This version is outdated by a newer approved version.
This version (16 Oct 2018 13:12) is a draft.
Approvals: 0/1
This version (16 Oct 2018 13:12) is a draft.Approvals: 0/1
This is an old revision of the document!
Table of Contents
EVAL-CN0411-ARDZ Shield Overview
CN-0411 is a total dissolved solids measurement (TDS) system using direct measurement of conductivity of the solution. The system can measure low to high conductivity levels ranging from 1 μS to 0.1 S and can accommodate 2-wire conductivity probes of different cell constants from 0.01 to 10. Temperature compensation is performed using either a 100 Ω or 1000 Ω 2-wire RTD. CN-0411 generates a bipolar square wave excitation across the conductivity probe using the AD5683R, a 16-bit SPI voltage DAC, and the ADG884, ultra-low on-resistance CMOS Dual 2:1 SPDT switch. The frequency of the excitation is controlled by a PWM signal from the microcontroller which can be set to either 2.4 kHz or 94 Hz via the system software. The range of conductivity measurement can be adjusted using gain resistors switched using the ADG1608, a 16:1 multiplexer. The conductivity cell signal is measured by the AD8220, a low-input current JFET instrumentation amplifier. Then, A track-and-hold amplifier implemented using AD8628, a zero-drift rail-to-rail single supply op amp, samples the signal for the AD7124-8, a low noise low power 24-bit Sigma-Delta ADC. With the software calibration, the calibrated system accuracy is less than 2% for all conductivity ranges from 1 μS to 0.01 S and less than 7% for conductivity ranges greater than 0.01 S. This makes the system reliable for conductivity measurement used to compute TDS.
This design uses a combination of components that allow for single supply operation which minimize circuit complexity, making this suitable for low-power and portable instrument applications. Applications include chemical water analysis for field research, and monitoring water systems and natural bodies of water.
Total Dissolved Solids Measurement
The measurement of the total dissolved solids in a solution relies primarily on the conductivity and the temperature of the solution. Furthermore, the TDS factor, used to compute the TDS from the temperature-compensated conductivity value, varies at a defined range for different types of solutions based on the type of dissolved solids. The temperature coefficient used for compensation also depends on the type of the dissolved solid. Thus, total dissolved solids remains a general measure for water quality and cannot distinguish between the constituents of the dissolved solids in the solution.
Conductivity Measurement
The system measures conductivity using a 2-wire conductivity probe to be immersed in the solution as shown below.
It is preferable that the conductivity probe be positioned at the center of the container to maximize the accuracy of the measurement. The cell constant of a 2-wire conductivity probe is the distance between its two cells or electrodes divided by their inner surface area. The cell constant of the conductivity probe sets the range of conductivity measurements it is suitable to use. Proper selection of the probe makes it easier for the system to measure at a certain conductivity range. Below is the table listing the range of conductivity measurements appropriate for the probe's cell constant.
The CN-0411 connects to the EVAL-ADICUP360 using the Arduino mating headers. Shown below is the CN-0411, connected to the EVAL-ADICUP360, with labels for the hardware connections and jumper headers.
resources/eval/user-guides/eval-adicup360/hardware/cn0411.1539688357.txt.gz · Last modified: 16 Oct 2018 13:12 by Angelo Nikko Catapang