as life support) where a failure of the TI product would reasonably be expected to cause severe personal injury or death, unless officers of the parties have executed an agreement specifically governing such use. Buyers represent that they have all necessary expertise in the safety and regulatory ramifications of their applications, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of TI products in such safety-critical applications, notwithstanding any applications-related information or support that may be provided by TI. Further, Buyers must fully indemnify TI and its representatives against any damages arising out of the use of TI products in such safety-critical applications. TI products are neither designed nor intended for use in military/aerospace applications or environments unless the TI products are specifically designated by TI as military-grade or "enhanced plastic." Only products designated by TI as military-grade meet military specifications. Buyers acknowledge and agree that any such use of TI products which TI has not designated as military-grade is solely at the Buyer's risk, and that they are solely responsible for compliance with all legal and regulatory requirements in connection with such use. TI products are neither designed nor intended for use in automotive applications or environments unless the specific TI products are designated by TI as compliant with ISO/TS 16949 requirements. Buyers acknowledge and agree that, if they use any non-designated products in automotive applications, TI will not be responsible for any failure to meet such requirements. Following are URLs where you can obtain information on other Texas Instruments products and application solutions: Products Applications Audio www.ti.com/audio Automotive and Transportation www.ti.com/automotive Amplifiers amplifier.ti.com Communications and Telecom www.ti.com/communications Data Converters dataconverter.ti.com Computers and Peripherals www.ti.com/computers DLP® Products www.dlp.com Consumer Electronics www.ti.com/consumer-apps DSP dsp.ti.com Energy and Lighting www.ti.com/energy Clocks and Timers www.ti.com/clocks Industrial www.ti.com/industrial Interface interface.ti.com Medical www.ti.com/medical Logic logic.ti.com Security www.ti.com/security Power Mgmt power.ti.com Space, Avionics and Defense www.ti.com/space-avionics-defense Microcontrollers microcontroller.ti.com Video and Imaging www.ti.com/video RFID www.ti-rfid.com OMAP Mobile Processors www.ti.com/omap Wireless Connectivity www.ti.com/wirelessconnectivity TI E2E Community Home Page e2e.ti.com Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2012, Texas Instruments Incorporated