CN-0067: Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC, the...

Circuit Note
CN-0067
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
AD8220
High CMRR Bipolar In-Amp
ADuM5401
Quad-Channel isoPower® Digital Isolator
AD8601
Single-Supply Op Amp
Fully Isolated Input Module Based on the AD7793 24-Bit Σ-Δ ADC, the ADuM5401
Digital Isolator, and a High Performance In-Amp
CIRCUIT FUNCTION AND BENEFITS
CIRCUIT DESCRIPTION
This circuit provides a complete solution for an industrial
control input module. This design is suitable for process control
programmable logic controllers (PLCs) and distributed control
system (DCS) modules that must digitize standard 4 mA to 20 mA
current inputs and unipolar input voltage ranges. The AD8220
in-amp is used to level shift the bipolar signals to provide a 0 V
to 5 V input signal to the AD7793 ADC. The ADuM5401
provides all the necessary signal isolation and power between
the microcontroller and the ADC. The circuit also includes
standard external protection and has been tested and verified to
be fully compliant with IEC 61000 specifications.
For industrial control modules, analog input voltage and
current ranges include 0 V to 5 V, 0 V to 10 V, 4 mA to 20 mA,
and 0 mA to 20 mA. A resistor divider (R2-R3) on the input is
used to attenuate high voltage inputs so that they match the
maximum input range of the AD8220 (−0.1 V to +2.9 V when
operating on a single +5 V supply). A good variation of this
circuit is to use the AD8226, which has a wider maximum input
range (−0.1 V to +4.1V). The AD8220 is used to level shift the
input signal and provide gain so that it matches the input range
of the AD7793. This circuit is specifically designed for unipolar
analog inputs. Other boards have been designed for bipolar
operation, which requires bipolar supplies (see Scott Wayne,
“iCoupler® Digital Isolators Protect RS-232, RS-485, and CAN
Buses in Industrial, Instrumentation, and Computer Applications,”
Analog Dialogue, October 2005).
ALTERNATE
INPUT
S3
ISO
RG
RG
S4
R2 200pF
100kΩ
S1
AINx(+)
0.1µF
ISO
0.1µF
0.1µF
10Ω
ISO
(0.5V)
ISO
+
10nF
+5VISO
AVDD/DVDD
CS
SCLK
AD7793
DIN
AINx(–)
REFIN(+) REFIN(–) GND DOUT
1kΩ
REF S2 1kΩ
ISO
10µF
ADuM5401
0.1µF 10nF
ISO
AD8220
RG
51kΩ
470pF
R1
ISO
250Ω, 0.1%
25ppm/°C
+ 10µF 0.1µF
10nF
ISO
ISO
0.1µF
ISO
AD8601
0.1µF
ISO
0.1µF
0.1µF 10µF
10nF
5V
+
MBR0530
CS
SCLK
5V
DOUT
0V
+2.5VISO
5.1kΩ
ISO
VDD1
GND1
VIA
VIB
VIC
VOD
RCOUT
GND1
ISO
25kΩ
ISO
VISO
GNDISO
VOA
VOB
VOC
VID
VSEL
GNDISO
INTERFACE TO
MICROCONTROLLER
1nF
0.1µF
10µF +
2.5V
ADR441
VOUT
+
10µF 0.1µF
VIN
ISO
0.1µF+ 10µF
ISO
08349-001
TVS
20V
1N4148
INPUT
600Ω AT 100MHz
FERRITE BEAD
20V TVS:
R3
SMBJ20CA
330kΩ 1.5nF
+5VISO
ISO
Figure 1. 24-Bit Isolated Industrial Control Voltage Input Module (Simplified Schematic)
Rev. B
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CN-0067
Circuit Note
The AD8220 also has the excellent CMRR needed in these
applications. Jumpers (shown here as switches for clarity) are
used to switch between current and voltage ranges in hardware
and to set the gain of the AD8220. For example, when the input
is set to receive a 4 mA-to-20 mA current, the switches are
configured to provide a 250 Ω load resistor (R1) on the input,
providing a full-scale voltage of 5 V.
The AD8220 output is biased with a common-mode signal
connected to the REF pin. This reference signal is generated
from the same reference as the AD7793, divided down to
provide a 0.42 V bias voltage that is buffered using the AD8601
and connected to the REF pin of the AD8220. The REF input to
the AD8220 should always be driven with a low impedance,
heavily decoupled source.
Figure 2 shows a noise plot of the input module when a 2.5 V
input signal is connected using the 0 V to +10 V input
range. The measured noise on the output is in peak-to-peak
resolution. The 19.5 bit peak-to-peak noise-free code resolution
means there are 4.5 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 19.5 bits. In terms of
contributing to the overall system error, the measured noise of
4.5 bits contributes ~0.00014% error with respect to the fullscale range of the ADC. The integral nonlinearity error of
the AD7793 is 0.0015%.
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 that can provide 500 mW of regulated isolated power
at either 5 V or 3.3 V. This design uses 5 V provided from the
ADuM5401 to supply all the analog circuitry on the input
module. Four wires are used: three for transmit (CS, SCLK, and
DIN) and one for receive (DOUT), which connect to the
standard SPI interface.
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. The circuit shown in Figure 1 draws ~4.8 mA.
08349-002
The ADR441 is the chosen reference for this circuit. The
ADR441 has excellent accuracy specifications of 0.04% and drift
specifications of 3 ppm/°C maximum.
Figure 2. Input Module Histogram, 4.7 Hz Update Rate,
Gain =1, Input = 2.5 V Reference
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).
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
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
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.
Rev. B | Page 2 of 3
Circuit Note
CN-0067
Test Item
EN and IEC 61000-4-5
Description
Power line surge, ±2 kV
EN and IEC 61000-4-6
Immunity test on power cord,
10 V/m for 30 minutes
Immunity test on I/O cable,
10 V/m for 30 minutes
1
Result
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.
Data Sheets and Evaluation Boards
LEARN MORE
PLC Demo System.
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, Inc.
AD7793 Data Sheet.
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise? Analog Devices,
Inc.
ADR441 Data Sheet.
ADuM5401 Data Sheet.
AD8220 Data Sheet.
AD8226 Data Sheet.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices, Inc.
AD8601 Data Sheet.
REVISION HISTORY
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications. Analog Devices, Inc.
Slattery, Colm, Derrick Hartmann, and Li Ke. “PLC Evaluation
Board Simplifies Design of Industrial Process Control
Systems.” Analog Dialogue (April 2009).
Wayne, Scott. “iCoupler Digital Isolators Protect RS-232,
RS-485, and CAN Buses in Industrial, Instrumentation, and
Computer Applications.” Analog Dialogue (October 2005).
5/11—Rev. A to Rev. B
Changes to Circuit Functions 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
2/10—Rev. 0 to Rev. A
Changes to Figure 1 ..........................................................................1
Changes to Circuit Description Section.........................................1
7/09—Revision 0: Initial Version
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CN08349-0-5/11(B)
Rev. B | Page 3 of 3
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