CN0337 Datasheet, PDF (4/8 Pages) Analog Devices – EVALUATION AND DESIGN SUPPORT

CN-0337
Circuit Note
For any other temperature ranges or for any other temperature
sensor (for example Pt200, Pt500, Pt1000, Pt2000) the resistor
values must be recalculated as follows:
1. Choose a value for R3 (for example, 1 kΩ), and then make
R5 = R6 = 2R3.
2. Choose the excitation current through the sensor IR and
then calculate VR = IR × RX_low
where RX_low = the resistance of the RTD at the lowest
temperature of the range.
3. Choose a value for R9 (for example, R9 = 1 kΩ), and then
calculate R8:
R8 = ^{V REF −V R} × R9
V_R
where VREF = 2.5 V = ADC reference voltage.
4. Calculate A = 0.0435 × (RX_high – RX_low)
where
A = a temporary constant needed for this calculation
procedure.
RX_high = the resistance of the RTD at the highest
temperature of the range.
5. Calculate R0 = RX_low – A.
6. Calculate R0 = R1 × R2/(R1 + R2) and choose the values for
R1 and R2.
It is recommended to choose a standard value for R1 that is
equal to RX_low, and then calculate R2.
7. Calculate
B = ^0.2R0
V_R × A
where B= a temporary constant needed for this calculation
procedure.
8. Calculate R4 = B × R3, and ensure that R12 = R4.
Accuracy Analysis
Equation 1 shows that all resistors influence the total error. If
these values are chosen carefully, the overall error due to
substituting standard value resistors can be made less than a few
percent. However, use Equation 1 to recalculate the U1A op
amp output for 100 Ω and 212.05 Ω inputs to ensure that the
required headroom is preserved. In the actual circuit the nearest
available standard resistors values were chosen. The Resistors
Rl, R2, R8, and R9 are 0.1%, 25 ppm/°C. The other resistors in
the circuit are 1%, 100 ppm/°C: R3, R4, R5, R6, and R12.
The absolute accuracy in this type of circuit is primarily
determined by the resistors, and therefore gain and offset
calibration is required to remove the error due to standard value
substitution and resistor tolerances.
Effect of Resistor Temperature Coefficients on Overall Error
Equation 1 shows that the output voltage is a function of nine
resistors: R1, R2, R3, R4, R5, R6, R8, R9, and R12.
The sensitivity of the full-scale output voltage at TP1 to small
changes in each of the nine resistors was calculated using a
simulation program. The input RTD resistance to the circuit
was 212 Ω. The individual sensitivities calculated were SR1 = 1.83,
SR2 = 0.09, SR3 = 0.94, SR4 = 0.94, SR5 = 1.35, SR6 = 1.28. SR8 = 0.97,
SR9 = 0.96, and SR12 = 0.07. Assuming that the individual tempera-
ture coefficients combine in a root-sum-square (rss) manner, then
the overall full-scale drift 25 ppm/°C resistors for R1, R2, R8, R9,
and 100 ppm/°C resistors for R3, R4, R5, R6, R12 is approximately:
Full scale drift
= 25 ppm/°C√[(SR1)² +(SR2)² +(4SR3)² + (4SR4)² + (4SR5)² +
(4SR6)² + (SR8)² + (SR9)² + (4SR12)²)]
= 25 ppm/°C√(1.83² +0.09² + 3.76² + 3.76² + 5.4² + 5.12² +
0.97² +0.96² + 0.28²)
= 236 ppm/°C
The full-scale drift of 236 ppm/°C corresponds to 0.024% FSR/°C.
For a ±10°C change in temperature, the error is ±0.24% FSR.
Using 25 ppm/°C resistors for all nine resistors reduces the full-
scale drift to approximately 80 ppm/°C, or 0.008% FSR/°C.
The error caused by the tolerances of the resistors, the offset of the
AD8608 op amps (75 µV), and the ADC AD7091R is eliminated
after the calibration procedure. It is still necessary to calculate
and verify that the op amp output is within the required range.
Effect of Active Component Temperature Coefficients on
Overall Error
The dc offsets of the AD8608 op amps (75 µV) and the
AD7091R ADC are eliminated by the calibration procedure.
The offset drift of the ADC AD7091R internal reference is
4.5 ppm/°C typical and 25 ppm/°C maximum.
The offset drift of the AD8608 op op amp is 1 μV/°C typical and
4.5 μV/°C maximum.
Note that resistor drift is the largest contributor to total drift if
50 ppm/°C or 100 ppm/°C resistors are used, and the drift due
to active components can be neglected.
Lead Wire Resistance Compensation
The circuit in Figure 1 realizes full compensation for the lead
wire resistances (r1, r2, and r3). However, if there is any mismatch in
Equation 3, the lead wires r1 and r2 add errors to the measurement.
The third lead wire r3 does not have any effect on the circuit
because it is connected to the high impedance input of U1D.
The linearity of the circuit is not affected by the lead wires r1
and r2, even if there is mismatch in Equation 3.
RTD Linearization
The circuit in Figure 1 is linear with respect to the resistance
change of the RTD. However, the transfer function of the RTD
(resistance vs. temperature) is nonlinear. Therefore, linearization is
needed to eliminate the nonlinearity error of the RTD. For systems
in which a microcontroller is involved, this linearization is typically
done in the software. The AN-709 Application Note discusses
some linearization techniques for Pt100 RTD sensor. The same
techniques are used in the CN0337 evaluation software to
eliminate the nonlinearity error of the Pt100 sensor.
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