Total Dissolved Solids Measurements Demo

The ADuCM360_demo_cn0411 is a Total Dissolved Solids (TDS) measurements demo project, for the EVAL-ADICUP360 base board with additional EVAL-CN0411-ARDZ shield, created using the GNU ARM Eclipse Plug-ins in Eclipse environment.

General Description/Overview

The ADuCM360_demo_cn0411 project uses the EVAL-CN0411-ARDZ shield which is a single supply, low power, high precision complete solution for Total Dissolved Solids measurements, including temperature compensation. The circuit is optimized for conductivity measurements used to determine the TDS values, using conductivity cells with BNC plug.

The circuit is divided into three independent measurement front ends: TDS, conductivity and temperature. After signal conditioning, the three channels share an ad7124-8, 24-bit sigma-delta (Σ-Δ) ADC. The ad7124-8, is a low power, low noise, completely integrated analog front end for high precision measurement applications.

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For temperature compensation can be used an RTD PT100 sensor, 2-wire. The ADuCM360_demo_cn0411 application processes ADC outputs for all 5 channels (RTD, Vpeak+ and Vpeak-, VDAC, VR20S, VR200S), calculates conductivity and TDS values using as input RTD temperature value and the peak-to-peak voltage. Those data are sent to serial interface, using UART communication (115200 baud rate and 8-bits data length). The 24-bits ADC data are received using SPI interface of the EVAL-ADICUP360 board.

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The temperature value is calculated based on the RTD resistance:

Rrtd = (CODE* Rref) / (2^24 -1)                         Rref - Reference resistor (4.02kΩ)
                                                        CODE - ADC output

1. RTD resistance > 100Ω

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2. RTD resistance ≤ 100Ω

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In order to compute the total dissolved solids parameter a premeasurement procedure is run in the first place that aims to select the proper gain resistance for the measurement.

The multiplexer is set to the highest gain resistance (20MΩ) and the DAC output to a value set by the user (initially set to 400mV). Then, the positive and negative input voltage are captured via ADC channel 1 and 2. If the following formula is met:

Vp = positive input voltage
Vp + Vn > 0.3 * 2 * Vexc                 Vn = negative input voltage
Vexc = DAC output voltage

The excitation voltage used for computing tds is set to:

Vexc = 0.4 * Vexc / (Vp + Vn)

Otherwise, the gain resistor is dropped by 1 decade and the premeasurement process is repeated.

After the process is finished, the peak-to-peak voltage is measured again an the peak-to-peak current is computed:

Ipp = peak-to-peak current
Ipp = (2 * Vexc - (Vp + Vn)) / Rgain       Vexc = excitation voltage computed in the premeasurement procedure
Vp = positive input voltage
Vn = negative input voltage
Rgain = gain resistor set via multiplexer

Based on the peak-to-peak current the electrical conductance is computed, also removing the offset resistance (optional) that is obtained via the software command “refres” found in the list of available commands :

g = Ipp / ((Vp + Vn) - (Ipp * Roff))       Roff = offset resistance
                                           g = electrical conductance

The electrical conductivity is computed using the conductance and the cell constant which can be set accordingly for low conductivities, normal conductivities and high conductivities via software commands. A temperature compensation is also performed taking into account the temperature measured via RTD resistance.

s = electrical conductivity
s = k * g                                          s_cal = temperature compensated electrical conductivity
temp_coeff = solution temperature coefficient
s_cal = s / (1 + temp_coeff * (temp - t_cal))      temp = measured temperature
t_cal = reference temperature (25°C)

The calculation of total dissolved solids is the product between the temperature compensated conductivity and the tds factor corresponding to the solution that is used.

tds = k_e * s_cal                                  k_e = tds factor
                                                   tds = total dissolved solids

Demo Requirements

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

  • Hardware

    • EVAL-ADICUP360

    • EVAL-CN0411-ARDZ

    • Conductivity cell with BNC Connector

    • PT100/PT1000 RTD probe

    • Mirco USB to USB cable

    • PC or Laptop with a USB port

  • Software

    • ADuCM360_demo_CN0411 software

    • CrossCore Embedded Studio (2.7.0 or higher)

    • ADuCM36x DFP (1.0.2 or higher)

    • CMSIS ARM Pack (4.3.0 or higher)

    • Serial Terminal Program

      • Such as Putty or Tera Term

Setting up the Hardware

  • To program the base board, set the jumpers/switches as shown in the next figure. The important jumpers/switches are highlighted in red.

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  • Connect the EVAL-CN0411-ARDZ Shield to the Arduino connectors P2, P5, P6, P7, P8 of the EVAL-ADICUP360 board.

  • Connect the conductivity cell to the J1 connector of the EVAL-CN0411-ARDZ.

  • Connect the RTD sensor to the P3 connector of the EVAL-CN0411-ARDZ.

  • Connect PIN1 and PIN2 on P5 connector and PIN1 and PIN2 on P6 connector to read data from the conductivity cell.

  • Plug in the USB cable from the PC to the EVAL-ADICUP360 base board via the Debug USB.(P14)

Obtaining the Source Code

There are two basic ways to program the ADICUP360 with the software for the CN0411.

  • Dragging and Dropping the .Bin to the MBED drive

  • Building, Compiling, and Debugging using CCES

Using the drag and drop method, the software is going to be a version that Analog Devices creates for testing and evaluation purposes. This is the EASIEST way to get started with the reference design.

Importing the project into CrossCore is going to allow you to change parameters and customize the software to fit your needs, but will be a bit more advanced and will require you to download the CrossCore toolchain.

The software for the ADuCM360_demo_cn0411 demo can be found here:

Download

Prebuilt CN0411 Bin File

Complete CN0411 Source Files

Note

For more information on importing, debugging, or other tools related questions, please see the tools user guide.

Configuring the Software Parameters

  • DAC default output value - DAC_OUT_DEFAULT_VAL - set default output voltage for the DAC. (CN0411.h).

#define DAC_OUT_DEFAULT_VAL     0.4

  • KCl solution TDS factor - TDS_KCL - set the TDS factor for the KCl solution. (CN0411.h).

#define  TDS_KCL                0.5

  • NaCl solution TDS factor - TDS_NACL - set the TDS factor for the NaCL solution. (CN0411.h).

#define  TDS_NACL               0.47

  • KCl solution temperature coefficient - TEMP_COEFF_KCL - set the temperature coefficient for the KCl solution. (CN0411.h).

#define  TEMP_COEFF_KCL         1.88

  • NaCl solution temperature coefficient - TEMP_COEFF_NACL - set the temperature coefficient for the NaCl solution. (CN0411.h).

#define  TEMP_COEFF_NACL        2.14

Outputting Data

Serial Terminal Output

  • In order to view the data, you must flash the program to the EVAL-ADICUP360.

  • Once complete you will need to switch the USB cable from the DEBUG USB (P14) to the USER USB (P13).

  • Then follow the UART settings below with the serial terminal program.

Following is the UART configuration.

Select COM Port
Baud rate: 115200
Data: 8 bit
Parity: none
Stop: 1 bit
Flow Control: none

  • The user must press the <ENTER> key to start the program.

  • To get to the command menu the user must type <help> into the serial program.

  • Semihosting must be enabled to see data at the console window.

Available commands

Command

Description

help

Display available commands

syscal

Perform ADC system zero-scale calibration. Before calibration, short terminals 5 & 6 in jumper P5.

refres

Perform Referencing to a Precision Resistance. Before referencing, short terminals 3 & 4 in jumper P5.

convmod(sing/cont)

set single/continuous conversion mode for ADC.

autoset

Autoset Gain Resistance.

setdac<val>

Set DAC value (Volts).

gainres <val>

Set Gain Resistor value (Ω). <val> = 20/200/2K/20K/200K/2M/20M

rtdval <val>

Set RTD value (Ω). <val> = values 100, 1000

pwmfreq <val>

Set PWM frequency value (Hz), <val> = values 94, 2400

cellconst (low/normal/high/<val>)

Set cell constant for conductivity types.

solution (kcl/nacl/<val_tmp_coeff,val_tds_factor>)

Set parameters for specific solution.

temp

Display temperature value.

vinput (pos/neg)

Display Positive/Negative input voltage.

readdac

Read DAC value (Volts).

rdr20s

Read Voltage on R20S (Volts).

rdr200s

Read Voltage on R200S (Volts).

readdac

Read DAC value (Volts).

readdac

Read DAC value (Volts).

rdres

Read Input Resistance (Volts).

cond

Display conductivity value.

tds

Display TDS value.
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How to use the Tools

The official tool we promote for use with the EVAL-ADICUP360 is CrossCore Embedded Studio. For more information on downloading the tools and a quick start guide on how to use the tool basics, please check out the Tools Overview page.

Importing

For more detailed instructions on importing this application/demo example into the CrossCore Embedded Studios tools, please view our How to import existing projects into your workspace section.

Debugging

For more detailed instructions on importing this application/demo example into the CrossCore Embedded Studios tools, please view our How to configure the debug session section.

Project Structure

The ADuCM360_demo_cn0411 is a C++ project that uses ADuCM36x C/C++ Project structure.

This project contains: system initialization part - disabling watchdog, setting system clock, enabling clock for peripherals; port configuration for ADC, SPI read/write; for configuring and reading from AD7124, UART via P0.6/P0.7; UART read/write functions; for calibration and displaying the results.

In the src and include folders you will find the source and header files related to CN0411 software application. The Communication.c files contain SPI and UART specific data, meanwhile the CN0411.c files contain the calculation part, the AD7124.c files contain ADC channels handling. The default parameters are set at the run time, after initialization in the terminal window will appear information messages about the initial setup.

https://media.githubusercontent.com/media/analogdevicesinc/system-level/main/docs/solutions/reference-designs/eval-adicup360/images/cn0411_demo_3.png

The RTE folder contains device and system related files:

  • Device Folder – contains low levels drivers for ADuCM360 microcontroller.(try not to edit these files)

  • system.rteconfig - Allows the user to select the peripherial components they need, along with the startup and ARM cmsis files needed for the project.