OPA727 datasheet(12/28 Pages) TI

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RAIL-TO-RAIL OUTPUT

75W

OPA727

ADS8342

16-BitADC

AIN

Common

330pF

±2.5V

+5V

-5V

+5V

-5V

TRANSIMPEDANCE AMPLIFIER

CAPACITIVE LOAD AND STABILITY

20W

OPA727

OUT

V+

DRIVING FAST 16-BIT ADCs

OPA727

OUT

10MW

+5V

(1)

<1pF

NOTE:(1)C isoptionaltopreventgainpeaking.

Itincludesthestraycapacitanceof

OPA727,, OPA2727

OPA4727, OPA728

SBOS314H – SEPTEMBER 2004 – REVISED APRIL 2007

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 from the supply rail, while still

maintaining excellent linearity (AOL > 110dB). With

Ω resistive loads, the output is specified to swing

to within 250mV from the supply rails with excellent

Figure 31. OPA727 Driving an ADC

linearity (see the Typical Characteristics curve,

Output Voltage Swing vs Output Current).

Wide bandwidth, low input bias current, and low

Capacitive load drive is dependent upon gain and

input voltage and current noise make the OPA727 an

the

overshoot

requirements

the

application.

ideal wideband photodiode transimpedance amplifier.

Increasing the gain enhances the ability of the

Low-voltage noise is important because photodiode

amplifier to drive greater capacitive loads (see the

capacitance causes the effective noise gain of the

Typical

Characteristics

curve,

Small-Signal

circuit to increase at high frequency.

Overshoot vs Capacitive Load).

The key elements to a transimpedance design, as

One method of improving capacitive load drive in the

shown in

Figure

32,

are

the

expected

diode

unity-gain configuration is to insert a 10

Ω to 20Ω

capacitance (CD), which should include the parasitic

resistor inside the feedback loop, as shown in

input common-mode and differential-mode input

Figure 30. This reduces ringing with large capacitive

capacitance (4pF + 5pF for the OPA727); the desired

loads while maintaining DC accuracy.

transimpedance gain (RF); and the GBW for the

OPA727 (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.

Figure 30. Series Resistor in Unity-Gain Buffer

Configuration Improves Capacitive Load Drive

The OPA727 series is optimized for driving fast

16-bit ADCs such as the ADS8342. The OPA727 op

amps buffer the converter input capacitance and

resulting charge injection, while providing signal gain.

Figure 31 shows the OPA727 in a single-ended

method

interfacing

the

ADS8342

16-bit,

250kSPS, 4-channel ADC with an input range of

±2.5V. The OPA727 has demonstrated excellent

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

acquisition time of the ADS8342. The RC filter,

shown in Figure 31, has been carefully tuned for best

noise and settling performance. It may need to be

Figure 32. Dual-Supply Transimpedance

adjusted for different op amp configurations. Refer to

Amplifier

the ADS8342 data sheet (available for download at

www.ti.com)

for

additional

information

this

product.

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