Circuit Note

CN-0411

Circuits from the Lab® reference designs are engineered and

tested for quick and easysystemintegrationto helpsolve today’s

analog, mixed-signal, and RF design challenges. For more

informationand/orsupport, visit www.analog.com/CN0411.

Devices Connected/Referenced

AD7124-8

8-Channel, Low Noise, Low Power, 24-Bit,

Sigma-Delta ADC with PGA and Reference

AD8628

Zero-Drift, Single-Supply, RRIO Op Amp

ADG836

0.5

Ω CMOS, 1.65 V to 3.6 V Dual SPDT/2:1

MUX

AD8220

JFET Input Instrumentation Amplifier with Rail-

to-Rail Output

ADG884

0.5 Ω CMOS Dual 2:1 MUX/SPDT

ADG1608

4.5 Ω RON, 8-Channel ±5 V, +12 V, +5 V, and

+3.3 V Multiplexer

AD5683R

Tiny 16-Bit SPI nanoDAC+, with ±2 (16-Bit) LSB

INL and 2 ppm/°C External Reference

TDS Measurement System for Water Quality Monitoring

Rev. 0

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

CN-0411 Circuit Evaluation Board (EVAL-CN0411-ARDZ)

ADICUP360 Development Board (EVAL-ADICUP360)

Design and Integration Files

Schematics, Layout Files, Bill of Materials, Software

CIRCUIT FUNCTION AND BENEFITS

The total dissolved solids (TDS) present in a water system is

composed of inorganic salts and small amounts of organic

matter that are dissolved in water, and is an important measure

of water quality. TDS can be derived from the electrical

conductivity (or conductivity) of the solution by a factor

dependent on the properties, temperature and, number of ions

present. By measuring the conductivity of the solution,

determining the TDS of the system is faster, economical, and

less complicated in contrast to the more accurate gravimetric

method. The latter method involves evaporating the water and

weighing the residue, which is applicable in laboratory settings

but impractical in the field.

The circuit shown in Figure 1 is a TDS measurement system

based on the conductivity of the solution. 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.

The simplest method of measuring the conductivity of the solution

uses a 2-wire conductivity cell. Conductivity measurements

require temperature compensation for measurements taken at

temperatures other than 25˚C (or other reference temperature).

This system can reference the conductivity measurement to

room temperature using either a 100 Ω or 1000 Ω, 2-wire

resistance temperature device (RTD) and can accommodate

2-wire conductivity cells of various cell constants and operating

parameters.

The capacitance and polarization effects of the electrodes in the

conductivity cell require that the excitation signal be a bipolar

square wave with a sufficiently high frequency to reduce

polarization effects but also with sufficiently long periods to

reduce capacitance effects. To avoid damaging the conductivity

electrodes, the signal must have a very low to zero dc offset and

magnitude.

The circuit can measure the range of conductivity values from

1 µS to 0.1 S. A multiplexer switches between seven precision

resistors of different values to set the gain when measuring the

conductivity probe signal. The system can automatically

determine the gain setting of the conductivity measurement

through an auto-ranging procedure implemented in software.

The system can also be calibrated in the high conductivity range

to increase its accuracy