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

DESIGN OPTIMIZATION WITH RAIL-TO-RAIL INPUT OP AMPS
In most applications, operation is within the range of only
one differential pair. However, some applications can
subject the amplifier to a common-mode signal in the
transition region. Under this condition, the inherent mis-
match between the two differential pairs may lead to
degradation of the CMRR and THD. The unity-gain buffer
configuration is the most problematic—it will traverse
through the transition region if a sufficiently wide input
swing is required. A design option would be to configure
the op amp as a unity-gain inverter as shown below and
hold the noninverting input at a set common-mode voltage
outside the transition region. This can be accomplished
with a voltage divider from the supply. The voltage divider
should be designed such that the biasing point for the
noninverting input is outside the transition the region.
R
R
V_OUT
V_IN
V_CM
FIGURE 3. Design Optimization.
COMMON-MODE REJECTION
The CMRR for the OPA349 is specified in two ways so the
best match for a given application may be used. First, the
CMRR of the device in the common-mode range below the
transition region (V_CM < (V+) – 1.5V) is given. This specifi-
cation is the best indicator of the capability of the device when
the application requires use of one of the differential input
pairs. Second, the CMRR at V_S = 5V over the entire common-
mode range is specified.
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output.
Loads that connect to single supply ground (or the V- supply
pin) can cause the op amp to oscillate if the output voltage
is driven to the low limit (Figure 4). Similarly, loads that can
cause current to flow out of the output pin when the output
voltage is near V– can cause oscillations. The op amp will
recover to normal operation a few milliseconds after the
output is driven positively out of the rail.
Some op amp applications can produce this condition even
without a load connected to V– The integrator in Figure 4a
shows an example. Assume that the output ramps nega-
tively, and saturates near 0V. Any negative-going step at
V_IN will produce a positive output current pulse through R1
and C1. This may incite the oscillation. Diode, D1, prevents
the input step from pulling output current when the output is
saturated at the rail, thus preventing the oscillation.
a)
V+
OPA349
V_IN
V_O
R_L
0V
FIGURE 4. Output Driven to Negative Rail.
b)
R₁
1M
V+
C₁
1nF
V_IN
2V
D1
0V
1N4148
OPA349
0V
(No Load)
1V
®
7
OPA349