OPA378_15 Datasheet, PDF (13/29 Pages) Texas Instruments – Low-Noise, 900kHz, RRIO,Precision OPERATIONAL AMPLIFIER Zerø-Drift Series

OPA378
OPA2378
www.ti.com
An ESD event produces a short duration,
high-voltage pulse that is transformed into a short
duration, high-current pulse as it discharges through
a semiconductor device. The ESD protection circuits
are designed to provide a current path around the
operational amplifier core to prevent it from being
damaged. The energy absorbed by the protection
circuitry is then dissipated as heat.
When an ESD voltage develops across two or more
of the amplifier device pins, current flows through one
or more of the steering diodes. Depending on the
path that the current takes, the absorption device
may activate. The absorption device has a trigger, or
threshold voltage, that is above the normal operating
voltage of the OPA378 but below the device
breakdown voltage level. Once this threshold is
exceeded, the absorption device quickly activates
and clamps the voltage across the supply rails to a
safe level.
When the operational amplifier connects into a circuit
such as that illustrated in Figure 30, the ESD
protection components are intended to remain
inactive and not become involved in the application
circuit operation. However, circumstances may arise
where an applied voltage exceeds the operating
voltage range of a given pin. Should this condition
occur, there is a risk that some of the internal ESD
protection circuits may be biased on, and conduct
current. Any such current flow occurs through
steering diode paths and rarely involves the
absorption device.
Figure 30 depicts a specific example where the input
voltage, V_IN, exceeds the positive supply voltage
(+V_S) by 300mV or more. Much of what happens in
the circuit depends on the supply characteristics. If
+V_S can sink the current, one of the upper input
steering diodes conducts and directs current to +V_S.
Excessively high current levels can flow with
increasingly higher V_IN. As a result, the datasheet
specifications recommend that applications limit the
input current to 10mA.
If the supply is not capable of sinking the current, V_IN
may begin sourcing current to the operational
amplifier, and then take over as the source of positive
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.
SBOS417D – JANUARY 2008 – REVISED OCTOBER 2009
Another common question involves what happens to
the amplifier if an input signal is applied to the input
while the power supplies +V_S and/or –V_S are at 0V.
Again, it depends on the supply characteristic while at
0V, or at a level below the input signal amplitude. If
the supplies appear as high impedance, then the
operational amplifier supply current may be supplied
by the input source via the current steering diodes.
This state is not a normal bias condition; the amplifier
most likely will not operate normally. If the supplies
are low impedance, then the current through the
steering diodes can become quite high. The current
level depends on the ability of the input source to
deliver current, and any resistance in the input path.
APPLICATION IDEAS
Figure 31 shows the basic configuration for a bridge
amplifier.
A low-side current shunt monitor is shown in
Figure 32. R_N are optional resistors used to isolate
the ADS8325 from the noise of the digital two-wire
bus. Because the ADS8325 is a 16-bit converter, a
precise reference is essential for maximum accuracy.
If absolute accuracy is not required, and the 5V
power supply is sufficiently stable, the REF3330 may
be omitted.
Figure 33 shows a high-side current monitor. The
load current develops a voltage drop across R_SHUNT.
The noninverting input monitors this voltage and is
duplicated on the inverting input. R_G then has the
same voltage drop as R_SHUNT. R_G can be sized to
provide whatever current is most convenient to the
designer based on design constraints. The current
from R_G then flows through the MOSFET and to
resistor R_L, creating a voltage that can be read. Note
that R_L and R_G set the voltage gain of the circuit.
The supply voltage for the op amp is derived from the
zener diode. For the OPA378 V_S must be between
2.2V and 5.5V. Two possible methods to bias the
zener are shown in the circuit of Figure 33: the
customary resistor bias and the current monitor. The
current monitor biasing achieves the lowest possible
voltage. Resistor R₁ and the diode on the
noninverting input provide short-circuit protection.
V_EX
RR
RR
R₁
+5V
OPA378
V_OUT
R₁
V_REF
Figure 31. Single Op Amp Bridge Amplifier
Copyright © 2008–2009, Texas Instruments Incorporated
Product Folder Link(s): OPA378 OPA2378
Submit Documentation Feedback
13