OPA705, 2705, 4705

9

SBOS182A

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”).

CAPACITIVE LOAD AND STABILITY

The OPA705 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 OPA705

features no phase inversion when the inputs extend beyond

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

FIGURE 4. OPA705—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

APPLICATION CIRCUITS

The OPA705 series op amps are optimized for driving

medium-speed sampling data converters. Figure 6 shows the

OPA2705 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 combi-

nation ideal for automatic test equipment, data acquisition

systems, and other low-power space-limited applications.

R

OPA705

10mA max

+V

V–

V

IN

V

OUT

I

OVERLOAD

R

S

20

Ω

OPA705

C

L

R

L

V

IN

V

OUT