AD420
AD420 PROTECTION
TRANSIENT VOLTAGE PROTECTION
BOARD LAYOUT AND GROUNDING
The AD420 contains ESD protection diodes which prevent
damage from normal handling. The industrial control environ-
ment can, however, subject I/O circuits to much higher tran-
sients. In order to protect the AD420 from excessively high
voltage transients such as those specified in IEC 801, external
power diodes and a surge current limiting resistor may be re-
quired, as shown in Figure 14. The constraint on the resistor is
that during normal operation the output voltage level at I
OUT
must remain within its voltage compliance limit
(I
OUT
×
(Rp +
R
LOAD
)
≤
V
CC
– 2.5
V)
and the two protection diodes and resistor must have appropri-
ate power ratings.
V
CC
The AD420 ground pin, designated GND, is the “high quality”
ground reference point for the device. Any external loads on the
REF OUT and V
OUT
pins of the AD420 should be returned to
this reference point. Analog and digital ground currents should
not share a common path. Each signal should have an appropri-
ate analog or digital signal return routed close to it. Using this
approach, signal loops enclose a small area, minimizing the
inductive coupling of noise. Wide PC tracks, large gauge wire,
and ground planes are highly recommended to provide low
impedance signal paths.
POWER SUPPLIES AND DECOUPLING
V
CC
AD420
I
OUT
R
P
R
LOAD
GND
The AD420 supply pins, V
CC
(Pin 23) and V
LL
(Pin 2), should
be decoupled to GND with 0.1
µF
capacitors to eliminate high
frequency noise that may otherwise get coupled into the analog
system. High frequency ceramic capacitors are recommended.
The decoupling capacitors should be located in close proximity
to the pins and the ground line to have maximum effect. Further
reductions in noise, and improvements in performance, may be
achieved by using a larger value capacitor on the V
LL
pin.
Figure 14. Output Transient Voltage Protection
–10–
REV. F
AD [ ANALOG DEVICES ]
