CN0336 Datasheet, PDF (4/7 Pages) Analog Devices – Devices Connected

CN-0336
Test Data Before and After Two-Point Calibration
To perform the two-point calibration, 4 mA is first applied to
the input, and the ADC output code is recorded as Code_1.
Then 20 mA is applied to the input, and the ADC output code
is recorded as Code_2. The gain factor is calculated by
GF =
16 mA
.
(10)
Code_2 − Code_1
The input current can now be calculated corresponding to any
output code, Code_x, using the equation:
I _IN = 4 mA + GF ( Code_x − Code_1) ^.
(11)
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 resistors having ±1%
tolerance. The test results do not include temperature changes.
The graph in Figure 3 shows test results for percent error (FSR)
before and after calibration at ambient temperature. As it is
shown, the maximum error before calibration is about 0.25%
FSR. After calibration, the error decreases to ±0.02% FSR,
which approximately corresponds to 1 LSB error of the ADC.
0.30
0.25
ERROR BEFORE CALIBRATION
0.20
0.15
0.10
0.05
ERROR AFTER CALIBRATION
0
–0.05
0
5
10
15
20
INPUT CURRENT (mA)
Figure 3. Circuit Test Error Before and After Room Temperature 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 information on layout and grounding and the
MT-101 Tutorial for information on decoupling techniques.
Decouple the power supply to the AD8606 with 10 μF and 0.1 μF
Circuit Note
capacitors to properly suppress noise and reduce ripple. Place the
capacitors as close to the device as possible with the low ESR value,
0.1 μF capacitor. Ceramic capacitors are advised for all high
frequency decoupling. Power supply lines must 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 the ADuM5401
data sheet.
A complete documentation package including schematics,
board layout, and bill of materials (BOM) can be found at
www.analog.com/CN0336-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.
COMMON VARIATIONS
The circuit is proven to work with good stability and accuracy
with component values shown. Other precision op-amps and
other ADCs can be used in this configuration to convert the
4 mA-to-20 mA input to a digital output and for other various
applications for this circuit.
The circuit in Figure 1 can be recalculated for other than 4 mA-
to-20 mA input current range, following the recommendations,
given in the Circuit Design section. In these cases, when the low
limit of the range is zero (0 mA to 20 mA, 0 mA to10 mA, 0 mA
to 5 mA), the conversion does not require level shifting, and the
input circuit can be simplified, as is shown in Figure 4.
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