OPA378 datasheet(13/27 Pages) TI

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OUT

REF

OPA378

R R

+5V

OPA378

OPA2378

www.ti.com

SBOS417D – JANUARY 2008 – REVISED OCTOBER 2009

ESD

event

produces

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 +VS and/or –VS 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. RN 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

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 RSHUNT.

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. RG then has the

absorption device.

same voltage drop as RSHUNT. RG can be sized to

Figure 30 depicts a specific example where the input

provide whatever current is most convenient to the

voltage, VIN, exceeds the positive supply voltage

designer based on design constraints. The current

(+VS) by 300mV or more. Much of what happens in

from RG then flows through the MOSFET and to

the circuit depends on the supply characteristics. If

resistor RL, creating a voltage that can be read. Note

+VS can sink the current, one of the upper input

that RL and RG set the voltage gain of the circuit.

steering diodes conducts and directs current to +VS.

The supply voltage for the op amp is derived from the

Excessively

high

current

levels

can

flow

with

zener diode. For the OPA378 VS must be between

increasingly higher VIN. 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, VIN

current monitor biasing achieves the lowest possible

may

begin

sourcing

current

the

operational

voltage.

Resistor

and

the

diode

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

+VS and –VS are applied. If this event happens, a

direct current path is established between the +VS

and –VS 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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