sp8126b Exar Corporation, sp8126b Datasheet - Page 3

• Manufacturer Part Number: sp8126b
• Description: High Speed Differential Apc Amplifier
• Manufacturer: Exar Corporation

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
V
Rev:B Date: 3/18/04
Unless otherwise noted: V
(Nominal Gain), -20°C  T
PARAMETER
Output Settling Time (780nm)
(1% of Final Value)
Output Slew Rate
Output Overshoot
Power Supply Current
Gain Adjust Range
Gain Select Input Voltage
VIL (TTL Level)
Gain Select Input Voltage
VIH (TTL Level)
OUT
PIN NUMBER
- and V
1
2
3
4
5
6
7
8
OUT
+ are set to a reference voltage
CC
A
 +85°C, Output measured differentially.
= 5.0V, C
NAME
R
R
R
V
GAIN
V
GND
V
GCOM
GAIN1
GAIN2
OUT
OUT
CC
+
-
CONDITIONS
2V
2V
R
2V
-20°C < T
225 < R
SP8126B High Speed Differential APC Amplifier
GAIN
LOAD
PP
PP
PP
Step
Step,
Step
= 1230 (High Gain)
= 50pF to GND, R
A
GAIN
< +85°C
FUNCTION
Supply Voltage
Gain Select
Common connection point for R
Power Ground
Gain Adjust 2 (Gain Select = HIGH)
Output Voltage -
Output Voltage +
Gain Adjust 1 (Gain Select = LOW) or OPEN
< 1230
ELECTRICAL SPECIFICATIONS: Continued
3
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.
LOADP
= R
LOADN
MIN
200
2.0
THEORY OF OPERATION
-6
= 1k to GND, R
© Copyright 2004 Sipex Corporation
TYP
GAIN
300
10
15
15
PIN DESCRIPTION
1 and R
MAX
GAIN
0.8
13
18
20
5
6
GAIN
= 510
2
UNITS
V/µs
mA
mA
dB
ns
ns
%
V
V