CN0326 Datasheet, PDF (4/7 Pages) Analog Devices – Devices Connected

CN-0326
Pt1000
RTD
DIN 43760
CLASS A
210µA
10kΩ
1µF
R_LEAD
P1
R_LEAD
210µA
10kΩ
1µF
5kΩ, 0.1%
IOUT2
AIN2(+)
AD7793
AIN2(–)
RFIN(+)/AIN3(+)
RFIN(–)/AIN3(–)
GND
Figure 3. 2-Wire Pt RTD Connections (Simplified Schematic: All Connections
and Decoupling Not Shown)
Another possibility for eliminating wiring resistance errors is
the 3-wire RTD configuration that is described in detail in
Circuit Note CN-0287.
Output Coding
The output code for an input voltage on either channel is
Code
= 2 ^{N –1}



AIN × GAIN
V _REF
+1



where:
AIN is the analog input voltage.
GAIN is the in-amp setting.
N = 24
The EVAL-SDP-CB1Z system demonstration platform board
and the PC processes the data output from the AD7793.
Digital and Power Isolation
The ADuM5401 isolates the ADC digital signals and also
supplies isolated regulated 3.3 V power to the circuit. The input
to the ADuM5401 (VDD1) should be between 3.0 V and 3.6 V.
Take care with the layout of the ADuM5401 to minimize
EMI/RFI problems. For more details, please refer to Application
Note AN-1109, Recommendations for Control of Radiated
Emissions with iCoupler Devices.
System Calibration
In order to accurately measure the RTD resistance, the ±5%
variation in the IOUT2 current must be taken into account. The
AIN3(+) input to the AD7793 is used to measure the voltage
dropped across the precision 5 kΩ 0.1% resistor. The exact
IOUT2 current is then determined by dividing this voltage by 5 kΩ.
The RTD resistance is calculated by dividing the voltage across the
RTD by the exact IOUT2 current.
A two-point calibration procedure shown in Figure 4 is used to
calibrate the pH meter in the EVAL-CN0326-PMDZ evaluation
software.
Circuit Note
Figure 4. Evaluation Software Calibration Settings Window
The user is required to use a minimum of two buffer solutions,
where a neutral pH buffer with a value of pH-7 should be used
to remove the offset introduced by the pH probe and by the system.
The neutral buffer solution can be used to set the first point for
calibration. The pH of the second buffer solution depends on
the pH of the solution to be measured. A pH-10 buffer solution
can be used when measuring alkaline base solutions, and a pH-
4 buffer can be used when measuring acidic solutions. For more
precise measurement, a three point calibration can be
performed. This can be done by using two different sets of
buffer solutions in Step 2 and in Step 3 as shown in Figure 4,
where the pH-7 solution is used to remove the offset.
The software includes a list of buffer solution recommended by
the NIST. Each buffer solution described in the list has its own
temperature coefficient from 0°C to 95°C, which can be found
in “pH Theory and Practice” by Radiometer Analytical. The
software uses this table to correlate the mV input from the pH
probe to the correct pH value that correspond to the temperature
read from the RTD sensor using linear interpolation to fill in
the gaps in the table. The user is given an option to enable/
disable the option for continuous temperature compensation by
clicking the green button as shown in Figure 4.
Buffer solutions are commonly found in the market for pH
sensor calibration. Other NIST-certified pH reference can also
be used for calibration. Because of the variety of buffer solutions
available, the software also provides the user an option to use
their desired NIST-certified pH reference for calibration as
shown in Figure 4.
The software also provides the user an option to use other RTD
resistance values, but by default it is set to 1000 Ω.
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