Non-dispersive infrared (NDIR) spectrometers are often used to detect gases and measure the concentration of carbon oxides such as carbon monoxide and carbon dioxide. An infrared beam passes through the sampling chamber, and each gas component in the sample absorbs infrared rays of a specific frequency. The concentration of the gas component can be determined by measuring the amount of infrared absorption at the corresponding frequency. The reason why this technique is non-dispersive is because the wavelength through the sampling cavity is not pre-filtered; instead, the optical filter is placed in front of the detector to filter out all rays outside the wavelength that the selected gas molecule can absorb. .
The circuit shown in Figure 1 is a complete circuit of a thermopile gas sensor based on the NDIR principle. The circuit is optimized for carbon dioxide detection, but the thermopile with different filters can also accurately measure the concentration of multiple gases.
The printed circuit board (PCB) is sized with the Arduino expansion board and interfaces with the Arduino compatible platform board EVAL-ADICUP360. Signal Tuning Reasons The AD8629 and ADA4528-1 low noise amplifiers are implemented along with the precision analog microcontroller ADuCM360, which integrates a programmable gain amplifier, dual 24-bit sigma-delta analog-to-digital converter (ADC), and ARM® Cortex® -M3 processor.
Thermopile sensors consist of a large number of thermocouples that are typically connected in series (or occasionally in parallel). The output voltage of the series thermocouple depends on the temperature difference between the thermocouple junction and the reference junction. This principle is called the Seebeck effect and is named after its discoverer, Thomas Johann Seebeck.
This circuit uses the AD8629 op amp to amplify the thermopile sensor output signal. Thermopile output voltages are relatively small (from a few hundred microvolts to a few millivolts), requiring high gain and very low offset and drift to avoid DC errors. The high internal resistance of the thermopile (typically 84 kΩ) requires an amplifier with low input bias current to minimize errors, while the AD8629 has a bias current of only 30 pA (typ). The device drifts very slowly with time and temperature and does not introduce additional errors after the calibration temperature is measured. A pulsed source that is synchronized with the ADC sampling rate minimizes errors caused by low frequency drift and flicker noise.
The voltage noise spectral density of the AD8629 at 1 kHz is only 22 nV/√Hz, which is lower than the voltage noise density of the thermopile 37 nV/√Hz.
The AD8629's current noise spectral density at 10 Hz is also very low, typically 5 fA/√Hz. This current noise flows through the 84 kΩ thermopile, and the noise contribution at 10 Hz is only 420 pV/√Hz.
Figure 1. NDIR gas detection circuit (schematic diagram: not showing all connections and decoupling)
The low-noise amplifier ADA4528-1 is used as a buffered sensor with a common-mode voltage of 200mV, so the NTC and thermopile signal outputs meet the ADuCM360 buffer mode input requirements: ADuCM360 ADC buffer mode input is AGND + 0.1 V to approximately AVDD - 0.1 V. CN-0338 The Arduino expansion board is compatible with other types of Arduino compatible platforms with single-ended input ADCs.
The circuit's chopping frequency range is from 0.1 Hz to 5 Hz and is software selectable. The low dropout regulator ADP7105 l generates a stable 5 V output voltage to drive the IR source and is controlled by the ADuCM360. The ADP7105 features a soft-start feature that eliminates inrush currents generated by cold-start sources.
The ADuCM360 integrates a dual, 24-bit, sigma-delta ADC that simultaneously samples dual thermopile cells over a programmable rate range of 3.5 Hz to 3.906 kHz. The data sampling rate range of the NDIR system is limited to between 3.5 Hz and 483 Hz for optimum noise performance.
Thermopile detector working principleIn order to understand the thermopile, it is necessary to review the basic theory of thermocouples.
If two different metals are connected at any temperature above absolute zero, a potential difference (thermoelectric EMF or contact potential) is generated between the two metals. This potential difference is a function of junction temperature (see thermoelectric EMF in Figure 2). Circuit).
If the two wires are connected at two locations, two junctions are formed (see the thermocouple connected to the load in Figure 2). If the temperatures at the two junctions are different, a net EMF is generated in the circuit and a current flows through it, which is determined by the EMF and the total resistance of the circuit (see Figure 2). If one of the wires is broken, the voltage at the breakpoint is equal to the net thermoelectric EMF of the circuit; and if the voltage is measurable, it can be used to calculate the temperature difference between the two junctions (see thermocouple in Figure 2). Voltage measurement). Remember that a thermocouple measures the temperature difference between two junctions, not the absolute temperature at a junction. The temperature at the measurement junction can only be measured if another junction (often referred to as a reference junction or cold junction) is known.
However, it is difficult to measure the voltage generated by the thermocouple. Assume that the voltmeter is connected to the first thermocouple measurement circuit (see the actual thermocouple voltage measurement shown in Figure 2 for cold junction). The wires connected to the voltmeter form more thermocouples at the junction. If these additional junction temperatures are the same (regardless of temperature), the intermediate metal rule indicates that they have no net contribution to the total EMF of the system. If their temperatures are different, an error occurs. Thermoelectric EMF is produced for each pair of different contact metals – including copper/solder joints, kovar/copper sheets (can be an alloy for IC leadframes) and aluminum/corinable (welding in ICs) ) - In practical circuits, the problem is more complicated, and it is necessary to be extremely careful to ensure that all junction pairs of the thermocouple peripheral circuit (except the measurement node and the reference node itself) have the same temperature.
Figure 2. Thermocouple principle
The thermopile is made up of a large number of thermocouples connected in series, as shown in Figure 3. The thermoelectric voltage generated by the thermopile is much higher than that of a single thermocouple.
Figure 3. Multiple thermocouples forming a thermopile
In NDIR applications, filtered pulsed infrared light is applied to the series active nodes; therefore, the junctions heat up, producing a smaller thermoelectric voltage. The temperature of the reference junction is measured by the thermistor.
The positive or negative charge center transients or steady states of many gases do not coincide. In the infrared spectrum, gases can absorb specific frequencies, which can be used for gas analysis. When infrared radiation is injected into the gas, and when the self-resonant frequency of the molecule matches the infrared wavelength, the gas molecules will resonate with the incident infrared light according to the energy level transition of the atom.
For most infrared gas detection applications, the composition of the target gas is known and therefore does not require GC analysis. However, if the absorption lines of different gases overlap, the system must deal with the mutual interference between these gases.
Carbon dioxide has an absorption peak between 4200 nm and 4320 nm, as shown in Figure 4.
Figure 4. Absorption spectrum of carbon dioxide (CO2)
The output wavelength range of the infrared source and the absorption spectrum of the water also determine the choice of detection wavelength. Below 3000 nm, and between 4500 nm and 8000 nm, water has a strong absorption. If there is moisture in the target gas (high humidity), the detected gas will be affected by strong interference within these ranges. Figure 5 shows the carbon dioxide absorption spectrum overlapping the absorption spectrum of water. (All absorption data comes from the HITRAN database).
Figure 5. Carbon dioxide and water absorption spectrum overlap
If infrared light is applied to the dual thermopile sensor and a pair of filters is installed such that one of the filters has a center wavelength of 4260 nm and the other center wavelength is 3910 nm, then the voltages of the two thermopiles are measured. The ratio of carbon dioxide can be measured. A filter whose center wavelength overlaps with the absorption wavelength of carbon dioxide is used as a measurement channel, and a filter whose center wavelength is outside the absorption wavelength of carbon dioxide is used as a reference channel. When the reference channel is used, the measurement error caused by dust or radiation intensity attenuation can be eliminated. It is important to note that carbon dioxide and water vapor do not absorb almost all of the 3910 nm infrared light; this makes the area an ideal location for the reference channel.
The thermopile used in the NDIR test has a relatively high internal resistance, while 50 Hz/60 Hz power line noise is coupled into the signal path. The internal resistance of the thermopile may be around 100 kΩ, causing thermal noise to become the main noise in the system. For example, the thermopile sensor voltage noise density selected in the system of Figure 1 is 37 nV/√Hz. In order for the system to have the best performance, the sensor should be output with as large a signal as possible and a lower gain in the circuit.
The best way to maximize the signal from the thermopile sensor is to use a chamber with high reflection characteristics, which ensures that as much radiation as possible enters the detector without being absorbed by the chamber. The use of a reflective chamber to reduce the amount of radiation absorbed by the chamber also reduces system power consumption because a low power radiation source can be used.
Bill-Lambert law for NDIR gas absorptionThe infrared intensity of the measurement channel sensor is decremented exponentially. This relationship is called the Beer-Lambert law:
among them:
I indicates the intensity of the emitted light.
I0 represents the incident light intensity.
k represents the absorption coefficient of a specific gas and filter combination.
l represents the equivalent optical path length between the source and the detector.
x represents the gas concentration.
For the measurement channel sensor output, there is a corresponding output voltage change V0 – V:
among them:
FA represents the relative absorption rate.
V0 indicates that the incident light intensity corresponds to the sensor output.
V indicates that the outgoing light intensity corresponds to the sensor output.
Organize the formula and combine the two previous formulas to get:
If k and l remain unchanged, the FA can be plotted against x, as shown in Figure 6 (where kl = 115, 50, 25, 10, and 4.5). The FA value increases with c but eventually saturates at high gas concentrations.
Figure 6. Typical relative absorption rate (kl = 4.5, 10, 25, 50, 115)
This relationship indicates that for any fixed setting, the effect of gas on the relative absorption rate is higher than the high concentration at low concentrations; however, k and l can be adjusted to provide optimal absorption for the desired gas concentration range. This means that longer optical paths are more suitable for low gas concentrations, while shorter optical paths are more suitable for high gas concentrations.
A two-point calibration step is described below, which is necessary in the case of determining the kl constant using the ideal Beer-Lambert formula. If b = kl, then
The first step in the calibration requires the application of a low concentration of carbon dioxide gas (or pure nitrogen, or 0% carbon dioxide gas) to the sensor assembly.
ACTLOW indicates the peak-to-peak output of the measurement channel sensor in a low concentration gas environment.
REFLOW represents the peak-to-peak output of the reference channel sensor in a low concentration gas environment.
TLOW indicates the temperature of the low concentration gas.
The second step of the calibration requires the application of a known concentration (xCAL) of carbon dioxide gas to the assembly. Typically, the xCAL concentration level selects the maximum value within the concentration range (eg, for industrial air quality ranges, 0.5% volume concentration is selected).
ACTCAL indicates the peak-to-peak output of the channel sensor when the calibration gas concentration is xCAL.
REFCAL indicates the peak-to-peak output of the reference channel sensor when the calibration gas concentration is xCAL.
This way you can write the following simultaneous equations with two unknowns (I0 and b):
Solve the I0 and b of the two equations,
Then, for gases of unknown concentration (x), where:
ACT represents the peak-to-peak output of the measurement channel sensor in an unknown gas environment.
REF represents the peak-to-peak output of the reference channel sensor in an unknown gas environment.
T represents the temperature of the unknown gas in K.
The coefficient T/TLOW compensates for the effect of temperature changes on the gas concentration (the ideal gas law is used here).
Amend the Bill-Lambert lawFor practical reasons, when using NDIR, you need to modify the Beer-Lambert law to get an accurate reading, as shown below:
Since not all infrared radiation reaching the thermopile has experienced ideal gas absorption (even if the gas concentration is high), the SPAN factor is introduced. Due to the fine structure of the filter bandwidth and absorption spectrum, SPAN is less than one.
Variations in the length of the optical path and scattering of light require an increase in the exponential term c in order to make the equation exactly match the actual absorption data.
The b and SPAN constant values ​​are also dependent on the measured concentration range. Typical concentration ranges are as follows:
Industrial Gas Quality (IAQ): 0 to 0.5% vol. (5000 ppm). Note that the concentration of carbon dioxide in ambient air is approximately 0.04% vol., or 400 ppm.
Safety protection: 0 to 5% vol.
Combustion: 0 to 20% vol.
Process control: 0 to 100% vol.
The actual values ​​of b and c for a particular system are typically obtained from a data point on the curve of FA versus concentration x using a curve fitting program.
For a given system where the b and c constants have been determined, the values ​​of ZERO and SPAN can be calculated using a two-point calibration method.
The first step in this process is to inject a low concentration of xLOW gas and record the following:
ACTLOW: Measure the peak-to-peak output of the channel sensor in a low concentration gas environment.
REFLOW: Peak-to-peak output of the reference channel sensor in a low concentration gas environment.
TLOW: The temperature of the low concentration gas in K.
The second step of the calibration requires the application of a known concentration (xCAL) of carbon dioxide gas to the assembly. Typically, the xCAL concentration level selects the maximum value within the concentration range (eg, for industrial air quality ranges, 0.5% volume concentration is selected). Record the following:
ACTCAL: Measures the peak-to-peak output of the channel sensor when the calibration gas concentration is xCAL.
REFCAL: Peak-to-peak output of the reference channel sensor when the calibration gas concentration is xCAL.
This way you can write the following simultaneous equations with two unknowns (I0 and SPAN):
Solve ZERO and SPAN in two equations:
Then, for gases of unknown concentration (x), where:
ACT represents the peak-to-peak output of the measurement channel sensor in an unknown gas environment.
REF represents the peak-to-peak output of the reference channel sensor in an unknown gas environment.
T represents the temperature of the unknown gas in K.
This equation assumes TLOW = TCAL.
Environmental temperature effectThermopile sensors detect temperature by absorbing radiation, but also respond to changes in ambient temperature, resulting in increased spurious and interfering signals. For this reason, many thermopile sensors have integrated thermistors in the package.
Radiation absorption is related to the number of target molecules in the chamber, not the absolute percentage of the target gas. Therefore, absorption is expressed using the ideal gas law at standard atmospheric pressure.
It is necessary to record the temperature data in both the calibration state and the measurement state at the same time:
among them:
x represents the gas concentration at the time of no temperature compensation.
TLOW indicates the gas temperature at the time of calibration, and the unit is K.
T represents the temperature at the time of sampling, and the unit is K.
xT represents the gas concentration at a temperature T.
Under the ideal gas law, except that the concentration will vary with temperature, SPAN and FA will also change slightly with temperature, and may need to be corrected for highly accurate concentration measurements.
This article does not cover SPAN and FA temperature corrections, but can be taken from SGX Sensor tech application notes 1, application notes 2, application notes 3, application notes 4 and application notes 5, and Alphasense Limided application notes A AN-201, A AN Get details in -202, A AN-203, AAN-204 and AAN-205.
Thermopile driverEach channel of the HTS-E21-F3.91/F4.26 thermopile (Heimann Sensor, GmbH) has an internal resistance of 84 kΩ. The single-channel equivalent drive circuit is shown in Figure 7. The internal 84 kΩ thermopile internal resistance and the external 8.2 nF capacitor form an RC low-pass noise filter with a -3 dB cutoff frequency of:
Changing the C11 and C15 of different thermopiles also changes the noise performance and response time.
Figure 7. Thermopile Driver Equivalent Circuit, G = 214.6
The step-function 22-bit settling time of the 84 kΩ/8.2 nF filter is approximately:
The gain of the AD8629 non-inverting amplifier is set to 214.6, and the -3 dB cutoff frequency is:
The 22-bit setup time is approximately:
The NDIR maximum chopping frequency is 5 Hz, so the minimum half-cycle pulse width is 100 ms. The 22-bit settling time is approximately 0.1 times the minimum chopping pulse width.
The AD8629's 0.1 Hz to 10 Hz input voltage noise is 0.5 μV pp. Ignoring sensor voltage noise and AD8629 current noise, the thermopile's 1 mV pp signal output has the following signal-to-noise ratio (SNR):
One of the thermopiles is connected to the ADuCM360 ADC1/ADC3 input pins with a pseudo differential input and the other to the ADC2/ADC3 input pins. The ADC3 input pin is connected to a 200 mV common-mode voltage and is driven by the low noise amplifier ADA4528-1. The ADA4528-1's 0.1 Hz to 10 Hz input voltage noise is 99 nV pp. To keep the ADC input pin above 0.1 V, a 200 mV common-mode voltage is required.
The AD8629 stage has a gain of 214.6. The ADuCM360's internal PGA gain is automatically set by software from 1 to 128, ensuring that the input signal matches the full-scale range of the ADC input (ie ±1.2 V). The peak-to-peak signal from the thermopile ranges from a few hundred microvolts to a few millivolts. For example, assuming a full-scale thermopile signal of 1 mV pp, PGA gain 4 produces an 860 mV pp ADC input signal.
Thermopiles with different sensitivities may require different gains for the AD8629 stage. If you need to connect the Arduino expansion board to an Arduino platform that does not have an integrated PGA inside the ADC, you may need higher gain.
The easiest way to change the gain of the AD8629 is to change R6 and R10; this does not affect the main pole frequency determined by R5/R8 and C9/C10.
The software can select the thermopile output data processing algorithm. The user can choose between a peak-to-peak algorithm and an averaging algorithm.
For more information on signal acquisition, source pulse timing, and temperature compensation processing algorithms, see the CN-0338 source code in . .
NTC thermistor driverThe characteristics of the integrated NTC temperature sensor in the thermopile are as follows:
RTH = 100 kΩ
β = 3940
See Figure 8 for the Thevenin equivalent circuit of the thermistor driver. The R3 and R4 divider resistors provide a 670.3 mV voltage source in series with a 103.6 kΩ resistor. The drive voltage is 670.3 mV -200 mV = 470.3 mV.
Figure 8. NTC thermistor driver equivalent circuit
When RTH = 100 kΩ (25 °C), the voltage across the thermistor is 231 mV, so the PGA gain is set to 4 when measuring.
The flexible input multiplexer and dual ADC in the ADuCM360 support simultaneous sampling of thermopile signals and temperature sensor signals to compensate for drift.
Infrared light source driverInternaTIonal Light Technologies MR3-1089 is used as the infrared source with a polished aluminum reflector that requires a drive voltage of 5.0 V at 150 mA to maximize infrared radiation and achieve optimum system performance. The heat from the lamp keeps the temperature of the light reflector above ambient temperature, helping to prevent condensation in the humid environment.
At lower temperatures (turning off the light), the filament has a lower resistance, which causes a current surge at the moment the light is turned on. A voltage regulator with a soft-start function is useful for solving this problem.
The low dropout regulator has a programmable enable pin that connects to the DuCM360's general purpose input/output pins for switching control of the source. The 10 nF soft-start capacitor C6 has a soft-start time of 12.2 ms, which is approximately equal to 0.125 times the minimum chop step time of 100 ms.
The on-current (~150 mA) of the lamp is large, so the circuit design and layout must be carefully designed to prevent the switching pulse of the lamp from coupling to the tiny thermopile output signal.
Carefully ensure that the return path of the lamp does not flow through the sensitive thermopile sensor ground return path. The current loop of the lamp may not overlap with the current loop of the processor, otherwise a voltage offset error may occur. It is highly recommended to use a separate voltage regulator for the lamp drive and for the signal conditioning portion of the system.
The ADP7105 light source driver is powered directly from an external power supply connected to the EVAL-ADICUP360 board.
Software considerations for synchronous chopping and samplingTo measure the gas concentration, the peak-to-peak signal values ​​in the reference and measurement channels must be sampled. The ADuCM360 integrates two 24-bit, sigma-delta ADCs that operate in continuous sampling mode. The ADC is driven by a programmable gain amplifier with gain options of 1, 2, 4, 8, 16, 32, 64, and 128.
The default chopping frequency is set to 0.25 Hz and the default sampling rate is set to 10 Hz. However, you can set the chopping frequency in the software from 0.1 Hz to 5 Hz; you can also set the ADC sampling rate from 3.5 Hz to 483 Hz. The software guarantees a sampling rate of at least 30 times the chopping frequency.
For a default chopping frequency of 0.25 Hz, the thermopile data is obtained at a 10 Hz sampling rate in the second 1.5 seconds of the 2 second half cycle, ensuring that the signal is fully established. The first 500 ms of data (blanking time) is ignored. The blanking time can also be set in the software, and the rising and falling edges can be set separately. Note that NTC thermistor data is obtained during blanking.
Calibration procedure: ideal Bill-Lambert equation
Due to the different characteristics of the lamp and thermopile, the circuit must be calibrated for initial use and when changing the thermopile or lamp.
It is recommended to place the entire assembly in a sealed chamber and to inject a known concentration of carbon dioxide gas into it until all of the original gas in the chamber is expelled. After a few minutes of stabilization, you can start measuring.
The calibration method and algorithm for the ideal Beer-Lambert equation are shown in the following steps:
1. Enter the following command: sbllcalibrate (standard Bill-Lambert calibration).
2. Inject a low concentration (xLOW) or zero concentration gas (nitrogen) and stabilize the gas in the chamber.
3. Enter the carbon dioxide concentration at the terminal.
4. The system measures ACTLOW, which represents the peak-to-peak output of the measurement channel sensor in a low concentration gas.
5. The system measures REFLOW, which represents the peak-to-peak output of the reference channel sensor in the low concentration gas.
6. The system measures the temperature of the low concentration gas TLOW.
7. Inject a high concentration of carbon dioxide at a concentration of xCAL into the chamber.
8. Enter the CO2 concentration at the terminal.
9. The system measures ACTCAL, REFCAL and calibration temperature TCAL.
10. The system calculates the ZERO and b values:
To measure an unknown concentration of carbon dioxide gas using the ideal Beer-Lambert equation, follow these steps:
1. Inject the chamber with an unknown concentration of gas and stabilize it.
2. Measure ACT, which represents the peak-to-peak output of the measurement channel sensor.
3. Measure REF, which represents the peak-to-peak output of the reference channel sensor.
4. Measure the temperature T in K.
5. Use the calibrated ZERO value.
6. Use the calibrated b value.
7. Calculate the relative absorption rate:
Calculate the concentration and apply the temperature compensation under the ideal gas law:
This step assumes TLOW = TCAL.
Note that CN-0338 software will automatically perform steps 2 through 7.
Calibration procedure: Correction of the Beer-Lambert equation
If the values ​​of the constants b and c are obtained by measurement, the following steps are used.
1. Enter the following command: mbllcalibrate (corrected Bill-Lambert calibration).
2. Enter the b and c constants.
3. Inject low concentration (xLOW) carbon dioxide gas (nitrogen) and stabilize the gas in the chamber.
4. Enter the CO2 concentration at the terminal.
5. The system measures ACTLOW, which represents the peak-to-peak output of the measurement channel sensor in a low concentration gas.
6. The system measures REFLOW, which represents the peak-to-peak output of the reference channel sensor in the low concentration gas.
7. The system measures the temperature TLOW.
8. Inject a high concentration of carbon dioxide at a concentration of xCAL into the chamber.
9. Enter the CO2 concentration at the terminal.
10. The system measures ACTCAL, REFCAL and calibration temperature TCAL.
11. The system calculates ZERO and SPAN:
To measure an unknown concentration of carbon dioxide gas using the modified Beer-Lambert equation, follow these steps:
1. Inject the chamber with an unknown concentration of gas and stabilize it.
2. Measure ACT, which represents the peak-to-peak output of the measurement channel sensor.
3. Measure REF, which represents the peak-to-peak output of the reference channel sensor.
4. Measure the temperature T in K.
5. Use the calibrated ZERO and SPAN values.
6. Use the previously determined b and c values.
7. Calculate the relative absorption rate:
Calculate the concentration and apply the temperature compensation under the ideal gas law:
This step assumes TLOW = TCAL.
NTC thermistor algorithm and calculation
The NTC thermistor equivalent circuit is shown in Figure 9.
Figure 9. NTC thermistor circuit
The voltage across the thermistor is:
among them:
VCC is 3.3 V.
RNTC is the thermistor value.
The NTC thermistor value can be expressed as:
among them:
RTH represents the thermistor value at a temperature of T0.
β is a parameter in the NTC thermistor data sheet.
RNTC represents the thermistor value at temperature T.
Combine the above two equations to get:
During the chopping interval of each lamp, the ADC switches to NTC sampling, as shown in Figure 10.
Figure 10. NTC and thermopile sampling timing and chopping user interaction interface for the lamp
The EVAL-ADICUP360 platform board connects to the PC via a USB port. The board is shown as a virtual COM device. Any type of serial terminal can interact with the EVAL-ADICUP360 board for development and debugging. See Circuit Note CN-0338 for more information on software operation.
Figure 11 shows the relative absorbance (FA) of a typical plate as a function of carbon dioxide concentration.
Figure 11. Relative absorption rate of typical EVAL-CN0338-ARDZ plate versus carbon dioxide concentration
The complete design support package for the EVAL-CN0338-ARDZ board includes layout files, bill of materials, schematics, and source code.
The functional block diagram of the test setup is shown in Figure 12. The EVAL-CN0338-ARDZ Arduino expansion board and the EVAL-ADICUP360 Arduino compatible platform board are shown in Figure 13.
Figure 12. Test setup functional block diagram
Figure 13. Summary of EVAL-CN0338-ARDZ board and EVAL-ADICUP360 board photo
The analog electronics required to implement NDIR measurements include precision low noise amplifiers and high resolution analog to digital converters. The circuit described in this article is a highly integrated solution that utilizes the precision analog microcontroller ADuCM360 to perform precision PGA functions, precision sigma-delta ADC conversion, and digital control and processing.
Arduino's extended compatibility capabilities enable rapid development of NDIR design prototypes and custom software tailored to specific application requirements.
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