OPA341, 2341

10

SBOS202A

FIGURE 3. Input Current Protection for Voltages Exceeding

the Supply Voltage.

FIGURE 4. Series Resistor in Unity-Gain Configuration Improves Capacitive Load Drive.

A double-folded cascode adds the signal from the two input

pairs and presents a differential signal to the class AB output

stage. Normally, input bias current is approximately 600fA,

however, input voltages exceeding the power supplies by

more than 300mV can cause excessive current to flow in or

out of the input pins. Momentary voltages greater than

300mV beyond the power supply can be tolerated if the

current on the input pins is limited to 10mA. This is easily

accomplished with an input resistor, as shown in Figure 3.

Many input signals are inherently current-limited to less

than 10mA, therefore, a limiting 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. For light resistive loads

(> 50k

Ω), the output voltage is typically a few millivolts

from the supply rails. With moderate resistive loads (2k

Ω to

50k

Ω), the output can swing to within a few tens of milli-

volts from the supply rails and maintain high open-loop

gain. See the typical characteristic “Output Voltage Swing

vs Output Current.”

CAPACITIVE LOAD AND STABILITY

OPA341 series op amps can drive a wide range of capacitive

loads. However, all op amps under certain conditions may

become unstable. Op amp configurations, gain, and load

value are just a few of the factors to consider when determin-

ing stability. An op amp in unity-gain configuration is the

most susceptible to the effects of capacitive load. The

capacitive load reacts with the op amp’s output resistance,

along with any additional load resistance, to create a pole in

the small-signal response which degrades the phase margin.

In unity gain, OPA341 series op amps perform well, with a

pure capacitive load up to approximately 1000pF. Increasing

gain enhances the amplifier’s ability to drive more capaci-

tance. See the typical characteristic “Small-Signal Over-

shoot vs Capacitive Load.”

One method of improving capacitive load drive in the unity-

gain configuration is to insert a 10

Ω to 20Ω resistor in series

with the output, as shown in Figure 4. This significantly

reduces ringing with large capacitive loads. However, if there

creates a voltage divider. This introduces a DC error at the

output and slightly reduces output swing. This error may be

there is only about a 0.2% error at the output.

DRIVING A/D CONVERTERS

OPA341 series op amps are optimized for driving medium

speed (up to 100kHz) sampling A/D converters. However,

they also offer excellent performance for higher-speed con-

verters. The OPA341 series provides an effective means of

buffering the A/D converter’s input capacitance and result-

ing charge injection while providing signal gain. For appli-

cations requiring high accuracy, the OPA340 series is rec-

ommended.

The OPA341 implements a power-saving shutdown feature

particularly useful for low-power sampling applications. Figure 5

shows the OPA341 driving the ADS7816, a 12-bit micro-power

sampling converter available in the tiny MSOP-8 package. With

the OPA341 in non-inverting configuration, an RC network at the

amplifier’s output is used as an anti-aliasing filter. By tying the

enable of the OPA341 to the shutdown of the ADS7816, addi-

tional power-savings can be used for sampling applications. To

effectively drive the ADS7816, timing delay was introduced

between the two devices, see Figure 5. Alternative applications

may need additional timing adjustments.

Figure 6 shows the OPA341 configured as a speech band-

pass filter. Figure 7 shows the OPA341 configured as a

transimpedance amplifier.

OPAx341

10mA max

V+

V

IN

V

OUT

I

OVERLOAD

10

Ω to

20

Ω

OPAx341

V+

V

ENABLE

V

IN

V

OUT

R

S

R

L

C

L