OPA342_06 Datasheet, PDF (9/13 Pages) Burr-Brown (TI) – Low-Cost, Low-Power, Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series

DESIGN OPTIMIZATION WITH RAIL-TO-RAIL INPUT OP AMPS
Rail-to-rail op amps can be used in virtually any op amp With a unity-gain buffer, for example, signals will traverse
configuration. To achieve optimum performance, how- this transition at approximately 1.3V below V+ supply
ever, applications using these special double-input-stage and may exhibit a small discontinuity at this point.
op amps may benefit from consideration of their special
behavior.
The common-mode voltage of the non-inverting ampli-
fier is equal to the input voltage. If the input signal always
In many applications, operation remains within the com- remains less than the transition voltage, no discontinuity
mon-mode range of only one differential input pair. will be created. The closed-loop gain of this configura-
However some applications exercise the amplifier through tion can still produce a rail-to-rail output.
the transition region of both differential input stages.
Although the two input stages are laser trimmed for
excellent matching, a small discontinuity may occur in
this transition. Careful selection of the circuit configura-
tion, signal levels and biasing can often avoid this transi-
Inverting amplifiers have a constant common-mode volt-
age equal to V_B. If this bias voltage is constant, no
discontinuity will be created. The bias voltage can gener-
ally be chosen to avoid the transition region.
tion region.
G = 1 Buffer
Non-Inverting Gain
Inverting Amplifier
V+
V+
V_B
V+
V_IN
V_O
V_IN
V_CM = V_IN = V_O
V_O
V_IN
V_CM = V_IN
V_O
V_B
V_CM = V_B
FIGURE 3. Design Optimization with Rail-to-Rail Input Op Amps.
COMMON-MODE REJECTION
The CMRR for the OPA342 is specified in several 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+) – 1.8V) is given. This speci-
fication 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 = 5.5V over the entire
common-mode range is specified. Third, the CMRR at V_S =
2.7V over the entire common-mode range is provided. These
last two values include the variations seen through the
transition region.
INPUT VOLTAGE BEYOND THE RAILS
If the input voltage can go more than 0.3V below the
negative power supply rail (single-supply ground), special
precautions are required. If the input voltage goes suffi-
ciently negative, the op amp output may lock up in an
inoperative state. A Schottky diode clamp circuit will pre-
vent this—see Figure 4. The series resistor prevents exces-
sive current (greater than 10mA) in the Schottky diode and
in the internal ESD protection diode, if the input voltage can
exceed the positive supply voltage. If the signal source is
limited to less than 10mA, the input resistor is not required.
RAIL-TO-RAIL OUTPUT
A class AB output stage with common-source transistors is
used to achieve rail-to-rail output. This output stage is
capable of driving 600Ω loads connected to any potential
OPA342, 2342, 4342
SBOS106A
between V+ and ground. For light resistive loads (> 50kΩ),
the output voltage can typically swing to within 1mV from
supply rail. With moderate resistive loads (2kΩ to 50kΩ),
the output can swing to within a few tens of milli-volts from
the supply rails while maintaining high open-loop gain. See
the typical performance curve “Output Voltage Swing vs
Output Current.”
I_OVERLOAD
10mA max
V_IN
1kΩ
V+
OPA342
V_OUT
IN5818
Schottky diode is required only
if input voltage can go more
than 0.3V below ground.
FIGURE 4. Input Current Protection for Voltages Exceed-
ing the Supply Voltage.
CAPACITIVE LOAD AND STABILITY
The OPA342 in a unity-gain configuration can directly drive
up to 250pF pure capacitive load. Increasing the gain en-
hances the amplifier’s ability to drive greater capacitive
loads. See the typical performance curve “Small-Signal
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