LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications General Description Features 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 proportional 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 solutions. 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 interface. The I2C interface can also be used for sensor diagnostics. 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 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 5.25 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 chemical cells ■ Programmable cell bias voltage ■ Low bias voltage drift 2.75kΩ to 350kΩ ■ Programmable TIA gain ■ Sink and source capability ■ I2C compatible digital interface -40°C to 85°C ■ Ambient operating temperature 14 pin LLP ■ Package ■ Supported by Webench Sensor AFE Designer Applications ■ Chemical species identification ■ Amperometric applications ■ Electrochemical blood glucose meter Typical Application 30132505 AFE Gas Detector © 2011 National Semiconductor Corporation 301325 www.national.com LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications September 12, 2011 LMP91000 Ordering Information Package Package Marking Part Number Transport Media LMP91000SD 14-Pin LLP NSC Drawing 1k Units Tape and Reel LMP91000SDE 250 Units Tape and Reel L91000 LMP91000SDX SDA14B 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 6 VDD Not Internally Connected 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.national.com Voltage Reference input Connect to AGND 2 If Military/Aerospace specified devices are required, please contact the National Semiconductor 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 LMP91000 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 5.25V -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 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 µA Potentiostat Bias_RW Bias Programming range Percentage of voltage referred to VREF or VDD (differential voltage between RE pin and WE pin) ±24 Bias Programming Resolution First two smallest step ±1 All other steps ±2 -90 -800 90 800 VDD=5.25V; Internal Zero 50% VDD -90 -900 90 900 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; en_RW % VDD=2.7V; Internal Zero 50% VDD IRE AOL_A1 % -750µA≤ICE≤750µA 104 120 Low Frequency integrated noise 0.1Hz to 10Hz, Zero Bias between RE pin and WE pin (Note 10) 3.4 0.1Hz to 10Hz, with Bias (Note 10, Note 11) 5.1 3 pA µA mA dB µVpp www.national.com LMP91000 Symbol Parameter Conditions Min Typ Max (Note 7) (Note 6) (Note 7) Units 0% VREF Internal Zero=20% VREF 0% VREF Internal Zero=50% VREF -550 550 ±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 -4 4 ±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 0% VREF Internal Zero=67% VREF VOS_RW WE Voltage Offset referred to RE BIAS polarity (Note 12) µV 0% VREF Internal Zero=20% VREF 0% VREF Internal Zero=50% VREF 0% VREF Internal Zero=67% VREF TcVOS_RW TIA_GAIN WE Voltage Offset Drift referred BIAS polarity to RE from -40°C to 85°C (Note 12) (Note 8) Transimpedance gain accuracy Linearity Programmable TIA Gains 7 programmable gain resistors Maximum external gain resistor www.national.com 4 µV/°C 5 % ±0.05 % 2.75 3.5 7 14 35 120 350 350 kΩ TIA_ZV Parameter Min Typ Max (Note 7) (Note 6) (Note 7) Conditions Internal zero voltage 3 programmable percentages of VREF 20 50 67 3 programmable percentages of VDD 20 50 67 Internal zero voltage Accuracy RL Programmable Load 4 programmable resistive loads Power Supply Rejection Ratio at RE pin 2.7 ≤VDD≤5.25V % ±0.04 % 10 33 50 100 Ω 5 % 110 dB Load accuracy PSRR Units Internal zero 20% VREF Internal zero 50% VREF 80 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 Sensitivity TA=-40˚C to 85˚C -3 3 -8.2 Power on time °C mV/°C 1.9 ms VDD V External reference specification VREF External Voltage reference range 1.5 Input impedance I2C Interface 10 MΩ (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 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 14) 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 13) tSU;DAT Data Setup time tf SDA fall time (Note 14) tSU;STO Set-up time for STOP condition Conditions After this period, the first clock pulse is generated Min IL ≤ 3mA; Max Units 100 kHz 250 CL ≤ 400pF 4.0 5 Typ ns µs www.national.com LMP91000 Symbol LMP91000 Symbol Parameter Conditions Min Typ Max tBUF Bus free time between a STOP and START condition 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 600 ns tEN;HIGH time between consecutive I2C interface enabling and disabling 600 ns µs 4.7 Interface Disabling Units 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: 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 www.national.com 6 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) -100 -100 VDD = 2.7V VDD = 3.3V VDD = 5V -120 -140 -120 -140 -160 -160 -180 -180 VOS (μV) VOS (μV) Input VOS_RW vs. VDD (Vbias 0mV) -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 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132563 30132562 IWE Step current response (fall) NORMALIZED OUTPUT TIA (200mV/DIV) IWE (50μA/DIV) IWE 2.75kΩ 3.5kΩ 7kΩ 14kΩ 35kΩ 120kΩ 350kΩ IWE IWE (50μA/DIV) NORMALIZED OUTPUT (200mV/DIV) IWE Step current response (rise) 2.75kΩ 3.5kΩ 7kΩ 14kΩ 35kΩ 120kΩ 350kΩ TIME (200μs/DIV) TIME (200μs/DIV) 30132564 30132566 Temperature sensor output vs. VDD (Temperature = 30°C) AC PSRR vs. Frequency 140 1320 1318 120 VOUT (mV) PSRR (dB) 130 110 100 1316 1314 1312 90 80 1310 10 100 1k 10k FREQUENCY (Hz) 100k 2.5 30132560 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132569 7 www.national.com LMP91000 Typical Performance Characteristics SUPPLY CURRENT (μA) 0.9 Supply current vs. VDD (Deep Sleep Mode) 1.0 VDD = 2.7V VDD = 3.3V VDD = 5V 0.9 SUPPLY CURRENT (μA) 1.0 0.8 0.7 0.6 0.5 0.4 0.3 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.1 -25 0 25 50 75 TEMPERATURE (°C) 100 85°C 25°C -40°C 0.8 0.2 -50 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132591 30132597 Supply current vs. temperature (Standby Mode) SUPPLY CURRENT (μA) 7.25 Supply current vs. VDD (Standby Mode) 7.50 VDD = 2.7V VDD = 3.3V VDD = 5V 7.25 SUPPLY CURRENT (μA) 7.50 7.00 6.75 6.50 6.25 6.00 5.75 -50 85°C 25°C -40°C 7.00 6.75 6.50 6.25 6.00 5.75 5.50 5.50 -25 0 25 50 75 TEMPERATURE (°C) 100 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 30132587 10.8 10.6 11.0 VDD = 2.7V VDD = 3.3V VDD = 5V 10.8 10.4 10.2 10.0 9.8 9.6 9.4 9.2 10.4 10.2 10.0 9.8 9.6 9.4 9.0 -25 0 25 50 75 TEMPERATURE (°C) 100 2.5 30132586 www.national.com 10.6 85°C 25°C -40°C 9.2 9.0 -50 Supply current vs. VDD (3-lead amperometric Mode) SUPPLY CURRENT (μA) 11.0 5.5 30132592 Supply current vs. temperature (3-lead amperometric Mode) SUPPLY CURRENT (μA) LMP91000 Supply current vs. temperature (Deep Sleep Mode) 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132593 8 SUPPLY CURRENT (μA) 16.5 16.0 VDD = 2.7V VDD = 3.3V VDD = 5V 15.8 SUPPLY CURRENT (μA) 17.0 Supply current vs. VDD (Temp Measurement TIA ON) 16.0 15.5 15.0 14.5 14.0 13.5 15.4 15.2 15.0 14.8 14.6 14.4 14.2 13.0 -50 15.6 85°C 25°C -40°C 14.0 -25 0 25 50 75 TEMPERATURE (°C) 100 2.5 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 30132588 30132594 Supply current vs. temperature (Temp Measurement TIA OFF) SUPPLY CURRENT (μA) 12.5 Supply current vs. VDD (Temp Measurement TIA OFF) 13.0 VDD = 2.7V VDD = 3.3V VDD = 5V SUPPLY CURRENT (μA) 13.0 12.0 11.5 11.0 10.5 10.0 9.5 12.5 85°C 25°C -40°C 12.0 11.5 11.0 10.5 10.0 9.0 -50 5.5 -25 0 25 50 75 TEMPERATURE (°C) 2.5 100 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132595 30132589 Supply current vs. temperature (2-lead ground referred amperometric Mode) SUPPLY CURRENT (μA) 7.25 7.00 9.0 VDD = 2.7V VDD = 3.3V VDD = 5V 8.5 SUPPLY CURRENT (μA) 7.50 Supply current vs. VDD (2-lead ground referred amperometric Mode) 6.75 6.50 6.25 6.00 5.75 5.50 8.0 7.5 7.0 6.5 6.0 5.25 5.5 5.00 5.0 -50 -25 0 25 50 75 TEMPERATURE (°C) 100 2.5 30132590 85°C 25°C -40°C 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 30132596 9 www.national.com LMP91000 Supply current vs. temperature (Temp Measurement TIA ON) LMP91000 0.1Hz to 10Hz noise, 0V bias 0.1Hz to 10Hz noise, 300mV bias 2.5 1.5 2.0 1.5 0.5 EN_RW (μV) EN_RW (μV) 1.0 0.0 -0.5 1.0 0.5 0.0 -0.5 -1.0 -1.5 -1.0 -2.0 -1.5 -2.5 0 1 2 3 4 5 6 TIME (s) 7 8 9 10 0 1 2 3 4 5 6 TIME (s) 7 8 9 10 30132598 30132599 0.1Hz to 10Hz noise, 600mV bias A VOUT step response 100 ppm to 400 ppm CO (CO gas sensor connected to LMP91000) 2.5 2.0 2.0 1.5 1.8 1.0 0.5 1.7 0.0 1.6 VOUT (V) EN_RW (μV) LMP91000 1.9 -0.5 -1.0 1.5 1.4 -1.5 1.3 -2.0 1.2 -2.5 1.1 0 1 2 3 4 5 6 TIME (s) 7 8 9 10 1.0 RTIA=35kΩ, Rload=10Ω, VREF=5V 0 25 50 75 100 TIME (s) 125 150 301325100 30132568 www.national.com 10 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 sensors. 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 voltage 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 applications, the LMP91000 AFE works over a voltage range of 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 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 LMP91000 between 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 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. Variable Bias The Variable Bias block circuitry (Figure 2) provides the amount of bias voltage required by a biased gas sensor between 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 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. 11 www.national.com LMP91000 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 sensor 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. Function Description LMP91000 The Internal zero is provided through an internal voltage divider (Vref divider box in Figure 2). The divider is programmed through the I2C interface. Temperature sensor The embedded temperature sensor can be switched off during gas concentration measurement to save power. The temperature 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). -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 Temperature Sensor Transfer Table 7 1504 70 987 8 1496 71 979 9 1488 72 971 10 1480 73 962 Temperature (°C) Output Voltage (mV) Temperature (°C) Output Voltage (mV) -40 1875 23 1375 11 1472 74 954 -39 1867 24 1367 12 1464 75 945 -38 1860 25 1359 13 1456 76 937 -37 1852 26 1351 14 1448 77 929 -36 1844 27 1342 15 1440 78 920 -35 1836 28 1334 16 1432 79 912 -34 1828 29 1326 17 1424 80 903 -33 1821 30 1318 18 1415 81 895 -32 1813 31 1310 19 1407 82 886 -31 1805 32 1302 20 1399 83 878 -30 1797 33 1293 21 1391 84 870 -29 1789 34 1285 22 1383 85 861 -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 www.national.com 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 calculated 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 function 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 capacitance and operating speed. The LMP91000 comes with a 7 bit bus fixed address: 1001 000. 12 30132572 (a) Register write transaction 30132571 (b) Pointer set transaction 30132570 (c) Register read transaction FIGURE 3. READ and WRITE transaction 13 www.national.com LMP91000 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 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). 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 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 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 operation, 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 LMP91000 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. 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 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 R/W N Y 0x01 LOCK 0x02 through 0x09 RESERVED 0x10 TIACN 0x1C R/W 0x11 REFCN 0x00 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 [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.national.com Function Write protection 0 Registers 0x10, 0x11 in write mode 1 Registers 0x10, 0x11 in read only mode (default) 14 The parameters in the TIA control register allow the configuration of the transimpedance gain (RTIA) and the load resistance (RLoad). Bit Name [7:5] RESERVED [4:2] TIA_GAIN Function RESERVED TIA feedback resistance selection 000 External resitance 001 2.75kΩ 010 3.5kΩ 011 7kΩ 100 14kΩ 101 35kΩ 110 120kΩ 111 350kΩ (default) RLoad selection [1:0] RLOAD 00 10Ω (default) 01 33Ω 10 50Ω 11 100Ω 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 7 REF_SOURCE [6:5] INT_Z 4 BIAS_SIGN [3:0] BIAS Function Reference voltage source selection 0 Internal (default) 1 external Internal zero selection (Percentage of the source reference) 00 20% (default) 01 50% 10 67% 11 Internal zero circuitry bypassed (only in O2 ground referred measurement) Selection of the Bias polarity 0 Negative (VWE – VRE)<0V (default) 1 Positive (VWE –VRE)>0V 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.national.com LMP91000 TIACN -- TIA Control Register (address 0x10) 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 7 FET_SHORT [6:3] RESERVED [2:0] OP_MODE Function Shorting FET feature 0 Disabled (default) 1 Enabled 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. 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 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 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, Reference 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. www.national.com 16 LMP91000 30132583 FIGURE 4. 3-Lead Amperometric Cell this configuration the Control Amplifier (A1) is turned off, and the output of the gas sensor is amplified by the Transimpedance 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 internal feedback resistor should be programmed to “external”. 2-lead Galvanic Cell In Ground Referred Configuration When the LMP91000 is interfaced to a galvanic cell (for instance 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 electrode 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 17 www.national.com LMP91000 30132575 FIGURE 5. 2-Lead Galvanic Cell Ground Referred Control Amplifier (A1) is ON and provides the internal zero voltage. The transimpedance amplifier (TIA) is also ON and it is configured as an inverting amplifier, the gain is set according the following formula: Gain= -(RTIA/RLoad) 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 LMP91000 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 LMP91000 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 LMP91000. The LMP91000 is then configured in 3-lead amperometric cell mode (as for amperometric cell). In this configuration the www.national.com 18 LMP91000 30132584 FIGURE 6. 2-Lead Galvanic Cell In Potentiostat Configuration SCL) are shared, while the MENB of each LMP91000 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 LMP91000s connected to the I2C bus. Only the enabled device will acknowledge 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. 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 communication 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 30132581 FIGURE 7. More than one LMP91000 on I2C bus 19 www.national.com LMP91000 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 commands. 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. SMART GAS SENSOR ANALOG FRONT END The LMP91000 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 LMP91000's configuration (content of registers 10h, 11h, 12h). At startup the microcontroller reads the EEPROM's content and configures the LMP91000. A typical smart gas sensor AFE is shown in Figure 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 30132582 FIGURE 9. SMART GAS SENSOR AFEs on I2C bus www.national.com 20 Power Consumption Scenario Deep Sleep StandBy 3-Lead Temperature Amperometric Measurement Cell TIA OFF Temperature Measurement TIA ON 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 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 Total 7.95 Notes and finally the bias is set again at 0mV since this is the normal operation condition for this sensor. 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 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 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 INPUT PULSE (100mV/DIV) OUTPUTT VOLTTAGE (1V/DIV) LMP91000 OUTPUT TEST PULSE TIME (25ms/DIV) 30132561 FIGURE 10. TEST PROCEDURE EXAMPLE 21 www.national.com LMP91000 -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 approximately 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 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 Scenario table. This has the following assumptions: -Power On only happens a few times over life, so its power consumption can be ignored LMP91000 Physical Dimensions inches (millimeters) unless otherwise noted NS Package Number SDA14B www.national.com 22 LMP91000 Notes 23 www.national.com LMP91000 Sensor AFE System: Configurable AFE Potentiostat for Low-Power Chemical Sensing Applications Notes For more National Semiconductor product information and proven design tools, visit the following Web sites at: www.national.com Products Design Support Amplifiers www.national.com/amplifiers WEBENCH® Tools www.national.com/webench Audio www.national.com/audio App Notes www.national.com/appnotes Clock and Timing www.national.com/timing Reference Designs www.national.com/refdesigns Data Converters www.national.com/adc Samples www.national.com/samples Interface www.national.com/interface Eval Boards www.national.com/evalboards LVDS www.national.com/lvds Packaging www.national.com/packaging Power Management www.national.com/power Green Compliance www.national.com/quality/green Switching Regulators www.national.com/switchers Distributors www.national.com/contacts LDOs www.national.com/ldo Quality and Reliability www.national.com/quality LED Lighting www.national.com/led Feedback/Support www.national.com/feedback Voltage References www.national.com/vref Design Made Easy www.national.com/easy www.national.com/powerwise Applications & Markets www.national.com/solutions Mil/Aero www.national.com/milaero PowerWise® Solutions Serial Digital Interface (SDI) www.national.com/sdi Temperature Sensors www.national.com/tempsensors SolarMagic™ www.national.com/solarmagic PLL/VCO www.national.com/wireless www.national.com/training PowerWise® Design University THE CONTENTS OF THIS DOCUMENT ARE PROVIDED IN CONNECTION WITH NATIONAL SEMICONDUCTOR CORPORATION (“NATIONAL”) PRODUCTS. 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