6

OPA349

®

FIGURE 2. Simplified Schematic.

FIGURE 1. AC-Coupled Amplifier.

V

BIAS1

V

BIAS2

V

V

Class AB

Control

Circuitry

V

O

V–

(Ground)

V+

Reference

Current

APPLICATIONS INFORMATION

OPA349 series op amps are unity gain stable and can operate

on a single supply, making them highly versatile and easy to

use. Power supply pins should be by passed with 0.01

µF

ceramic capacitors.

OPA349 series op amps are fully specified and guaranteed

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

operating voltages or temperature are shown in the Typical

Performance Curves.

The ultra low quiescent current of the OPA349 requires

careful applications circuit techniques to achieve low overall

current consumption. Figure 1 shows an ac-coupled ampli-

fier biased with a voltage divider. Resistor values must be

very large to minimize current. The large feedback resistor

value reacts with input capacitance and stray capacitance to

produce a pole in the feedback network. A feedback capaci-

tor may be required to assure stability and limit overshoot or

gain peaking. Check circuit performance carefully to assure

that biasing and feedback techniques meet your signal and

quiescent current requirements.

RAIL-TO-RAIL INPUT

The input common-mode voltage range of the OPA349 series

extends 200mV beyond the supply rails. This is achieved with

a complementary input stage—an N-channel input differen-

tial pair in parallel with a P-channel differential pair (see

Figure 2). The N-channel pair is active for input voltages close

to the positive rail, typically (V+) – 1.3V to 200mV above the

positive supply, while the P-channel pair is on for inputs from

200mV below the negative supply to approximately (V+) –

1.3V. There is a small transition region, typically (V+) – 1.5V

to (V+) – 1.1V, in which both pairs are on. This 400mV

transition region can vary 300mV with process variation.

Thus, the transition region (both stages on) can range from

(V+) – 1.8V to (V+) – 1.4V on the low end, up to (V+) – 1.2V

to (V+) – 0.8V on the high end. Within the 400mV transition

region PSRR, CMRR, offset voltage, offset drift, and THD

may be degraded compared to operation outside this region.

For more information on designing with rail-to-rail input op

amps, see Figure 3 “Design Optimization with Rail-to-Rail

Input Op Amps.”

V

OUT

OPA349

G = 11

C

F

3pF

+1.8 to 5.5V

R

3

2M

R

4

2M

R

5

10M

R

1

10M

R

2

10M

10nF

CF may be required

for best stability or to

reduce frequency

peaking—see text.