SP8126 datasheet(3/8 Pages) SIPEX

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Rev:B Date: 3/18/04

SP8126B High Speed Differential APC Amplifier

PIN DESCRIPTION

PIN NUMBER

NAME

FUNCTION

1VCC

Supply Voltage

GAIN

Gain Select

3RGCOM

Common connection point for RGAIN1 and RGAIN2

GND

Power Ground

5RGAIN1

Gain Adjust 1 (Gain Select = LOW) or OPEN

6RGAIN2

Gain Adjust 2 (Gain Select = HIGH)

7VOUT-

Output Voltage -

8VOUT+

Output Voltage +

Internal Operation

The SP8126B APC circuit has an integrated

photo detector and is designed with nominal

sensitivities of 3mV/

µW for both 650nm and

780nm wavelength laser light. The part's sensi-

tivity can also be adjusted continuously and

independently for two different gain modes via

two external resistors over a range of

±6dB. The

two gain modes are controlled by a TTL com-

patible logic input, called GAIN SELECT. This

logic pin selects between the two external gain

setting resistors to allow independent control

and settings for the two gain functions.

The system is a two stage design, consisting of

a Trans-Impedance Amplifier (TIA) and an out-

put buffer stage. In dark condition the outputs

VOUT- and VOUT+ are set to a reference voltage

that is defined internally as Vcc/2. With 650nm

or 780nm wavelength laser light falling on the

photodetector, the output will swing differen-

tially around Vcc/2 proportional with the light

power according to the chosen gain.

TIA and Gain control

The first stage is a differential TIA used to

convert the photodetector current to a balanced

differential voltage. The traditional fixed feed-

back resistors have been replaced with an active

resistor circuit that sets the trans-impedance value.

A Resistor Control Block that senses the value

of the external gain setting resistor controls the

value of the equivalent feedback resistor.

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Output Settling Time (780nm)

2VPP Step

(1% of Final Value)

2VPP Step,

RGAIN = 1230Ω (High Gain)

Output Slew Rate

200

300

V/µs

Output Overshoot

2VPP Step

Power Supply Current

-20°C < TA < +85°C20

Gain Adjust Range

225Ω < RGAIN < 1230Ω

-6

Gain Select Input Voltage

0.8

VIL (TTL Level)

Gain Select Input Voltage

2.0

VIH (TTL Level)

THEORY OF OPERATION

Unless otherwise noted: VCC = 5.0V, CLOAD = 50pF to GND, RLOADP = RLOADN = 1kΩ to GND, RGAIN = 510Ω

(Nominal Gain), -20°C ≤ TA ≤ +85°C, Output measured differentially.

ELECTRICAL SPECIFICATIONS: Continued

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