CN-0349

Circuit Note

CIRCUIT DESCRIPTION

The circuit shown in Figure 1 incorporates the AD5934 12-bit

impedance converter, the ADG715 octal single-pole, single-throw

(SPST) switch, the AD8606 rail-to-rail op amp, the ADuM1250

dual I2C isolator, and the ADuM5000 isolated dc-to-dc converter to

create a complete data acquisition system for conductivity

measurements. The circuit has an 8-pin IMOD connector on

board that can be used for connection to a customer

microprocessor or a field programmable gate array (FPGA).

The AD5934 is a high precision impedance converter system

solution that combines an on-chip, programmable direct digital

synthesizer (DDS) with a 12-bit, 250 kSPS analog-to-digital

converter (ADC). The tunable frequency generator allows an

external complex impedance to be excited with a known frequency.

The AD5933 is a similar device with a 1 MSPS ADC. The on-

board ADC monitors the voltage and current of the unknown

impedance. An on-board digital signal processor (DSP) engine

calculates the discrete Fourier transform (DFT). The DFT

algorithm returns a real (R) and an imaginary (I) data-word at

each output frequency.

The AD8606 op amp was chosen because of its low offset

voltage (65 µV maximum), low bias current (1 pA maximum),

and low noise (12 nV/√Hz maximum).

The ADG715 is a complementary metal oxide semiconductor

(CMOS), octal SPST switch that is controlled via a 2-wire serial

interface that is compatible with the I2C interface standard. Low

power consumption, a low operating supply range of 2.7 V to

5.5 V, low on-resistance (typically 2.5 Ω), and a small 24-lead

TSSOP package make this device ideal for many applications.

The ADuM5000 is an isolated dc-to-dc converter with a 3.3 V

or 5 V output, based on the Analog Devices isoPower® technology,

and packaged in a 16-lead SOIC.

The ADuM1250 is a hot swappable, digital isolator with non-

latching, bidirectional communication channels that are compatible

with I2C interfaces, based on the iCoupler chip scale transformer

technology from Analog Devices, Inc., and packaged in an

8-lead SOIC.

Conductivity Theory

The resistivity, ρ, of a material or liquid is defined as the

resistance of a cube of the material with perfectly conductive

contacts on opposite faces. The resistance for other shapes can

be calculated by

R = ρL/A

where:

L is the distance between the contacts.

A is the area of the contacts.

Resistivity is measured in units of Ω cm. A 1 Ω cm material has

a resistance of 1 Ω when contacted on opposite faces of a 1 cm ×

1 cm × 1 cm cube.

Conductance is the reciprocal of resistance, and conductivity is

the reciprocal of resistivity.

All aqueous solutions conduct electricity to some degree. The

measure of the ability of a solution to conduct electricity is

called conductance, and it is the reciprocal of resistance. The

unit of measurement of conductance is Siemens (S). Adding

electrolytes such as salts, acids, or bases to pure water increases

the conductance (and decreases resistance). Resistivity is

expressed in Ω cm, and conductivity is expressed in S/cm,

mS/cm, or µS/cm.

For the purposes of this circuit note, Y is used as the general

symbol for conductivity measured in S/cm, mS/cm, or µS/cm.

However, in many cases, the distance term is dropped for

convenience, and the conductivity is simply expressed as S,

mS, or µS.

A conductivity system measures conductance by means of

electronics connected to a sensor immersed in a solution. The

analyzer circuitry impresses an alternating voltage on the sensor

and measures the size of the resulting current, which is related

to the conductivity. Because conductivity has a large temperature

coefficient (up to 4%/°C), an integral temperature sensor is

incorporated into the circuitry to adjust the reading to a

standard temperature, usually 25°C (77°F). When measuring

solutions, the temperature coefficient of the conductivity of the

water itself must be considered. To compensate accurately for

the temperature, a second temperature sensor and

compensation network must be used.

Conductivity Sensors

The contacting type sensor usually consists of two electrodes

that are insulated from one another. The electrodes, typically

Type 316 stainless steel, titanium palladium alloy, or graphite,

are specifically sized and spaced to provide a known cell

constant. Theoretically, a cell constant of 1.0/cm describes two

electrodes, each sized one square centimeter in area, and spaced

one centimeter apart. Cell constants must be matched to the

analyzer for a given range of operation. For instance, if a sensor

with a cell constant of 1.0/cm is used in pure water with a

conductivity of 1 µS/cm, the cell has a resistance of 1 MΩ.

Conversely, the same cell in seawater has a resistance of 30 Ω.

Because the resistance ratio is so large, it is difficult for ordinary

instruments to accurately measure such extremes with only one

cell constant.

When measuring the 1 µS/cm solution, the cell is configured

with large electrodes spaced a small distance apart. For

example, a cell with a cell constant of 0.01/cm results in a cell

resistance of approximately 10,000 Ω, which can be measured

quite accurately. Therefore, the measuring instrument can

operate over the same range of cell resistance for both ultra-

pure water and high conductivity seawater, by using cells with

different cell constants.

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