OPA4340-EP datasheet(11/17 Pages) TI

Part #	OPA4340-EP

Description	SINGLE-SUPPLY RAIL-TO-RAIL OPERATIONAL AMPLIFIERS

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CAPACITIVE LOAD AND STABILITY

DRIVING A/D CONVERTERS

10

to

W

20W

OPAx340

V+

V

IN

V

OUT

R

S

R

L

C

L

OPA340-EP

OPA2340-EP

OPA4340-EP

www.ti.com ................................................................................................................................................................................................ SBOS433 – AUGUST 2008

OPA340 series operational amplifiers can drive a

OPA340 series operational amplifiers are optimized

wide

range

of

capacitive

loads.

However,

all

for driving medium speed (up to 100 kHz) sampling

operational amplifiers under certain conditions may

A/D converters. However, they also offer excellent

become unstable. Op amp configuration, gain, and

performance

for

higher

speed

converters.

The

load value are just a few of the factors to consider

OPA340 series provides an effective means of

when determining stability. An operational amplifier in

buffering the A/D converter’s input capacitance and

unity gain configuration is most susceptible to the

resulting charge injection while providing signal gain.

effects of capacitive load. The capacitive load reacts

Figure 27 and Figure 28 show the OPA340 driving an

with the operational amplifier’s output resistance,

ADS7816. The ADS7816 is a 12-bit, micro-power

along with any additional load resistance, to create a

sampling converter in the tiny MSOP-8 package.

pole in the small-signal response which degrades the

When used with the miniature package options of the

phase

margin.

In

unity

gain,

OPA340

series

OPA340

series,

the

combination

is

ideal

for

operational amplifiers perform well, with a pure

space-limited and low-power applications. For further

capacitive

load

up

to

approximately

1000

pF.

information consult the ADS7816 data sheet. With the

Increasing gain enhances the amplifier’s ability to

OPA340 in a noninverting configuration, an RC

drive more capacitance. See the typical performance

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

curve Small-Signal Overshoot vs Capacitive Load.

high-frequency noise in the signal (see Figure 27). In

the

inverting

configuration,

filtering

may

be

One method of improving capacitive load drive in the

accomplished with a capacitor across the feedback

unity gain configuration is to insert a 10-

Ω to 20-Ω

resistor (see Figure 28).

resistor in series with the output, as shown in

Figure 26. 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

insignificant. For example, with RL = 10 kΩ and

RS = 20 Ω, there is only approximately 0.2% error at

the output.

Figure 26. Series Resistor in Unity-Gain

Configuration Improves Capacitive Load Drive

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Product Folder Link(s): OPA340-EP OPA2340-EP OPA4340-EP

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