OPA349 Datasheet, PDF (6/7 Pages) Burr-Brown (TI) – 1mA, Rail-to-Rail, CMOS OPERATIONAL AMPLIFIERS

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 0.01µF
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
+1.8 to 5.5V
R₁
R₃
10M
2M
C_F
3pF ^{CF may be required}
for best stability or to
reduce frequency
R₅
10M
peaking—see text.
10nF
OPA349
G = 11
V_OUT
R₂
R₄
10M
2M
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.”
FIGURE 1. AC-Coupled Amplifier.
V+
Reference
Current
V_IN+
V_IN–
V_BIAS1
Class AB
Control
V_O
Circuitry
V_BIAS2
V–
(Ground)
FIGURE 2. Simplified Schematic.
®
OPA349
6