OPA353NASLASH3K datasheet(9/33 Pages) TI2

9 / 33 page

OPA353, 2353, 4353

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 500fA.

However, large inputs (greater than 300mV beyond the

supply rails) can turn on the OPA353’s input protection

diodes, causing 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.

FEEDBACK CAPACITOR IMPROVES RESPONSE

For optimum settling time and stability with high-imped-

ance feedback networks, it may be necessary to add a

feedback capacitor across the feedback resistor, RF, as

shown in Figure 4. This capacitor compensates for the zero

created by the feedback network impedance and the

OPA353’s input capacitance (and any parasitic layout

capacitance). The effect becomes more significant with

higher impedance networks.

FIGURE 3. Input Current Protection for Voltages Exceeding

the Supply Voltage.

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

(>10k

Ω), the output voltage swing is typically ten millivolts

from the supply rails. With heavier resistive loads (600

Ω to

10k

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 performance curves “Output Voltage

Swing vs Output Current” and “Open-Loop Gain vs Output

Voltage.”

CAPACITIVE LOAD AND STABILITY

OPA353 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 impedance,

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

the small-signal response which degrades the phase margin.

In unity gain, OPA353 series op amps perform well with

large capacitive loads. Increasing gain enhances the

amplifier’s ability to drive more capacitance. The typical

performance curve “Small-Signal Overshoot vs Capacitive

Load” shows performance with a 1k

Ω resistive load. In-

creasing load resistance improves capacitive load drive ca-

pability.

FIGURE 4. Feedback Capacitor Improves Dynamic Perfor-

mance.

It is suggested that a variable capacitor be used for the

feedback capacitor since input capacitance may vary be-

tween op amps and layout capacitance is difficult to

determine. For the circuit shown in Figure 4, the value of

the variable feedback capacitor should be chosen so that

the input resistance times the input capacitance of the

OPA353 (typically 9pF) plus the estimated parasitic layout

capacitance equals the feedback capacitor times the feed-

back resistor:

RIN • CIN = RF • CF

where CIN is equal to the OPA353’s input capacitance

(sum of differential and common-mode) plus the layout

capacitance. The capacitor can be varied until optimum

performance is obtained.

DRIVING A/D CONVERTERS

OPA353 series op amps are optimized for driving medium

speed (up to 500kHz) sampling A/D converters. However,

they also offer excellent performance for higher speed

converters. The OPA353 series provides an effective means

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

charge injection while providing signal gain. For applica-

tions requiring high accuracy, the OPA350 series is recom-

mended.

Ω

OPAx353

10mA max

V+

OUT

OVERLOAD

OPA353

V+

OUT

IN • CIN = RF • CF

Where C

IN is equal to the OPA353’s input

capacitance (approximately 9pF) plus any

parastic layout capacitance.

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