CN-0066: Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC and the

Circuit Note
CN-0066
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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AD7793
24-Bit Σ-Δ ADC
ADR441
Precision 2.5 V Reference
ADuM5401
Quad-Channel isoPower® Digital Isolator
Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC and
the ADuM5401 Digital Isolator
CIRCUIT FUNCTION AND BENEFITS
The ADR441 is the chosen reference for this circuit. The
ADR441 has excellent accuracy specs of 0.04% and a drift of
3 ppm/°C maximum.
This circuit provides a complete solution for a single-supply
input circuit design requiring isolation. The AD7793 is a 24-bit
Σ-Δ ADC. The ADC includes an on-chip PGA and can,
therefore, accept small signal inputs from sensors directly. The
PGA gains can be set for 1, 2, 4, 8, 16, 32, 64, or 128. The
ADuM5401 provides all the necessary signal isolation and
power between the microcontroller and the input.
The ADuM5401 is a quad-channel isolator with integrated
isoPower technology, based on Analog Devices, Inc., iCoupler®
technology. It is used to provide isolation between the field side
and the system microcontroller, with an isolation rating of
2.5 kV rms. The ADuM5401 also has an integrated dc-to-dc
converter, which can provide 500 mW of regulated isolated
power at either 5 V or 3.3 V. This design uses the 5 V provided
from the ADuM5401 to supply all the analog circuitry on the
input module. All four data wires are used: three for transmit
(CS, SCLK, DIN) and one for receive (DOUT), which connects
to the standard SPI interface.
CIRCUIT DESCRIPTION
Sensor outputs are commonly small signal, such as
thermocouple or RTD. This design allows these small signal
inputs to be directly connected to the AD7793 input, which has
an internal PGA with a maximum gain of 128. The design can
be used as a complete solution to a non-loop-powered SMART
transmitter.
0.1µF 10nF
DIFFERENTIAL
VOLTAGE INPUT
+5VISO
VISO
GNDISO
VOA
VOB
VOC
VID
VSEL
GNDISO
ISO
1kΩ
1kΩ
0.1µF
CS
SCLK
DIN
GND DOUT
AVDD/DVDD
AINx(+)
AD7793
AINx(–)
REFIN(+)
REFIN(–)
ISO
10nF 10nF
ISO
VDD1
GND1
VIA
VIB
VIC
VOD
RCOUT
GND1
10nF 0.1µF 10µF
+
5V
MBR0530
CS
SCLK
DIN
DOUT
0V
INTERFACE TO
MICROCONTROLLER
ADuM5401
+ 10µF 0.1µF
0.1µF
ISO
2.5V
ADR441
+ 10µF 0.1µF
VOUT
VIN
0.1µF + 10µF
ISO
ISO
08348-001
ISO
ISO
ISO
Figure 1. 24-Bit Isolated Single Supply Input Circuit
Rev. A
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CN-0066
Circuit Note
The AD7793 is specified for an AVDD of 2.7 V to 5.25 V under
normal operation. For loads greater than 10 mA, the output
voltage of the ADuM5401 is 4.75 V to 5.25 V, although for
lighter loads, it is 4.7 V to 5.4 V. For this reason, the user can
either ensure that the isolated circuit draws more than 10 mA
during operation or place a diode in series with the AVDD supply
of the AD7793 to ensure that the AVDD voltage is within the
specification.
Figure 2 shows a histogram plot of the AD7793 output
performance when a 2.5 reference is connected to its inputs.
The measured noise on the output is measured in peak-to-peak
resolution. The 20.4-bit peak-to-peak resolution figure means
there are 3.6 bits of noise flicker on the output of the ADC with
respect to the ADC’s 24-bit resolution. Therefore, the ADC’s
noise-free code resolution is 20.4 bits. In terms of contribution
to the overall system error, the measured noise of 3.6 bits
contributes ~0.00007% error with respect to the full-scale range
of the ADC. The integral nonlinearity error of the AD7793 is
0.0015%.
08348-002
This circuit is from a portion of the PLC demo system. The PLC
demo system has been successfully tested to the IEC 61000
standards shown in Table 1 (see Colm Slattery, Derrick
Hartmann, and Li Ke, “PLC Evaluation Board Simplifies Design
of Industrial Process Control Systems,” Analog Dialogue (April
2009) for more discussion of external protection techniques).
Figure 2. Input Module Histogram of the AD7793, 512 Samples, 4.17 Hz Data
Rate, Gain = 1, Input = 2.5 V Reference
Table 1. Conformance to IEC Specifications 1
Test Item
EN55022
EN and IEC 61000-4-2
EN and IEC 61000-4-3
EN and IEC 61000-4-4
EN and IEC 61000-4-5
EN and IEC 61000-4-6
1
Description
Radiated emission Class A,
3 meter anechoic chamber
Electrostatic discharge (ESD)
±8 kV VCD
Electrostatic discharge (ESD)
±8 kV HCD
Radiated immunity 80 MHz to
1 GHz 18 V/m, vertical antenna
polarization
Radiated immunity 80 MHz to
1 GHz 18 V/m, horizontal antenna
polarization
Electrically fast transient (EFT)
±4 kV power port
Electrically fast transient (EFT)
±2 kV analog I/O ports
Power line surge, ±2 kV
Immunity test on power cord,
10 V/m for 30 minutes
Immunity test on I/O cable,
10 V/m for 30 minutes
Result
Passed and met −6 dB margin.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively −8 ppm, 10 ppm, and 13 ppm when there is interference.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively −8 ppm, 10 ppm, and 13 ppm when there is interference.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively 0.05%, 0.004%, and −0.13%. Performance automatically
resorted to ≤0.05% after interference. Class B.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively −0.09%, 0.003%, and −0.02%. Performance automatically
resorted to ≤0.05% after interference. Class B.
Passed Class B.
Passed Class B.
No board or part damage occurred, no performance degrade, passed with
Class A.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively 9.3%, 11%, and 3.4%. Passed Class B.
Maximum deviations in Input Channel 2, Input Channel 3, and Input Channel 4
are respectively 4.5%, 4.7%, and 1.4%. Performance automatically resorted to
≤0.05% when interference stopped.
A sample was tested during initial release of the PLC Demo system (V07) and met the test compliances listed in this table. These results should be viewed as typical
data taken at 25°C. For these tests, the DAC outputs were connected to the ADC inputs, that is, DAC_CH2 to ADC_CH2, DAC_CH3 to ADC_CH3, and DAC_CH4 to
ADC_CH4. The DAC outputs were set to 5 V, 6 V, and 10 mA, respectively. The ADC channels correspond to the circuit as in CN0067.
Rev. A | Page 2 of 3
Circuit Note
CN-0066
LEARN MORE
Data Sheets and Evaluation Boards
Cantrell, Mark. AN-0971 Application Note, Recommendations
for Control of Radiated Emissions with isoPower Devices.
Analog Devices.
ADuM5401 Evaluation Board.
Chen, Baoxing. 2006. iCoupler Products with isoPower
Technology: Signal and Power Transfer Across Isolation
Barrier Using Microtransformers. Analog Devices.
PLC Demo System.
AD7793 Data Sheet.
ADR441 Data Sheet.
ADuM5401 Data Sheet.
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise? Analog Devices.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications. Analog Devices.
Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation
Board Simplifies Design of Industrial Process Control
Systems.” Analog Dialogue (April 2009).
REVISION HISTORY
5/11—Rev. 0 to Rev. A
Changes to Circuit Function and Benefits Section....................... 1
Changes to Figure 1 .......................................................................... 1
Changes to Circuit Description Section......................................... 2
Changes to Table 1 ............................................................................ 2
Changes to Learn More Section ...................................................... 3
7/09—Revision 0: Initial Version
Wayne, Scott. “iCoupler Digital Isolators Protect RS-232,
RS-485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue (October 2005).
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CN08348-0-5/11(A)
Rev. A | Page 3 of 3
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