Circuit Note

CN-0337

Rev. 0 | Page 5 of 8

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

Code_1

Code_2











100

05

.

212

GF

.

The RTD resistance can now be calculated corresponding to

any output code, Code_x, using the equation:

)

(

100

Code_1

Code_x









GF

R

X

.

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.

RTD MEASUREMENT TEMPERATURE (°C)

0

0.30

0.25

0.20

0.15

0.10

0.05

0

–0.05

–0.10

50

100

150

200

250

300

ERROR BEFORE CALIBRATION

ERROR AFTER CALIBRATION

Figure 3. Circuit Error Before and After Calibration

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

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.