OPA656SLASH7 datasheet(10/26 Pages) TI2

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OPA381

OPA2381

SBOS313B − AUGUST 2004 − REVISED NOVEMBER 2004

www.ti.com

BIASING PHOTODIODES IN SINGLE-SUPPLY

CIRCUITS

The +IN input can be biased with a positive DC voltage

to offset the output voltage and allow the amplifier

output to indicate a true zero photodiode measurement

when the photodiode is not exposed to any light. It will

also prevent the added delay that results from coming

out of the negative rail. This bias voltage appears

across the photodiode, providing a reverse bias for

faster operation. An RC filter placed at this bias point will

reduce noise. (Refer to Figure 4.) This bias voltage can

also serve as an offset bias point for an ADC with range

that does not include ground.

OPA381

OUT =IDRF +VBIAS

100k

Ω

V+

10M

Ω

(1)

< 1pF

0.1

µF

NOTE: (1) C

F is optional to prevent gain peaking.

It includes the stray capacitance of R

BIAS

[0V to (V+)

− 1.8V]

Figure 4. Photodiode with Filtered Reverse Bias

Voltage

TRANSIMPEDANCE AMPLIFIER

Wide bandwidth, low input bias current and low input

voltage and current noise make the OPA381 an ideal

wideband photodiode transimpedance amplifier. Low

voltage noise

important

because

photodiode

capacitance causes the effective noise gain of the

circuit to increase at high frequency.

The key elements to a transimpedance design are

shown in Figure 5:

D the total input capacitance (CTOT), consisting of the

photodiode capacitance (CDIODE) plus the parasitic

common-mode

and

differential-mode

input

capacitance (2.5pF + 1pF for the OPA381);

D the desired transimpedance gain (RF);

D the Gain Bandwidth Product (GBW) for the

OPA381 (18MHz).

With these three variables set, the feedback capacitor

value (CF) can be set to control the frequency response.

CSTRAY is the stray capacitance of RF, which is 0.2pF for

a typical surface-mount resistor.

To achieve a maximally flat 2nd-order Butterworth

frequency response, the feedback pole should be set

to:

F CF

) C

STRAY

GBW

FCTOT

Bandwidth is calculated by:

f*3dB +

GBW

FCTOT

These

equations

will

result

maximum

transimpedance

bandwidth.

For

even

higher

transimpedance bandwidth, the high-speed CMOS

OPA380 (90MHz GBW), the OPA300 (150MHz GBW),

or the OPA656 (230MHz GBW) may be used.

For additional information, refer to Application Bulletin

AB−050 (SBOA055), Compensate Transimpedance

Amplifiers

Intuitively,

available

for

download

www.ti.com.

TOT

(3)

OPA381

OUT

−5V

10M

Ω

+5V

(1)

STRAY

(2)

NOTE: (1) C

F is optional to prevent gain peaking.

(2) C

STRAY is the stray capacitance of RF

(typically, 0.2pF for a surface−mount resistor).

(3) C

TOT is the photodiode capacitance plus OPA381

input capacitance.

P (optional

pulldown resistor)

Figure 5. Transimpedance Amplifier

(1)

(2)

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