Circuit Note

CN-0411

Rev. 0 | Page 9 of 12

Figure 14 shows the measured accuracy of the conductivity

measurements taken using precision resistors from 1 MΩ to

10 Ω, corresponding to conductivities of 1 µS to 0.1 S. An

increase in the system errors is visible in the 10 mS to 100 mS

range. For these ranges, a calibration method to on-board

precision resistances (shown in Figure 5) is required.

Figure 14. System Error (%) vs Conductivity from 1 µS to 0.1 S

Calibration is performed only once for each board and an offset

resistance is obtained, which accounts for the on-resistance of

the multiplexer. The software system stores the offset resistance

value and uses it for all succeeding conductivity measurements

until the calibration method is performed again. Figure 15

shows the error for high conductivities when calibrated to the

20 Ω precision resistance shown in Figure 5.

Figure 15. Calibrated System Accuracy (%) vs Conductivity

from 1 mS to 100 mS

System Noise Performance

As shown in Figure 16, the noise level of the system is only

15.99 nS for a 10 kΩ precision resistance corresponding to a

100 µS conductance.

Figure 16. CN-0411 Noise Levels for a 100 µS Conductance

COMMON VARIATIONS

An ADG884 analog switch is used to impose a bipolar

excitation across the conductivity cell, with both terminals of

the cell being driven. Alternative designs hold one terminal of

the cell at ground potential, which reduces the impact of

leakage at the grounded terminal; such designs require more

elaborate excitation circuits. The CN-0411 is further simplified

by only measuring the voltage signal across the conductivity

cell. Using a trans-impedance amplifier, to measure the current

through the cell eliminates the need to compute for the current

value from the gain resistance. Furthermore, using a variable

gain amplifier replaces the need for a multiplexer selecting the

gain resistors for the voltage divider network. The CN-0359 is

an example of a higher-performance conductivity measurement

solution.

0

5

10

15

20

25

30

35

1µ

10µ

100µ

1m

10m

100m

CONDUCTIVITY (S)

0

1

2

3

4

5

6

7

5m

50m

CONDUCTIVITY (S)

0

20

40

60

80

100

120

140

160

CONDUCTANCE (µS)

MEAN = 101.33

1μS