OPA379

OPA2379

OPA4379

SBOS347A − NOVEMBER 2005 − REVISED DECEMBER 2005

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7

APPLICATION INFORMATION

The OPA379 family of operational amplifiers minimizes

power consumption without compromising bandwidth or

noise.

Power-supply

rejection

ratio

(PSRR),

common-mode rejection ratio (CMRR), and open-loop

When designing for ultra-low power, choose system

components carefully. To minimize current consumption,

select large-value resistors. Any resistors will react with

stray capacitance in the circuit and the input capacitance

of

the

operational

amplifier.

These

parasitic

RC

combinations can affect the stability of the overall system.

A feedback capacitor may be required to assure stability

and limit overshoot or gain peaking.

Good layout practice mandates the use of a 0.1

µF bypass

capacitor placed closely across the supply pins.

OPERATING VOLTAGE

OPA379 series op amps are fully specified and tested from

+1.8V to +5.5V. Parameters that vary significantly with

supply voltage are shown in the Typical Characteristics

curves.

INPUT COMMON-MODE VOLTAGE RANGE

The input common-mode voltage range of the OPA379

family typically extends 100mV beyond each supply rail.

This rail-to-rail input is achieved using a complementary

input stage. CMRR is specified from the negative rail to 1V

below the positive rail. Between (V+) − 1V and (V+) + 0.1V,

the amplifier operates with higher offset voltage because

of the transition region of the input stage. See the typical

characteristic, Offset Voltage vs Common-Mode Voltage.

PROTECTING INPUTS FROM

OVER-VOLTAGE

Normally, input currents are 5pA. However, large inputs

(greater than 500mV beyond the supply rails) can cause

excessive current to flow in or out of the input pins.

Therefore, as well as keeping the input voltage below the

maximum rating, it is also important to limit the input

current to less than 10mA. This limiting is easily

accomplished with an input voltage resistor, as shown in

Figure 1.

5k

Ω

OPA379

10mA max

+5V

V

IN

V

OUT

I

OVERLOAD

Figure 1. Input Current Protection for Voltages

Exceeding the Supply Voltage

NOISE

Although micropower amplifiers frequently have high

wideband noise, the OPA379 series offer excellent noise

performance. Resistors should be chosen carefully

because the OPA379 has only 2.8

noise, and 80nV/

√Hz of wideband noise; otherwise, they

can become the dominant source of noise.

CAPACITIVE LOAD AND STABILITY

Follower configurations with load capacitance in excess of

30pF

can

produce

extra

overshoot

(see

typical

characteristic, Small-Signal Overshoot vs Capacitive

Load) and ringing in the output signal. Increasing the gain

enhances the ability of the amplifier to drive greater

capacitive loads. In unity-gain configurations, capacitive

load drive can be improved by inserting a small (10

Ω to

20

Figure 2. This resistor significantly reduces ringing while

maintaining DC performance for purely capacitive loads.

However, if there is a resistive load in parallel with the

capacitive load, a voltage divider is created, introducing a

Direct Current (DC) error at the output and slightly

reducing the output swing. The error introduced is

10

Ω to

20

Ω

OPA379

V+

V

IN

V

OUT

R

S

R

L

C

L

Figure 2. Series Resistor in Unity-Gain Buffer

Configuration Improves Capacitive Load Drive