EVAL-AD421EB Datasheet, PDF (7/7 Pages) Analog Devices – Evaluation Board for Loop Powered 4-20mA DAC

EVAL-AD421EB
EMI/EMC CONSIDERATIONS
On the AD421 board care was taken with the layout, grounding
and decoupling to reduce any effects that electromagnetic
disturbances might have on the AD421. Two ferrites, L1 and L2
are included on the AD421 board to provide an extra level of
security against EMI disturbances that come in on the loop(+)
and loop(-) lines. EMC susceptibility tests were carried out on
the AD421 board to check the level of performance obtained
from this board when subjected to various electromagnetic
disturbances. The aim of the susceptibility tests is to ensure that
the product has an adequate level of intrinsic immunity to
electromagnetic disturbance to enable it to operate as intended.
The EMI/EMC directive 89/336/EEC only applies to products
or systems and not to individual components. The AD421
evaluation board acts as a system in this test procedure. The
immunity tests performed included conducted immunity and
radiated immunity. Conducted immunity is a fast transient burst
test where fast transients are coupled onto the 4-20ma loop
twisted pair cable using a 1m capacitive clamp. Radiated
immunity testing involves subjecting the board under test to
various electric fields with frequencies varying from 30MHz to
1GHz.
Radiated Immunity (IEC 1000-4-3)
This set of experiments looked at the performance of the AD421
when subjected to various electric fields. Class B approval for
use in residential/commercial light industrial environments
requires the system to remain functional in a field of 3V/m from
30MHz to 1GHZ. Class A approval for use in industrial
environments requires the system to remain functional in a field
of 10V/m. This test is carried out in a stripline cell which is
essentially two metal plates placed in parallel, the AD421 board
is placed between these plates where it is subjected to an electric
field applied to the plates. The results obtained from this test
with the AD421 in its normal operating mode with Vcc =5V and
controlling the current in the loop to 10mA are as follows.
Board in Horizontal Orientation:
Field Strength Results Comments
3V/m
5V/m
10V/m
PASS Range from 30MHz to 1GHz
PASS Range from 30MHz to 1GHz
PASS Range from 30MHz to 1GHz
This is class 1 performance, i.e. normal operation maintained
during application on the field.
Board in Vertical Orientation:
Field Strength Results Comments.
3V/m
5V/m
10V/m
PASS Range from 30MHz to 1GHz
PASS Range from 30MHz to 1GHz
FAIL Device resets to 4mA at 860MHz
This is class 1 performance, ie normal operation maintained
during application of the fields of 3V/m and 5V/m. The classifi-
cation is class 3 at 10V/m as user intervention is required to get
the AD421 back to its programmed conditions.
Conducted Immunity (IEC 1000-4-4 Fast Transient Burst
Test)
This set of experiments involved the coupling of fast transients
onto the current loop unshielded twisted pair cable through a
1m capacitive clamp. The fast transient burst is specified to
have a rise time of 5ns and a duration of 50ns and is supplied
from a 50Ω source. Bursts of 15ms duration of these pulses at a
repetition rate of 5kHz are applied every 300ms. The voltage
level of the pulse is controlled from 250V to 5kV. Coupling onto
the cable is via a capacitive clamp which is essentially two metal
plates which sandwich the line under test to provide a distrib-
uted coupling capacitance. This clamp is fed from the transient
generator which is constructed with a spark gap driven from an
energy storage capacitor which enables the high voltages to be
generated with the fast rise times. The AD421 is programmed
so that 10mA flows in the loop during the experiments. The
results from this test were as follows:
Burst Amplitude Results Comments.
500V Positive
500V Negative
1kV Positive
1kV Negative
2kV Positive
2kV Negative
PASS
PASS
FAILS
FAILS
FAILS
FAILS
Normal Operation.
Normal Operation.
Loop current resets to 4mA.
Loop current resets to 4mA.
Loop current resets to 4mA.
Loop current goes to 20mA.
The board achieves class 1 performance for application of a
burst of 500V, ie it continues to work as normal. For all other
bursts the operation can be classified as class 3 where user
intervention is required to restore the part back to normal
operation after the burst has been removed.
Further development work on the AD421 evaluation board is
being planned to see if the above performance can be improved
upon.
REV. A
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