OPA342, 2342, 4342

9

SBOS106A

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

fication is the best indicator of the capability of the device

when the application requires use of one of the differential

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 600

Ω loads connected to any potential

between V+ and ground. For light resistive loads (> 50k

Ω),

the output voltage can typically swing to within 1mV from

supply rail. With moderate resistive loads (2k

Ω 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.”

V

O

V

IN

V

B

V+

Non-Inverting Gain

V

V

O

V

B

V

IN

V+

Inverting Amplifier

V

V

O

V

IN

V+

G = 1 Buffer

V

FIGURE 3. Design Optimization with Rail-to-Rail Input Op Amps.

Rail-to-rail op amps can be used in virtually any op amp

configuration. To achieve optimum performance, how-

ever, applications using these special double-input-stage

op amps may benefit from consideration of their special

behavior.

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.

DESIGN OPTIMIZATION WITH RAIL-TO-RAIL INPUT OP AMPS

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.

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.

Inverting amplifiers have a constant common-mode volt-

discontinuity will be created. The bias voltage can gener-

ally be chosen to avoid the transition region.

FIGURE 4. Input Current Protection for Voltages Exceed-

ing the Supply Voltage.

1k

Ω

OPA342

10mA max

V+

V

IN

V

OUT

I

OVERLOAD

IN5818

Schottky diode is required only

if input voltage can go more

than 0.3V below ground.

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