AN-1312: Applying the ADE7912/ADE7913 to 4 mA to 20 mA DC Measurements (Rev. 0) PDF

AN-1312
APPLICATION NOTE
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Applying the ADE7912/ADE7913 to 4 mA to 20 mA DC Measurements
by Zhen Zhu
INTRODUCTION
This application note describes how to apply the ADE7912/
ADE7913 to measure analog 4 mA to 20 mA current loops.
Compared with other communications methods, an output of
4 mA to 20 mA has several unique advantages.
Within the field of industrial process control, there has been
widespread adoption of digital signals and communications,
such as in the RS-485 interface. However, many industrial
process control applications still employ analog 4 mA to 20 mA
current loops for analog signaling and monitoring. The reason
is that the analog 4 mA to 20 mA current loop is a very robust
sensor signaling standard.
•
•
•
•
As a result, many process control instruments incorporate
analog 4 mA to 20 mA dc input measurement modules.
RWIRE
+
–
TRANSMITTER
4mA TO 20mA
24 V DC
SUPPLY
RWIRE
+
–
RRECEIVER
Figure 1. Current Loop Schematic
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12455-001
Current loops are ideal for data transmission because of their
inherent robustness to electrical noise. In an analog 4 mA to
20 mA current loop, all signaling current flows through all
components; the same current flows even if the wire terminations
are less than perfect. All the components in the loop drop voltage
due to the signaling current flowing through them. The signaling
current is not affected by these voltages as long as the power
supply voltage is greater than the sum of the voltage drops around
the loop at the maximum signaling current of 20 mA (see Figure 1).
•
The current signal has good anti-interference ability.
The current signal can be transmitted relatively long
distances.
A 4 mA to 20 mA output allows convenient determination
of a short circuit or of a broken circuit.
Because the minimum current is 4 mA, many transducers
can obtain power directly from two lines, simplifying
wiring connections.
Because the 0 point and full scale from different transducers
constitute similar signals, it is easy to quantify the conversion
correlations on a unitary basis.
AN-1312
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Calibration and Measurement Procedures ....................................4
Revision History ............................................................................... 2
Measurement Procedure: 4 mA to 20 mA .....................................4
Hardware Design .............................................................................. 3
Conclusion..........................................................................................5
REVISION HISTORY
7/14—Revision 0: Initial Version
Rev. 0 | Page 2 of 5
Application Note
AN-1312
HARDWARE DESIGN
This section explains the hardware design of the 4 mA to 20 mA
current loop sensing with the ADE7912/ADE7913.
major components: an ADC, an isolated power supply, and a
batch of high speed digital isolators.
For measuring an analog 4 mA to 20 mA current signal, an ADC
and an isolated power supply module are required. A diagram of
the traditional solution that uses an SPI communication interface
is shown in Figure 2.
Compared with the traditional platform, the ADE7912/
ADE7913 isolated ADCs contain all the core elements of a
traditional solution. As shown in Figure 3, the ADE7912/
ADE7913 integrate the isolated power supply and the signal
isolation circuits.
Sampling resistance is employed to convert a 4 mA to 20 mA
signal to a voltage signal, and an analog-to-digital converter
(ADC) then converts the voltage signal to a digital signal.
Because most interface circuits on industrial equipment require
isolation, an isolated dc-to-dc power supply must provide
power exclusively to the ADC. Because the ADC ground is
isolated from the microcontroller ground, a batch of high speed
digital isolators are placed in the communication path. As an
example, Figure 2 shows four isolated channels for an SPI
communication. Thus, the measurement system requires three
The operating temperature range of the ADE7912/ADE7913 is
−40°C to +85°C, the temperature requirements of the industrial
process control. The dynamic range of the analog 4 mA to 20 mA
is 5:1. The AN-1304 Application Note, ADE7912/ADE7913 DC
Measurement Performance, provides a typical temperature
coefficient of the dc measurements for a dynamic range of 10:1
equal to 62 ppm/°C. Thus, 62 ppm/°C is considered the typical
temperature coefficient of the analog 4 mA to 20 mA dc
measurement.
POWER
SUPPLY
ISOLATED DC/DC
MISO
MOSI
ADC
RRECEIVER
SCLK
4-CHANNEL
DIGITAL ISOLATOR
MCU
12455-002
CS
Figure 2. Block Diagram of a Traditional 4 mA to 20 mA DC Input Measurement Circuit
ISOLATION
BARRIER
1
VDDISO
2
GNDISO
LDO 8
19
VDD
20
GND
POWER
ISOLATION
LDO
TEMP
SENSOR
ADC
5
VM
4
V1P
6
IM
7
IP
REF
GNDISO
9
DATA
ADC
ADC
DATA
CLOCK ISOLATION
DATA
DIGITAL
BLOCK
AND
CLOCK SPI PORT
18
17
16
15
14
13
CS
SCLK
MOSI
MISO
XTAL2
XTAL1
12 CLKOUT/
DREADY
11
GND
VREF
10
Figure 3. Block Diagram of the ADE7912
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12455-003
V2P
3
AN-1312
Application Note
Using the ADE7912/ADE7913, the 4 mA to 20 mA current loop
sensing schematic can be simplified as shown in Figure 4. It is
recommended to use the V1 channel for this measurement;
therefore, the analog signals are provided at the V1P input.
Note that the three circuit blocks needed in a traditional 4 mA
to 20 mA dc input measurement element have been simplified
into a single chip, and no other auxiliary circuits are needed.
Additionally, the ADE7912/ADE7913 can be cascaded in
groups of four ICs maximum that share one crystal and one SPI
bus. This feature is useful because usually more than one 4 mA
to 20 mA current loop must be sensed in one application.
CALIBRATION AND MEASUREMENT PROCEDURES
To obtain one 4 mA to 20 mA dc measurement, read the V1WV
register at least 100 times in 1 second. The measurement result
is the average of these readings.
To prepare the ADE7912/ADE7913 for the 4 mA to 20 mA dc
measurement, the IC must be calibrated as a two-step process.
Supply a calibrated 4 mA signal. Read the V1WV register
at least 100 times in 1 second. The average of the readings
constitutes the A value.
Supply a calibrated 20 mA signal. Read the V1WV register
at least 100 times in 1 second. The average of the readings
constitutes the B value.
When a general analog 4 mA to 20 mA signal is measured,
average 100 readings over 1 second to obtain the C value and
use the following formula to calculate the current value, x:
x
16  C  4  B  20  C
BA
MEASUREMENT PROCEDURE: 4 mA TO 20 mA
To assess the performance of the ADE7912/ADE7913 in 4 mA
to 20 mA current loop applications, an evaluation platform was
utilized (see Figure 5). The AD5421 is a complete, loop powered,
4 mA to 20 mA digital-to-analog converter (DAC) designed to
meet the needs of smart transmitter manufacturers in the industrial
control industry.
The AD5421 is guaranteed monotonic to 16 bits. It provides
0.0015% integral nonlinearity, 0.0012% offset error, and
0.0006% gain error under typical conditions. The following
testing scenario uses the AD5421 evaluation kit as a 4 mA to
20 mA outputting source, and the ADE7913 evaluation kit sets
the measurement device.
ADE7912
VDDISO
GNDISO
VDD
POWER
SUPPLY
GND
POWER
ISOLATION
LDO
TEMP
SENSOR
ADC
ADC
VM
ADC
DATA
CLOCK ISOLATION
DIGITAL
BLOCK
AND
CLOCK SPI PORT
MOSI
MCU
SCLK
CS
VREF
12455-004
RRECEIVER
MISO
DATA
DATA
V1P
Figure 4. Block Diagram of a 4 mA to 20 mA DC Input Measurement Circuit
Using the ADE7912/ADE7913
LOOP+
AD5421
EVAL BOARD
V1PINA
RRECEIVER
LOOP–
ADE7913
EVAL BOARD
AGND_ADC1
12455-005
1.
2.
Figure 5. Block Diagram of 4 mA to 20 mA Measurement
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Application Note
AN-1312
Figure 6 shows the ADE7913 outputs obtained for different dc
inputs. To measure the linearity of the ADC outputs, calculate
the nonlinearity errors (see Figure 7). The goal is for these
errors to be very small; thus the calibration procedure is greatly
simplified. A line was traced between the 4 mA and 20 mA
readings, and errors were computed in relation to the ADC
outputs.
The ADE7912/ADE7913 exhibit less than 0.04% nonlinearity
error over the full range of 4 mA to 20 mA (see Figure 7).
0.02
DC MEASUREMENT ERROR (%)
An external resistor, RRECEIVER = 15.1 Ω, was used to sense the
current value, and its outputs were connected to the V1 channel
of the ADE7913. The testing procedure consisted of generating
controlled current values from the AD5421 evaluation board
and using the ADE7913 for sampling.
0
–0.01
–0.02
–0.04
0
5
10
15
20
INPUT CURRENT (mA)
3000000
25
12455-007
–0.03
4000000
Figure 7. Nonlinearity Error of the ADE7912/ADE7913
CONCLUSION
2000000
1000000
0
0
5
10
15
20
INPUT CURRENT (mA)
Figure 6. ADE7913 Readings with Different DC Inputs
25
12455-006
ADE7913 V1 CHANNE L OUTPUT (BITS)
0.01
The ADE7912/ADE7913 are fully isolated ADCs that can
perform analog 4 mA to 20 mA current loop measurements.
Because the ADC is integrated with the isolated dc-to-dc
converter and the digital isolators into a single chip, the circuit
design is greatly simplified. This enhances the reliability of the
system. The nonlinearity error is under 0.04% for a 5:1 dynamic
range at room temperature, which simplifies the calibration
procedure.
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AN12455-0-7/14(0)
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