OPA703, OPA704

11

SBOS180A

CAPACITIVE LOAD AND STABILITY

The OPA703 and OPA704 series op amps can drive up to

1000pF pure capacitive load. Increasing the gain enhances

the amplifier’s ability to drive greater capacitive loads (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 inside

the feedback loop, as shown in Figure 5. This reduces

ringing with large capacitive loads while maintaining DC

accuracy.

FIGURE 5. Series Resistor in Unity-Gain Buffer Configura-

tion Improves Capacitive Load Drive.

INPUT VOLTAGE

Device inputs are protected by ESD diodes that will conduct

if the input voltages exceed the power supplies by more than

approximately 300mV. Momentary voltages greater than

300mV beyond the power supply can be tolerated if the

current 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 always required. The OPA703

features no phase inversion when the inputs extend beyond

supplies if the input current is limited, as seen in Figure 4.

FIGURE 4. OPA703—No Phase Inversion with Inputs

Greater than the Power-Supply Voltage.

FIGURE 3. Input Current Protection for Voltages Exceeding

the Supply Voltage.

V

20

µs/div

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 1k

Ω loads connected to any point be-

tween V+ and ground. For light resistive loads (> 100k

Ω),

the output voltage can swing to 40mV from the supply rail.

With moderate resistive loads (20k

Ω), the output can swing

to within 75mV from the supply rails while maintaining high

open-loop gain (see the typical performance curve “Output

Voltage Swing vs Output Current”).

APPLICATION CIRCUITS

Figure 6 shows a G = 5 non-inverting amplifier implemented

with the OPA703 and OPA704 op amps. It demonstrates the

increased speed characteristics (bandwidth, slew rate and

settling time) that can be achieved with the OPA704 family

when used in gains of five or greater. Some optimization of

feedback capacitor value may be required to achieve best

dynamic response. Circuits with closed-loop gains of less

than five should use the OPA703 family for good stability

and capacitive load drive. The OPA703 can be used in gains

greater than five, but will not provide the increased speed

benefits of the OPA704 family.

The OPA703 series op amps are optimized for driving

medium-speed sampling data converters. The OPA703 op

amps buffer the converter’s input capacitance and resulting

charge injection while providing signal gain.

Figure 7 shows the OPA2703 in a dual-supply buffered

reference configuration for the DAC7644. The DAC7644 is

a 16-bit, low-power, quad-voltage output converter. Small

size makes the combination ideal for automatic test equip-

ment, data acquisition systems, and other low-power space-

limited applications.

R

OPA703

10mA max

+V

V–

V

IN

V

OUT

I

OVERLOAD

R

S

20

Ω

OPA703

C

L

R

L

V

IN

V

OUT