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
this transition at approximately 1.3V below V+ supply
and may exhibit a small discontinuity at this point.
configuration. To achieve optimum performance, how-
ever, applications using these special double-input-stage
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
remains less than the transition voltage, no discontinuity
will be created. The closed-loop gain of this configura-
tion can still produce a rail-to-rail output.
In many applications, operation remains within the com-
mon-mode range of only one differential input pair.
However some applications exercise the amplifier through
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-
tion region.
Inverting amplifiers have a constant common-mode volt-
age equal to VB. If this bias voltage is constant, no
discontinuity will be created. The bias voltage can gener-
ally be chosen to avoid the transition region.
G = 1 Buffer
Non-Inverting Gain
Inverting Amplifier
V+
V+
V+
VB
VIN
VO
VO
VO
VIN
VIN
VB
VCM = VB
VCM = VIN = VO
VCM = VIN
FIGURE 3. Design Optimization with Rail-to-Rail Input Op Amps.
COMMON-MODE REJECTION
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.”
The CMRR for the OPA344 and OPA345 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 (VCM < (V+) – 1.8V) is
given. This specification is the best indicator of the capabil-
ity of the device when the application requires use of one of
the differential input pairs. Second, the CMRR at VS = 5.5V
over the entire common-mode range is specified. Third, the
CMRR at VS = 2.7V over the entire common-mode range is
provided. These last two values include the variations seen
through the transition region.
V+
IOVERLOAD
10mA max
VOUT
OPA344
INPUT VOLTAGE BEYOND THE RAILS
VIN
1kΩ
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.
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
RAIL-TO-RAIL OUTPUT
The OPA344 in a unity-gain configuration and the OPA345
in gains greater than 5 can directly drive up to 250pF pure
capacitive load. Increasing the gain enhances the amplifier’s
ability to drive greater capacitive loads. See the typical
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
®
OPA344, 2344, 4344
10
OPA345, 2345, 4345
BB [ BURR-BROWN CORPORATION ]
