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Gas Sensor

Objective: To build a sensor solution for detecting Carbon Monoxide levels in home/building applications using a 3 electrode City Tech sensor- 4CF+.

Step 1: Click on “Select a Sensor” - select “CarbonMonoxide”->“4CF+”

Step 2: Click on “I2C Interface and Control Logic” block to select the interface as follows:

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Step 3: External Reference – click on the VREF switch to choose “REF_SOURCE = “1: External”, and make sure the “Vref_Ext” on the upper-left hand side is 4.1V. This supply will be sourced from another board, the ADC161S626BEB, which will be connected to the LMP91000SDEVAL.

 

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Step 4: Vref Divider - click on the “Vref Divider” block to select INT_Z = “000: 20% Vref”.  This sets the offset voltage for Vout.

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We chose 20% of VREF because the sensor's current (IWE) is flowing out of the TIA and thus Vout is high. If Vout is already high, then the offset voltage should be low. If the current were flowing into the TIA, then choose 65% of VREF instead.

How do you know if IWE is flowing into or out of the TIA? Look in the sensor database or the sensor's datasheet. If its sensitivity is > 0, then IWE is flowing out of the TIA; otherwise, IWE is flowing into the TIA. 

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Step 5: Variable Bias – click on the “Variable Bias” block to set the bias to “0% of VREF” and its sign to “Positive”. 

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This bias voltage information can also be obtained from the sensor database (or the sensor's datasheet) as seen below.

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Step 6: TIA GAIN – click on the “RTIA” resistor to select “35K Ohm”. 

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RTIA^MAX= (V^OUT_MAX - V^REF_DIV) / (I^WE)

RTIA^MAX= (4.1-0.82) / (70 nA/ppm * 500ppm)

RTIA^MAX= 93.7KOhm

Step 7: RLOAD – click on the “RE” resistor to set RLOAD to 10 ohm.

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This information can also be found in the sensor database.

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Step 8: Shorting FET – click on the FET switch to disable the FET

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Step 9: Performance - click on this step or the “Performance” mini-tab.  This tab displays the Estimated Device Performance base on the block diagram that you’ve configured.

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Step 10: Set up the Hardware per the instructions on the User guide and take the measurement .
LMP91000 Sensor AFE Evaluation Board User's Guide

Measurement Results:

Measured gas concentration in ppm:

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