Circuit Note

CN-0359

analog.com

Rev. C | 4 of 10

Figure 3. 2-Pole and 4-Pole Conductivity Cells

The 2-pole sensor is more suitable for low conductivity measure-

ments, such as purified water, and various biological and pharma-

ceutical liquids. The 4-pole sensor is more suitable for high conduc-

tivity measurements, such as waste water and seawater analysis.

The cell constants for 2-pole cells typically range from approximate-

ly 0.1/cm to 1/cm, and the cell constants for 4-pole cells typically

range from 1/cm to 10/cm.

The 4-pole cell eliminates the errors introduced by polarization of

the electrodes and field effects that can interfere with the measure-

ment.

The actual configuration of the electrodes can be that of parallel

rings, coaxial conductors, or others, rather than the simple parallel

plates shown in Figure 2.

Regardless of the type of cell, it is important not to apply a DC

voltage to any electrode, because ions in the liquid accumulate on

the electrode surface, thereby causing polarization, measurement

errors, and damage to the electrode.

Take special care with sensors that have shields, as in the case

of coaxial sensors. Connect the shield to the same potential as

the metal container holding the liquid. If the container is grounded,

connect the shield to the circuit board ground.

The final precaution is not to exceed the rated excitation voltage

or current for the cell. The CN0359 circuit allows programmable

excitation voltages from 100 mV to 10 V, and the 1 kΩ series

resistor limits the maximum cell current to 10 mA.

EXCITATION GENERATION

The excitation square wave for the conductivity cell is generated by

switching the ADG1419 between the +VEXC and −VEXC voltages

using the pulse-width modulation (PWM) output of the ADuCM360

microcontroller. It is important that the square wave has a precise

50% duty cycle and a very low DC offset. Even small DC offsets

can damage the cell over a period of time.

The +VEXC and −VEXC voltages are generated by the ADA4077-2

op amps, and their amplitudes are controlled by the DAC output of

the ADuCM360, as shown in Figure 4.

Figure 4. Excitation Voltage Sources

The ADA4077-2 has a typical offset voltage of 15 μV (A grade), a

0.4 nA bias current, and a 0.1 nA offset current, which minimizes

residual DC content of the excitation waveform. Output current is up

to ±10 mA, allowing use with very high conductivity liquids; and the

low 1.2 V dropout maximizes the available output swing.

The ADA4077-2 op amp has a closed-loop gain of 8.33 and

converts the ADuCM360 internal DAC output (0 V to 1.2 V) to

the +VEXC voltage of 0 V to 10 V. The second ADA4077-2 op

amp inverts the +VEXC and generates the −VEXC voltage. R22 is

chosen such that R22 = R24||R27 to achieve first-order bias current

cancellation. The error due to the 15 μV offset voltage of the first

ADA4077-2 op amp is approximately (2 × 15 μV) ÷ 10 V = 3 ppm.

The primary error introduced by the inverting stage is therefore the

error in the resistor matching between R24 and R27.

The ADG1419 is a 2.1 Ω, on-resistance SPDT analog switch with

an on-resistance flatness of 50 mΩ over a ±10 V range, making it

ideal for generating a symmetrical square wave from the ±VEXC

voltages. The ADG1419 introduces a symmetrical error of 50 mΩ ÷

1 kΩ = 50 ppm (Typical).

ANALOG SIGNAL CONDITIONING

The voltage applied to the conductivity cell is measured with the

AD8253 instrumentation amplifier. The positive input to the AD8253

is buffered by the ADA4622-1. The ADA4622-1 is chosen based pri-

marily on its bias current of 5 pA to minimize the error in measuring

low currents associated with low conductivities. The negative input

of the AD8253 does not require buffering.

The offset voltages of AD8253 and ADA4638-1 are removed by the

synchronous sampling stage and do not affect the measurement

accuracy.

The AD8253 instrumentation amplifiers have a high bandwidth of

10 MHz, programmable gains of 1, 10, 100, 1000, and a gain error

of less than 0.04%. The AD8253 also has a slew rate of 20 V/μs

and a settling time of 1.8 μs to 0.001% for G = 1000. The fixed,

selectable gains provide a wide measurement range, and the fast

slew rate ensures that the output is fully settled before sampling.

The ADA4627-1 stage is a precision current to voltage converter

that converts the current through the sensor to a voltage. The low

bias current and offset voltage of the ADA4627-1 has typical values