CO2 Sensor Analogue Output Voltage

Product catalog summary

Abstract
GSS sensors are designed to measure CO2 levels in various applications. Typically, data is accessed via UART or I2C interfaces, but some sensors offer an analogue output where voltage is proportional to CO2 levels. This document explains the functionality and optimization of this analogue output.

Method of Operation
The analogue CO2 level indication is an option for some GSS sensors, providing a voltage output proportional to the gas level. The PWM output is filtered to create an analogue signal, reducing noise and power consumption by up to 50% compared to continuous PWM output.

Analogue CO2 Signal Level
The output voltage range is determined by the sensor supply voltage. The formula for converting voltage to CO2 concentration is provided. The PWM signal is filtered to produce an analogue output.

CO2 Analogue Output Performance Characteristics
The output voltage range is from 0 to VDD. Repeatability is ±0.1% at 25°C, with a response time of 0.5 seconds. The internal resistance and capacitance affect the output, and load resistance should be greater than 10kΩ to avoid errors.

Output Linearity
The output voltage is linearly dependent on CO2 concentration, but accuracy is compromised near zero and full scale. Noise levels are specified for different bandwidths.

Specifying a Voltage Output Option
The CozIR®-A ordering guide is provided, showing options for measurement range, temperature, RH, CO2 voltage output, temperature range, and casing.

Important Notice
GSS products are subject to terms and conditions of sale. Changes to products and specifications can occur without notice. GSS products are not intended for life support systems or applications where failure could cause harm.

Address
Gas Sensing Solutions Ltd., Grayshill Road, Cumbernauld, G68 9HQ, United Kingdom.

Revision History
Version 1.0 was released on 13/05/2020, and version 1.1 was a re-write released on 18/05/2020.

Catalog excerpts

APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage ABSTRACT GSS sensors are designed to measure CO2 levels in variety of applications. Typically, measurement data is accessed through either a UART or I2C digital interface. In some circumstances, this interface mode may not be suitable. In such cases, some GSS sensors come with a true analogue CO2 level indicator output, where the analogue signal level voltage is directly proportional to the CO2 gas level. This application note describes how the GSS voltage level indicator functions and how to ensure the user maximises the performance of the sensor using this indication method. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 28 May 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage METHOD OF OPERATION The user can specify an analogue CO2 level indication option on a number of GSS CO2 sensors. If specified as an option on the sensor, the output voltage level will be directly proportional to the measured gas level. The on-board sensor microprocessor generates a PWM output with a duty cycle proportional to the CO2 signal level. There are a number of methods to use the PWM output. In some circumstances, the digital PWM signal can be used directly. This method is not supported by GSS sensors as it offers little advantage...

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage Sampling Pulse PWM Input + Analogue CO2 Level Output ANALOGUE CO2 SIGNAL LEVEL The voltage range of the CO2 level output from the sensor is determined by the sensor supply voltage. The sensor supply voltage defines the PWM signal levels. All voltage outputs are relative to the sensor input supply voltage. For example, if the supply voltage is 3.4V, then the full-scale output from the voltage pin will also be 3.4V, the half scale voltage will be 1.7V etc. To convert a voltage into a CO2 concentration, use the following formula. ݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐְݐ...

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APPLICATION NOTEAN006: CO2 Sensor Analogue Output Voltage CO2 ANALOGUE OUTPUT PERFORMANCE CHARACTERISTICS The CO2 analogue signal level accuracy is also affected by op-amp DC offsets and op-amp signal headroom. At low CO2 concentrations, the DC offset of approximately 14mV will increasingly affect the signal level accuracy. At high CO2 concentrations, the CO2 analogue signal level is unable to generate a voltage of more than VDD-50mV. The user is advised to take care when interpreting CO2 analogue signals below 50mV and or where the signal level is >VDD-50mV. Page | 5 For regular updates, sign...

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage Although great care has been taken to minimise sensor noise, there may be some high frequency noise present on the analogue voltage output. The typical noise present on the voltage output is as follows with the voltage output at half full scale: 140µVrms measured in a 20kHz bandwidth. 450µVrms measured in a 10MHz bandwidth. In a 10MHz bandwidth, the highest noise voltage amplitude is at least 70dB below the desired DC output voltage. OUTPUT LINEARITY The output voltage is linearly dependent on the CO2 concentration measured by the sensor....

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage SPECIFYING A VOLTAGE OUTPUT OPTION The ordering guide for the CozIR®-A is shown below, but an analogue CO2 signal output is available on several other GSS sensors. Refer to the specific sensor data sheet ordering information to determine if the option is available. The full-scale analogue output signal range always matches the selected measurement range of the sensor. This means when the output signal level is full scale, the CO2 level will be at full scale. COZIR-A-X-XXX-X Page | 7 For regular updates, sign up at https://gassensing.co.uk...

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage IMPORTANT NOTICE Gas Sensing Solutions Ltd. (GSS) products and services are sold subject to GSS’s terms and conditions of sale, delivery and payment supplied at the time of order acknowledgement. GSS warrants performance of its products to the specifications in effect at the date of shipment. GSS reserves the right to make changes to its products and specifications or to discontinue any product or service without notice. Customers should therefore obtain the latest version of relevant information from GSS to verify that the information...

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APPLICATION NOTE AN006: CO2 Sensor Analogue Output Voltage ADDRESS Gas Sensing Solutions Ltd. Grayshill Road Cumbernauld G68 9HQ United Kingdom Gas Sensing Solutions Ltd. Page | 9 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 28 May 2020 Copyright © 2021 Gas Sensing Solutions Ltd.

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