TLC080 datasheet(12/33 Pages) TI2

12 / 33 page

OPA725, OPA2725

OPA726, OPA2726

SBOS278B − SEPTEMBER 2003 − REVISED JANUARY 2004

www.ti.com

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 heavy loads connected to any point

between V+ and V−. For light resistive loads ( > 100k

Ω ),

the output voltage can swing to 150mV (175mV for dual)

from the supply rail, while still maintaining excellent

linearity (AOL > 110dB). With 1kΩ (2kΩ for dual) resistive

loads, the output is specified to swing to within 250mV from

the supply rails with excellent linearity (see the Typical

Characteristics curve Output Voltage Swing vs Output

Current).

CAPACITIVE LOAD AND STABILITY

Capacitive load drive is dependent upon gain and the

overshoot requirements of the application. Increasing the

gain enhances the ability of the amplifier to drive greater

capacitive loads (see the Typical Characteristics 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 3. This

reduces ringing with large capacitive loads while

maintaining DC accuracy.

Ω

OPA725

OUT

V+

Figure 3. Series Resistor in Unity-Gain Buffer

Configuration Improves Capacitive Load Drive

DRIVING FAST 16-BIT ADCs

The OPA725 series is optimized for driving fast 16-bit

ADCs such as the ADS8342. The OPA725 op amps buffer

the converter input capacitance and resulting charge

injection, while providing signal gain. Figure 4 shows the

OPA725 in a single-ended method of interfacing to the

ADS8342 16-bit, 250kSPS, 4-channel ADC with an input

range of

±2.5V. The OPA725 has demonstrated excellent

settling time to the 16-bit level within the 600ns acquisition

time of the ADS8342. The RC filter, shown in Figure 4, has

been carefully tuned for best noise and settling

performance. It may need to be adjusted for different op

amp configurations. Please refer to the ADS8342 data

sheet (available for download at www.ti.com) for additional

information on this product.

Ω

OPA725

ADS8342

16−Bit ADC

AIN

Common

330pF

±2.5V

+5V

−5V

+5V

−5V

Figure 4. OPA725 Driving an ADC

TRANSIMPEDANCE AMPLIFIER

Wide bandwidth, low input bias current, and low input

voltage and current noise make the OPA725 an ideal

wideband photodiode transimpedance amplifier. Low-

voltage noise is important because photodiode capaci-

tance causes the effective noise gain of the circuit to

increase at high frequency.

The key elements to a transimpedance design, as shown

in Figure 5, are the expected diode capacitance (CD),

which should include the parasitic input common-mode

and differential-mode input capacitance (4pF + 5pF for the

OPA725); the desired transimpedance gain (RF); and the

GBW for the OPA725 (20MHz). With these three variables

set, the feedback capacitor value (CF) can be set to control

the frequency response. CF includes the stray capacitance

of RF, which is 0.2pF for a typical surface-mount resistor.

OPA725

OUT

10M

Ω

+5V

−5V

(1)

<1pF

NOTE: (1) C

F is optional to prevent gain peaking.

It includes the stray capacitance of R

Figure 5. Dual-Supply Transimpedance Amplifier

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