Datasheet

OPA349, 2349
9
SBOS121B
www.ti.com
FIGURE 2. Simplified Schematic.
FIGURE 1. AC-Coupled Amplifier.
V
BIAS1
V
BIAS2
V
IN
+
V
IN
–
Class AB
Control
Circuitry
V
O
V–
(Ground)
V+
Reference
Current
APPLICATIONS INFORMATION
The 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 bypassed with
0.01µF ceramic capacitors.
The OPA349 series op amps are fully specified and tested
from +1.8V to +5.5V. Parameters that vary significantly with
operating voltages or temperature are shown in the Typical
Characteristic curves.
The ultra-low quiescent current of the OPA349 requires
careful application circuit techniques to achieve low overall
current consumption. Figure 1 shows an ac-coupled amplifier
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 pro-
duce a pole in the feedback network. A feedback capacitor
may be required to assure stability and limit overshoot or
gain peaking. Check circuit performance carefully to assure
that biasing and feedback techniques meet signal and quies-
cent 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 differential pair
in parallel with a P-channel differential pair (as shown in 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
+1.8V to 5.5V
R
3
2M
R
4
2M
R
5
10M
R
1
10M
R
2
10M
10nF
C
F
may be required
for best stability or to
reduce frequency
peaking—see text.