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®

OPA340/2340/4340

OPA340 series op amps are laser-trimmed to the reduce

offset voltage difference between the N-channel and

P-channel input stages, resulting in improved common-

mode rejection and a smooth transition between the

N-channel pair and the P-channel pair. However, within the

400mV transition region PSRR, CMRR, offset voltage,

offset drift, and THD may be degraded compared to opera-

tion outside this region.

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 200fA,

however, input voltages exceeding the power supplies by

more than 500mV can cause excessive current to flow in or

out of the input pins. Momentary voltages greater than

500mV 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.

CAPACITIVE LOAD AND STABILITY

OPA340 series op amps can drive a wide range of capacitive

loads. However, all op amps under certain conditions may

become unstable. Op amp configuration, 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, OPA340 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 performance curve “Small-Signal

Overshoot 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 is a resistive load in parallel with the capacitive load,

it creates a voltage divider introducing 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

OPA340 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

converters. The OPA340 series provides an effective means

of buffering the A/D’s input capacitance and resulting

charge injection while providing signal gain.

Figures 5 and 6 show the OPA340 driving an ADS7816.

The ADS7816 is a 12-bit, micro-power sampling converter

in the tiny MSOP-8 package. When used with the minia-

ture package options of the OPA340 series, the combina-

tion is ideal for space-limited and low power applications.

For further information consult the ADS7816 data sheet.

With the OPA340 in a noninverting configuration, an RC

network at the amplifier’s output can be used to filter high

frequency noise in the signal (Figure 5). In the inverting

configuration, filtering may be accomplished with a ca-

pacitor across the feedback resistor (Figure 6).

FIGURE 3. Input Current Protection for Voltages Exceeding

the Supply Voltage.

5k

Ω

OPAx340

10mA max

V+

V

IN

V

OUT

I

OVERLOAD

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 performanc curve “Output Voltage

Swing vs Output Current.”

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

10

Ω to

20

Ω

OPAx340

V+

V

IN

V

OUT

R

S

R

L

C

L