This version (07 Jan 2021 09:41) was approved by Ioana Chelaru.The Previously approved version (06 Jan 2021 09:58) is available.

Volatile Organic Compounds (VOC) Measurement Demo

The CN0395_example is a volatile organic compounds (VOC) detector demo project for the Arduino Uno base board with additional EVAL-CN0395-ARDZ Shield, created using the Arduino Genuino IDE.

General description

This project is a good example of using ADI shields with Arduino boards for fast and easy prototyping.

The CN0395_example project uses the EVAL-CN0395-ARDZ Shield which is a portable VOC detector which comes with a Figaro TGS8100 MOX sensor.

The TGS8100 sensor requires two voltage inputs: heater voltage (VH) and circuit voltage (VC). The heater voltage (VH) is applied to the integrated heater in order to maintain the sensing element at a specific temperature which is optimal for sensing. The EVAL-CN0395-ARDZ Shield circuit provides the heater voltage (VH), by using ADN8810 IDAC as a programmable current source. The default full scale current in the IDAC is 9.94mA and the default value of the RSN resistors is 41.2Ω.

The hardware also allows for two main modes of operation: heater mode and sensor resistance measurement mode. In heater mode, the AD7988-1 ADC receives as input the heater voltage (VH) while in sensor mode the input is the voltage from the sense circuit (VRS). The switching is done by using ADG884. The full scale voltage measured by the ADC is 4.096 V.

Moreover, the hardware includes a gain select circuit which can add additional overlapping ranges if needed when performing a resistance sensor measurement. ADG758 8-channel multiplexer is used to accomplish this task.

TGS8100 sensor has a temperature and humidity dependency, therefore temperature compensation is performed. The EVAL-CN0395-ARDZ shield includes a temperature and humidity sensor (Sensirion SHT-30), which operates on I2C bus.

The CN0395_example uses a UART interface (9600 baud rate and 8-bits data length), as a command line interpreter, to send the results to terminal window. Beside the interpreter processes several commands.

Demo Requirements

The following is a list of items needed in order to replicate this demo.

Hardware
- Arduino Uno board
- EVAL-CN0395-ARDZ
- USB type A to USB type B cable
- PC or Laptop with a USB port
Software
- CN0395_example software
- Arduino/Genuino IDE
- Arduino AVR Boards Built-In by Arduino package (1.6.23 or higher)

Setting up the hardware

Connect the EVAL-CN0411-ARDZ Shield to the Arduino connectors DIGITAL (PWM~), POWER, ANALOG IN and ICSP of the Arduino Uno board.
Plug in the USB cable from the PC to the Arduino Uno base board via the type B port on the board.

Obtaining the source code

We recommend not opening the project directly, but rather make a local copy in your workspace and open it using Arduino/Genuino IDE.

The source code and include files of the CN0395_example can be found on Github:

CN0395_example at Github

Configuring the Software Parameters

There are no configurations needed for this demo example.

Outputting Data

Serial Terminal Output

In order to view the data, you must flash the program to the Arduino Uno board by pressing the Upload button in the IDE.
Once complete Press the serial monitor button.
Make sure the UART settings are “Carriage return” and “9600 baud”.

Following is the UART configuration.

  Carriage return
  9600 baud

The application allows the user to select between the two modes of operation:

Heater mode (RH)
Sensor Resistance mode (RS)

Heater Mode (RH)

The user can further choose the subroutine which determines the heater current (IH):

voltage is the routine for setting heater voltage to constant voltage VH (the default value is VH = 1.8V). The relationship between heater resistance RH and heater current IH or heater voltage VH is nonlinear. Therefore the software runs in background several iterations in order to get VH to the desired accuracy with a 0.5% max error.

resistance is the routine for setting the heater resistance to constant resistance RH (the default is RH = 225Ω). For a heater resistance RH, set IH = (RH – 110Ω)/14375 [Note: The slope of the RH vs. IH curve is 115Ω/8mA = 14375Ω/A]. The software runs in background several iterations in order to get RH to the desired accuracy with a 0.5% max error.

temperature is the routine of setting the heater temperature to constant temperature TH (the default is TH = 360 C). This is done in three steps:

The desired heater resistance RH_T is computed from RH_T = RH_A [ 1 + ALPHA*(RH_0/RH_A)*(T – T_A)], where:
- RH_A is the heater ambient temperature (measured at power up)
- RH_0 is the default heater resistance (110Ω @ 20°C)
- T is the desired heater temperature (user input)
- T_A is the ambient temperature (measured at power up)
- ALPHA is the constant 0.003074
The resulted RH is used to calculate IH and VH with constant resistance routine.
The resulted VH is further adjusted by using the constant heater voltage routine.

current simply sets the IDAC to the desired current.

After the completion of the routine, the application displays the measured values: RH_A (Ambient Heater Res ), VH (heater voltage), IH (heater current), RH (heater resistance), T_A (ambient temperature), HUM (ambient humidity), PH (heater power consumption), TH (heater temperature), ADC data (raw data read from ADC in hex), Ro ( sensor resistance measured in clean air).

At power up, the application starts in constant current mode and sets the default current to 8mA. Furthermore, it is assumed that the measurement circuit is placed in clean air, therefore we measure and store the sensor resistance in clean air (Ro). After each heater measurement mode change, it is assumed that the board is placed in clean air, and the Ro value is updated. This is required, because Ro is a function of the heater temperature.

Sensor Resistance mode (RS)

Sensor measurement is performed. The application can switch at any time to this mode by pressing the <ENTER> key. The AD7988-1 ADC receives the voltage from the sense circuit (VRS). The switching is done by using ADG884.

In background every time the application runs the gain-ranging algorithm and determines RS and the gas concentration (C) measured in PPM (parts per million):

RS reading can also be performed by typing <operation RS>, but it does the same thing as pressing the <ENTER> key.

Factory Calibration The IDAC current from the ADN8810 is 1% accurate, therefore a factory calibration must be performed. The routine loads code 4095 into ADN8810 and reads the ADC, which ideally should be 9.94mA × 71.5Ω = 0.71V, or code [0.71/4.096] x 65,535 = 11,360. The gain correction factor k1 = 11,360/CODEFS. It is recommended that this is done only once. Follow the procedure:

Type <calibrate w>
Connect jumper P2 between P2-1 and P2-2, this connects the IOUT to the precision 71.5Ω resistor.
Press <c> key when ready
Power Off → On

From this point on, K1 is stored in permanent memory and applied to currents that are input. To read the gain correction factor from memory, type <calibrate r>.

Help

Type <help> to see the available commands:

Project structure

The CN0411_example is a C++ Arduino sketch.

This project contains: system initialization part - setting Digital IO pins in the right mode; port configuration for SPI1, UART via Digital pin 0/Digital pin 1, I2C via SDA/SCL pins; SPI, UART, I2C read/write functions; AD7988 control, ADN8810 control, SHT30 control and VOC concentration computation.

All files are in the same folder as the .ino file and include the source and header files related to CN0395 software application. The Communication.cpp/h files contain SPI, UART and I2C specific data, the AD7988.cpp/h files contain the ADC control, the ADN8810.cpp/h files contain the IDAC control, the SHT30.c/h files contain the temperature/humidity sensor control, and the CN0395.cpp/h files contain commands, configurations and computations specific to the VOC detector application.

End of Document

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