CN-0411

Circuit Note

Rev. 0 | Page 6 of 12

Temperature Compensation

Conductivity measurements are also temperature dependent;

conductance increases as temperature increases. Most

commercial conductivity probes have an integrated RTD,

facilitating temperature compensation.

The typical method to compensate the conductivity

measurement to a standard reference temperature is through a

linear function dependent on the temperature coefficient α,

which is based on the type of ions present in the solution as

shown in Equation 10. Usually, the reference temperature is set

at 25 ˚C.

(

)

1

REF

REF

Y

Y

TT

=

+α

−

(10)

where:

YREF is the conductivity referenced to TREF.

Y is the conductivity at temperature T.

TREF is the reference temperature.

T is the solution temperature.

α is the temperature coefficient.

The typical values of α for common salt solutions are shown in

Table 2.

Table 2. Temperature Coefficients for Common Salt

Solutions

Salt Solution

Temperature Coefficient (α)

Potassium Chloride (KCl)

1.88

Sodium Chloride (NaCl)

2.14

The TDS of the solution in mg/L is calculated from the referenced

conductivity measurement using Equation 11.

TDS = ke × YREF

(11)

where ke is the TDS factor.

The TDS factor is calculated by measuring the referenced

conductivity of a standard known TDS solution as shown in

Equation 12.

,

,

REF STD

e

REF STD

TDS

k

Y

=

(12)

where:

TDSREF,STD is the known TDS value of the solution at TREF.

YREF,STD is the measured conductivity of the solution at TREF.

The conductivity measurement cannot distinguish between

constituents and individual types of ions. Thus, the TDS factor

varies at a specific range, which is distinct for each type of

solution. Table 3 shows the typical range of the TDS factors of

common salt solutions.

Table 3. TDS Factors of Common Salt Solutions

Salt Solution

Range of TDS Factor (ke)

Potassium Chloride (KCl)

0.50 to 0.57

Sodium Chloride (NaCl)

0.47 to 0.50

Temperature Measurement

Conductivity varies significantly with the temperature of the

solution and the temperature coefficients are also significantly

for each type of solution. A simple method of measuring the

temperature of a solution uses a 2-wire RTD. The simplified

front-end schematic is shown in Figure 10.

Figure 10. 2-Wire RTD Simplified Schematic

The AD7124-8 contains two matched software configurable

constant current sources, which have eight selectable current

output values and can be made available to any of the analog

input channels. The excitation current to the RTD resistor

network is set to 250 µA from the AIN0 channel. Node A and

Node B are connected to the AD7124-8 external reference

input. Node B and Node C, which constitute the differential

RTD signal, are connected to the AD7124-8 analog input

channels. Both differential inputs are passed through two

identical and typical RC low-pass filter as shown in Figure 11.

Figure 11. Typical RC Low-Pass Filter for Differential Inputs

This low-pass filter topology provides attenuation of both

differential and common-mode noise signals. The large value of

R1 and R2 in Figure 11 also help to protect against 30 V

miswiring.C1 and C2 are the common-mode capacitors and are

set to a tenth of the differential-mode capacitor C3. This

attenuates the effects of noise due to mismatch between the

common-mode capacitors. The common mode cut-off

frequency (fCM) is approximately 20 times the differential mode

cut-off frequency (fDM), which are 53 kHz and 2.5 kHz,

respectively. The calculations are shown in Equation 13.

(

)

1

1

; for 

 and 

22

1

; for 

 or 

2

2

CM

1

2

1

2

11

2 2

DM

1

2

12

12

3

f

R = R

C =C

RC

R C

f

C =C

CC

R +R

C

=

=

ππ

=





π+









(13)

where:

fCM is the common mode cut-off frequency.

fDM is the differential mode cut-off frequency.

All resistance values for the RTD are referenced or capped to

4.02 kΩ and the reference voltage is 250 µA × 4.02 kΩ = 1.005 V.

A

4.02kΩ

C

RTD

IEXC

1.5kΩ

B

POSITIVE

ADC INPUT

NEGATIVE

ADC INPUT

POSITIVE

SIGNAL INPUT

NEGATIVE

SIGNAL INPUT

R1

3kΩ

R2

3kΩ

C1

1nF

C2

1nF

C3

10nF