OPA378 datasheet(9/14 Pages) TI

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APPLICATIONS INFORMATION

5kW

OPA378

10mAmax

+5V

OUT

OVERLOAD

Current-limitingresistor

requiredifinputvoltage

exceedssupplyrailsby

³ 0.5V.

INTERNAL OFFSET CORRECTION

GENERAL LAYOUT GUIDELINES

OPERATING VOLTAGE

INPUT VOLTAGE

OPA378

OPA2378

SBOS417 – JANUARY 2008

The OPA378 and OPA2378 are unity-gain stable and

free from unexpected output phase reversal. These

devices use a proprietary auto-calibration technique

to provide low offset voltage and very low drift over

time and temperature. For lowest offset voltage and

precision performance, circuit layout and mechanical

conditions should be optimized. Avoid temperature

gradients that create thermoelectric (Seebeck) effects

in the thermocouple junctions formed from connecting

dissimilar

conductors.

These

thermally-generated

potentials can be made to cancel by assuring they

Figure 23. Input Current Protection

are equal on both input terminals. Other layout and

design considerations include:

• Use low thermoelectric-coefficient conditions

(avoid dissimilar metals).

The OPA378 and OPA2378 op amps use an

• Thermally isolate components from power

auto-calibration technique with a time-continuous

supplies or other heat sources.

350kHz op amp in the signal path. This amplifier is

• Shield op amp and input circuitry from air

zero-corrected

every

µs using a proprietary

currents, such as cooling fans.

technique. Upon power-up, the amplifier requires

Following these guidelines reduces the likelihood of

approximately

100

µs to achieve specified V

junctions being at different temperatures, which can

accuracy. This architecture has no aliasing or flicker

cause thermoelectric voltages of 0.1

µV/°C or higher,

noise.

depending on materials used.

Attention

good

layout

practices

always

The OPA378 and OPA2378 op amps operate over a

recommended.

Keep

traces

short

and,

when

power-supply range of +1.8V to +5.5V (±0.9V to

possible, use a printed circuit board (PCB) ground

±2.75V). Supply voltages higher than +7V (absolute

plane with surface-mount components placed as

maximum) can permanently damage the device.

close to the device pins as possible. Place a 0.1

µF

Parameters

that

vary

over

supply

voltage

capacitor closely across the supply pins. These

temperature are shown in the Typical Characteristics

guidelines should be applied throughout the analog

section of this data sheet.

circuit

improve

performance

and

provide

benefits

such

reducing

the

electromagnetic

interference (EMI) susceptibility.

Operational amplifiers vary in susceptibility to radio

The OPA378 and OPA2378 input common-mode

frequency interference (RFI). RFI can generally be

voltage range extends 0.1V beyond the supply rails.

identified as a variation in offset voltage or dc signal

The

OPA378

designed

cover

the

full

levels with changes in the interfering RF signal. The

common-mode

range

without

the

troublesome

OPA378 has been specifically designed to minimize

transition region found in some other rail-to-rail

susceptibility to RFI and demonstrates remarkably

amplifiers.

low sensitivity compared to previous generation

Normally, input bias current is about 150pA; however,

devices. Despite this design, strong RF fields may

input voltages exceeding the power supplies can

cause varying offset levels. If the amplifier cannot be

cause excessive current to flow into or out of the

located away from sources of radiation, shielding may

input pins. Momentary voltages greater than the

be needed. Twisting wire input leads makes them

power supply can be tolerated if the input current is

more resistant to RF fields.

limited to 10mA. This limitation is easily accomplished

with an input resistor, as shown in Figure 23.

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