OPA727 datasheet(13/28 Pages) TI

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2pR

FCF

GBW

4pR

FCD

(1)

OPTIMIZING THE TRANSIMPEDANCE

*3dB +

GBW

2pR

FCD

(2)

OPA727

OUT

10MW

V+

(1)

<1pF

NOTE:(1)C isoptionaltopreventgainpeaking.

Itincludesthestraycapacitanceof

Bias

OPA727,, OPA2727

OPA4727, OPA728

SBOS314H – SEPTEMBER 2004 – REVISED APRIL 2007

To achieve a maximally-flat, 2nd-order Butterworth

For

additional

information,

refer

Application

frequency response, the feedback pole should be set

Bulletin (SBOA055), Compensate Transimpedance

to:

Amplifiers

Intuitively,

available

for

download

www.ti.com.

CIRCUIT

Bandwidth is calculated by:

To achieve the best performance, components

should be selected according to the following

guidelines:

1. For lowest noise, select RF to create the total

For even higher transimpedance bandwidth, the

required gain. Using a lower value for RF and

high-speed CMOS OPA380 (90MHz GBW), OPA354

adding gain after the transimpedance amplifier

(100MHz GBW), OPA300 (180MHz GBW), OPA355

generally produces poorer noise performance.

(200MHz GBW), or OPA656, OPA657 (400MHz

The noise produced by RF increases with the

GBW) may be used.

square-root

RF,

whereas

the

signal

For single-supply applications, the +IN input can be

increases linearly. Therefore, signal-to-noise

biased with a positive dc voltage to allow the output

ratio is improved when all the required gain is

to reach true zero when the photodiode is not

placed in the transimpedance stage.

exposed to any light, and respond without the added

2. Minimize photodiode capacitance and stray

delay that results from coming out of the negative

capacitance at the summing junction (inverting

rail; this is shown in Figure 33. This bias voltage also

input). This capacitance causes the voltage

appears across the photodiode, providing a reverse

noise

the

amp

amplified

bias for faster operation.

(increasing amplification at high frequency).

Using

low-noise

voltage

source

reverse-bias a photodiode can significantly

reduce its capacitance. Smaller photodiodes

have

lower

capacitance.

Use

optics

concentrate light on a small photodiode.

3. Noise increases with increased bandwidth.

Limit

the

circuit

bandwidth

only

that

required. Use a capacitor across the RF to

limit bandwidth, even if not required for

stability.

4. Circuit

board

leakage

can

degrade

the

performance of an otherwise well-designed

amplifier. Clean the circuit board carefully. A

circuit board guard trace that encircles the

summing junction and is driven at the same

voltage can help control leakage.

For additional information, refer to the Application

Bulletins Noise Analysis of FET Transimpedance

Amplifiers

(SBOA060),

and

Noise

Analysis

for

High-Speed Op Amps (SBOA066), available for

download at the TI web site.

Figure 33. Single-Supply Transimpedance

Amplifier

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