CN0337 Datasheet, PDF (5/8 Pages) Analog Devices – EVALUATION AND DESIGN SUPPORT

Circuit Note
Test Data Before and After Two-Point Calibration
To perform the two-point calibration, a 100 Ω precision resistor is
first applied to the input, and the ADC output code is recorded
as Code_1. Then a 212.05 Ω precision resistor is applied to the
input, and the ADC output code is recorded as Code_2. The
gain factor is calculated by
GF  ^{212.05  100  .}
Code_2  Code_1
The RTD resistance can now be calculated corresponding to
any output code, Code_x, using the equation:
R _X  100   GF ( Code_x  Code_1) .
The error before calibration is obtained by comparing the ideal
transfer function calculated using the nominal values of the
components, and real circuit transfer function without
calibration. The tested circuits have been built with ±1%,
±100 ppm/°C resistors with the exception of R1, R2, R8, and R9
which are ±0.1%, ±25 ppm/°C. The tests were conducted with
the printed circuit board (PCB) at room ambient temperature.
The graph in Figure 3 shows test results for few tested boards
before and after calibration (without temperature changes). As
it is shown, the maximum error before calibration is about
0.27% FSR. After calibration, the error decreases to
±0.037% FSR, which approximately corresponds to 1.5 LSB
error of the ADC.
0.30
ERROR BEFORE CALIBRATION
0.25
0.20
0.15
0.10
0.05
ERROR AFTER CALIBRATION
0
–0.05
–0.10
0
50
100
150
200
250
300
RTD MEASUREMENT TEMPERATURE (°C)
Figure 3. Circuit Error Before and After Calibration
CN-0337
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to
consider the power supply and ground return layout on the
board. The PCB should isolate the digital and analog sections as
much as possible. The PCB for this system was constructed in a
simple 2-layer stack up, but 4-layer stack up gives better EMS.
See the MT-031 Tutorial for more discussion on layout and
grounding and the MT-101 Tutorial for information on
decoupling techniques. Decouple the power supply to AD8608
with 10 μF and 0.1 μF capacitors to properly suppress noise and
reduce ripple. Place the capacitors as close to the device as
possible, with the 0.1 μF capacitor having a low ESR value.
Ceramic capacitors are advised for all high frequency decoupling.
Power supply lines should have as large trace width as possible
to provide low impedance path and reduce glitch effects on the
supply line. The ADuM5401 isoPower integrated dc-to-dc
converter requires power supply bypassing at the input and
output supply pins. Note that low ESR bypass capacitors are
required between Pin 1 and Pin 2 and between Pin 15 and Pin
16, as close to the chip pads as possible. To suppress noise and
reduce ripple, a parallel combination of at least two capacitors is
required. The recommended capacitor values are 0.1 μF and
10 μF for VDD1 and VISO. The smaller capacitor must have a low
ESR, for example, use of a ceramic capacitor is advised. The
total lead length between the ends of the low ESR capacitor and
the input power supply pin must not exceed 2 mm. Installing
the bypass capacitor with traces more than 2 mm in length may
result in data corruption. Consider bypassing between Pin 1
and Pin 8 and between Pin 9 and Pin 16 unless both common
ground pins are connected together close to the package. For
more information, see ADuM5401 datasheet.
A complete documentation package including schematics,
board layout, and bill of materials (BOM) can be found at
www.analog.com/CN0337-DesignSupport.
High Voltage Capability
This PCB is designed in adherence with 2500 V basic insulation
practices. High voltage testing beyond 2500 V is not recommended.
Appropriate care must be taken when using this evaluation
board at high voltages, and the PCB should not be relied on for
safety functions because it has not been high potential tested
(also known as hipot tested or dielectric withstanding voltage
tested) or certified for safety.
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