Datasheet

R
1
V
EX
V
OUT
V
REF
R
1
OPA378
R
R
R R
+5V
OPA378
OPA2378
www.ti.com SBOS417D –JANUARY 2008–REVISED OCTOBER 2009
An ESD event produces a short duration, Another common question involves what happens to
high-voltage pulse that is transformed into a short the amplifier if an input signal is applied to the input
duration, high-current pulse as it discharges through while the power supplies +V
S
and/or –V
S
are at 0V.
a semiconductor device. The ESD protection circuits Again, it depends on the supply characteristic while at
are designed to provide a current path around the 0V, or at a level below the input signal amplitude. If
operational amplifier core to prevent it from being the supplies appear as high impedance, then the
damaged. The energy absorbed by the protection operational amplifier supply current may be supplied
circuitry is then dissipated as heat. by the input source via the current steering diodes.
This state is not a normal bias condition; the amplifier
When an ESD voltage develops across two or more
most likely will not operate normally. If the supplies
of the amplifier device pins, current flows through one
are low impedance, then the current through the
or more of the steering diodes. Depending on the
steering diodes can become quite high. The current
path that the current takes, the absorption device
level depends on the ability of the input source to
may activate. The absorption device has a trigger, or
deliver current, and any resistance in the input path.
threshold voltage, that is above the normal operating
voltage of the OPA378 but below the device
APPLICATION IDEAS
breakdown voltage level. Once this threshold is
exceeded, the absorption device quickly activates
Figure 31 shows the basic configuration for a bridge
and clamps the voltage across the supply rails to a
amplifier.
safe level.
A low-side current shunt monitor is shown in
When the operational amplifier connects into a circuit
Figure 32. R
N
are optional resistors used to isolate
such as that illustrated in Figure 30, the ESD
the ADS8325 from the noise of the digital two-wire
protection components are intended to remain
bus. Because the ADS8325 is a 16-bit converter, a
inactive and not become involved in the application
precise reference is essential for maximum accuracy.
circuit operation. However, circumstances may arise
If absolute accuracy is not required, and the 5V
where an applied voltage exceeds the operating
power supply is sufficiently stable, the REF3330 may
voltage range of a given pin. Should this condition
be omitted.
occur, there is a risk that some of the internal ESD
Figure 33 shows a high-side current monitor. The
protection circuits may be biased on, and conduct
load current develops a voltage drop across R
SHUNT
.
current. Any such current flow occurs through
The noninverting input monitors this voltage and is
steering diode paths and rarely involves the
duplicated on the inverting input. R
G
then has the
absorption device.
same voltage drop as R
SHUNT
. R
G
can be sized to
Figure 30 depicts a specific example where the input
provide whatever current is most convenient to the
voltage, V
IN
, exceeds the positive supply voltage
designer based on design constraints. The current
(+V
S
) by 300mV or more. Much of what happens in
from R
G
then flows through the MOSFET and to
the circuit depends on the supply characteristics. If
resistor R
L
, creating a voltage that can be read. Note
+V
S
can sink the current, one of the upper input
that R
L
and R
G
set the voltage gain of the circuit.
steering diodes conducts and directs current to +V
S
.
The supply voltage for the op amp is derived from the
Excessively high current levels can flow with
zener diode. For the OPA378 V
S
must be between
increasingly higher V
IN
. As a result, the datasheet
2.2V and 5.5V. Two possible methods to bias the
specifications recommend that applications limit the
zener are shown in the circuit of Figure 33: the
input current to 10mA.
customary resistor bias and the current monitor. The
If the supply is not capable of sinking the current, V
IN
current monitor biasing achieves the lowest possible
may begin sourcing current to the operational
voltage. Resistor R
1
and the diode on the
amplifier, and then take over as the source of positive
noninverting input provide short-circuit protection.
supply voltage. The danger in this case is that the
voltage can rise to levels that exceed the operational
amplifier absolute maximum ratings. In extreme but
rare cases, the absorption device triggers on while
+V
S
and –V
S
are applied. If this event happens, a
direct current path is established between the +V
S
and –V
S
supplies. The power dissipation of the
absorption device is quickly exceeded, and the
extreme internal heating destroys the operational
amplifier.
Figure 31. Single Op Amp Bridge Amplifier
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