OPA656SLASH7 datasheet(8/26 Pages) TI2

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OPA381

OPA2381

SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004

www.ti.com

APPLICATIONS INFORMATION

BASIC OPERATION

The OPA381 is a high-precision transimpedance

amplifier with very low 1/f noise. Due to its unique

architecture, the OPA381 has excellent long-term input

voltage offset stability.

The OPA381 performance results from an internal

auto-zero amplifier combined with a high-speed

amplifier. The OPA381 has been designed with circuitry

to improve overload recovery and settling time over that

achieved by a traditional composite approach. It has

been specifically designed and characterized to

accommodate circuit options to allow 0V output

operation (see Figure 3).

The OPA381 is used in inverting configurations, with the

noninverting input used as a fixed biasing point.

Figure 1 shows the OPA381 in a typical configuration.

Power-supply pins should be bypassed with 1

µF

ceramic or tantalum capacitors. Electrolytic capacitors

are not recommended.

OPA381

OUT

(1)

(0.5V to 4.4V)

BIAS =0.5V

+5V

µF

NOTE: (1) V

OUT = 0.5V in dark conditions.

Figure 1. OPA381 Typical Configuration

OPERATING VOLTAGE

OPA381 series op amps are fully specified from 2.7V to

5.5V over a temperature range of −40

°C to +125°C.

Parameters that vary significantly with operating

voltages or temperature are shown in the Typical

Characteristics.

INTERNAL OFFSET CORRECTION

The OPA381 series op amps use an auto-zero topology

with a time-continuous 18MHz op amp in the signal

path. This amplifier is zero-corrected every 100

µs using

a proprietary technique. Upon power-up, the amplifier

requires approximately 400

µs to achieve specified VOS

accuracy, which includes one full auto-zero cycle of

approximately 100

µs and the start-up time for the bias

circuitry. Prior to this time, the amplifier will function

properly but with unspecified offset voltage.

This design has virtually no aliasing and low noise. Zero

correction occurs at a 10kHz rate, but there is virtually

no fundamental noise energy present at that frequency

due to internal filtering. For all practical purposes, any

glitches have energy at 20MHz or higher and are easily

filtered, if necessary. Most applications are not sensitive

to such high-frequency noise, and no filtering is

required.

INPUT VOLTAGE

The input common-mode voltage range of the OPA381

series extends from V− to (V+) –1.8V. With input signals

above this common-mode range, the amplifier will no

longer provide a valid output value, but it will not latch

or invert.

INPUT OVERVOLTAGE PROTECTION

Device inputs are protected by ESD diodes that will

conduct if the input voltages exceed the power supplies

by more than approximately 500mV. Momentary

voltages greater than 500mV beyond the power supply

can be tolerated if the current is limited to 10mA. The

OPA381 family features no phase inversion when the

inputs extend beyond supplies if the input is current

limited.

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