With the Air Discharge Method, an ESD voltage
is applied to the equipment under test (EUT)
throughair. Thissimulatesanelectricallycharged
person ready to connect a cable onto the rear of
the system only to find an unpleasant zap just
before the person touches the back panel. The
high energy potential on the person discharges
through an arcing path to the rear panel of the
system before he or she even touches the system.
This energy, whether discharged directly or
through air, is predominantly a function of the
discharge current rather than the discharge
voltage. Variables with an air discharge such as
approach speed of the object carrying the ESD
potential to the system and humidity will tend to
change the discharge current. For example, the
rise time of the discharge current varies with the
approach speed.
30A
15A
0A
t=0ns
t=30ns
t ➙
Figure 9. ESD Test Waveform for IEC1000-4-2
voltage stored in the capacitor is then applied
throughRS, thecurrentlimitingresistor, ontothe
device under test (DUT). In ESD tests, the SW2
switch is pulsed so that the device under test
receives a duration of voltage.
The Contact Discharge Method applies the ESD
current directly to the EUT. This method was
devised to reduce the unpredictability of the
ESD arc. The discharge current rise time is
constant since the energy is directly transferred
without the air-gap arc. In situations such as
handheldsystems,theESDchargecanbedirectly
dischargedtotheequipmentfromapersonalready
holdingtheequipment. Thecurrentistransferred
ontothekeypadortheserialportoftheequipment
directly andthentravelsthroughthePCBandfinally
to the IC.
FortheHumanBodyModel, thecurrentlimiting
resistor (R ) and the source capacitor (C ) are
1.5kΩ an 1S00pF, respectively. For IEC-10S00-4-
2,thecurrentlimitingresistor(RS)andthesource
capacitor (CS) are 330Ω an 150pF, respectively.
The higher C value and lower RS value in the
IEC1000-4-2Smodel are more stringent than the
HumanBodyModel. Thelargerstoragecapacitor
injects a higher voltage to the test point when
SW2 is switched on. The lower current limiting
resistor increases the current charge onto the test
point.
The circuit model in Figures 7 and 8 represent
the typical ESD testing circuit used for all three
methods. TheCS isinitiallychargedwiththeDC
power supply when the first switch (SW1) is on.
Now that the capacitor is charged, the second
switch(SW2)isonwhileSW1switchesoff. The
SP481E, SP485E
FAMILY
HUMAN BODY
MODEL
IEC1000-4-2
Air Discharge Direct Contact
Level
Driver Outputs
Receiver Inputs
±15kV
±15kV
±15kV
±15kV
±8kV
±8kV
4
4
Rev. 5/16/03
SP481E Low Power Half-Duplex RS485 Transceivers
© Copyright 2003 Sipex Corporation
8
SIPEX [ SIPEX CORPORATION ]
