Gas Sensor Design
LMP91000: Fully integrated, programmable potentiostat AFE for Electrochemical Gas detection:
The LMP91000 is perfect 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. This device provides all of the functionality for detecting changes in gas concentration based on a delta current at the working electrode.
The 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 AFE 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 adjustable cell bias and Transimpedance amplifier (TIA) gain are programmable through 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 AFE 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. The LMP91000 support dozens of different toxic gases and 100’s of different sensors. Configurability allows LMP91000 to address the critical parameters of each gas.
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Fundamental Blocks of LMP91000:
- Transimpedance Amplifier (TIA) – provides an output voltage that is proportional to the cell current. TIA provides 7 programmable internal gain resistors and also allow external gain resistor to be connected to the LMP91000.
- (Vref_div –Vout) / (RTIA) = Iwe
- Vout = (Vref_div) – (RTIA*Iwe)
- Input - The LMP91000 provides a three electrode(CE collecting electrode, RE reference electrode, WE working electrode)as well as a two electrode solution(short the CE and RE)
- Variable Bias – provides the amount of bias voltage required by a biased gas sensor between RE and WE. This bias voltage can be programmed to be 1% to 24% of the supply, or it can be VREF. The bias can also be negative or positive depending on the type of sensing element.
- Vref Divider – this is the voltage at the non-inverting pin at TIA. This can be programmed to be either 20%, 50%, or 67% of the supply, or it can be VREF. 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.
- How to select the appropriate Vref divider?
- If the current at pin WE (Iwe) is flowing into the TIA, then Vref divider should be set to 67% of Vref.
- If Iwe is flowing out of the TIA, then Vref divider should be set to 20% of Vref.
Assume Vref_divider is set to “20% of Vref”, Variable Bias is set to “2% of Vref”, and Vref = 4.1V, then Vref divider is 0.82V, and the non-inverting input to A1 is 0.902 V (22% of Vref).
- Control Amplifier A1 – differential amplifier used to compare the potential between WE (working electrodes) and RE (reference electrodes). The error signal is amplified and applied to the CE (counter electrode). Any changes in the impedance between the WE and RE will cause a change in the voltage applied to the CE, in order to maintain the constant voltage between WE and RE.
- Temperature Sensor – An on-board temperature sensor provides a ±3˚C accuracy. This can be used by an external µC to correct for performance over temperature.
- Serial Interface – Calibration and programming is done through the I2C digital interface. This enables calibration and state of health monitoring. As mentioned before, chemical cells can degrade over time and health monitoring is very important.
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