Circuit Note

CN-0411

Rev. 0 | Page 7 of 12

Also, the system can accommodate both Pt100 or Pt1000 RTDs.

The resistance of the RTD (RRTD) is computed from the 24-bit

unipolar ADC code using Equation 14.

24

21

CODE

REF

RTD

ADC

R

R

×

=

−

(14)

where:

ADCCODE is the 24-bit unipolar code of the signal sample.

RREF is the reference resistor. RREF = 4.02 kΩ.

The Callender-Van Dusen equation is used to define the

relation of the RTD resistance and temperature. For

temperatures greater than or equal to 0˚C or for resistances

greater than or equal to R0, the temperature in degrees Celsius

can be computed using Equation 15, which can be obtained

directly from the Callender-Van Dusen equation.

2

0

41

2

RTD

RTD

R

AA

B

R

T

B





−+

−

−









=

(15)

where:

A = 3.9083 ×10−3.

B = −5.775 ×10−7.

C = −4.183 × 10−12.

R0 is the RTD resistance at 0˚C.

RRTD is the RTD resistance at TRTD.

TRTD is the temperature in degrees Celsius.

For temperatures below 0˚C or for resistances below R0, a best

fit polynomial expression is used as shown in Equation 16.

RTD

1

2 RTD

3 RTD

4 RTD

5 RTD

6 RTD

T

C C RC RC RC RC R

=

+

+

+++

(16)

where:

C1 = -242.02.

C2 = 2.2228.

C3 = 2.589 ×10−3.

C4 = 48.26 × 10−3.

C5 = 1.5243 × 10−10.

RRTD is the RTD Resistance at TRTD.

TRTD is the temperature in degrees Celsius.

Software Operation Settings

The provided software configures six parameters for TDS

measurement:

•

Conductivity cell excitation voltage

•

Gain resistance selected by the multiplexer

•

Value of RTD used

•

Conductivity cell switching frequency

•

Conductivity probe cell constant

•

Type of solution being measured.

The DAC voltage can be set to any value from 0 to 2.5 V. At

startup, the DAC voltage is set to 400 mV.

The selected gain resistance can be set to open or any of the

seven resistor values: 20 Ω, 200 Ω, 2 kΩ, 20 kΩ, 200 kΩ, 2 MΩ,

and 20 MΩ. Initially, the gain resistance is set to open.

The type of RTD used can be set to either Pt100 or Pt1000.

Initially, the type of RTD used in temperature measurements is

set to Pt100.

The frequency of the PWM signal sets the frequency of the

bipolar pulse across the conductivity cell. The software can only

switch between two options for PWM frequency: 94 Hz and

2.4 kHz. Initially, the PWM frequency is set to 94 Hz.

There are three fixed options for conductivity probe cell

constants: 0.1, 1.0, and 10. A forth option allows the user to

enter a custom value for probes with cell constants other than

0.1, 1.0, and 10. By default, the cell constant is set to 1.0.

The type of solution sets the TDS factor and temperature

coefficient used in computing for TDS and temperature

compensation, respectively. The software has built-in settings

for only sodium chloride (NaCl) and potassium chloride (KCl)

solutions. However, the user can set custom values for the TDS

factor and temperature coefficient separately for other types of

solutions. Initially, the type of solution is set to NaCl.

Apart from these parameters, the software can also switch

between taking measurements with the ADC in single

conversion mode and with the ADC in continuous conversion

mode. In single conversion mode, the ADC goes to idle mode

whenever a read conductivity command is not being issued.

This decreases the power consumption of the board when it is

not active. Additionally, the DAC voltage value can be set to

zero in these times to further decrease the board’s consumption.

In continuous conversion mode, the ADC continuously samples

the conductivity cell signal. This decreases the measurement

time for each sample, which makes this effective when

continuously monitoring the conductivity of a solution.