CURRENT TO VOLTAGE CONVERTER FOR PIGTAILED
PHOTODIODE FIBER OPTIC POWER MONITORS
FEATURES
The DN135 is a current to voltage converter that is intended for use in
fiber optic power monitors. The device converts current from a
photodiode to voltage that is proportional to the incoming optical power in
dBm. The DN135 functions equally well with InGaAs and Silicon
photodiodes.
• 70dB DYNAMIC RANGE
• CONVERTS CURRENT LEVELS FROM 100pA to 2mA
• 0.500 VOLTS OUTPUT PER DECADE INCREASE IN OPTICAL POWER
• OPERATES FROM ± 5 VOLTS SUPPLIES
DATA SHEET
DN135
Therm
DN135
XXXX
1.25 Volts
Ref.
Calibration
Adjustment
10KΩ
6.81KΩ
Pigtailed
Photodiode
+5V
-5V
Output
OPTICAL
POWER IN
PIN
DN135
INPUT
POWER
Watts
dBm
Volts
1
100
10
1
100
10
1
mW
µ
W
µ
W
µ
W
nW
nW
nW
100 pW
0
-10
-20
-30
-40
-50
-60
-70
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
OUTPUT
VOLTAGE
INPUT
CURRENT
Amperes
Volts
1
100
10
1
100
10
1
mA
µ
A
µ
A
µ
A
nA
nA
nA
100
pA
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
OUTPUT
VOLTAGE
PIN #
FUNCTION
1
2
3
4
5
6
7
8
9
10
FACTORY CALIBRATION
IIN CURRENT INPUT
GROUND
VREF 1.25 VOLT REFERENCE
GROUND
+5 VOLTS
VC CALIBRATION INPUT
-5 VOLTS
VO VOLTAGE OUTPUT
FACTORY CALIBRATION
2
9
6
8
4
7
3, 5
The pigtailed photodiode and DN135 assembly
is calibrated by introducing an optical signal of
known magnitude into the fiber and adjusting
the calibration potentiometer so that the output
voltage corresponds to the input power level.
Example:
One hundred microwatts of optical power
@1550nm is introduced into a fiber that is
terminated in an InGaAs photodiode.* Adjust
the calibration control shown in Figure 1 until
the output voltage indicates 3.500 Volts. The
optical power monitor is now calibrated at
1550nm.
The calibration control can be driven from an
external voltage source such as the output of a
digital to analog converter. This would allow
the power monitor to be automatically
calibrated in a systems application.
*A typical InGaAs photodiode may have a
responsivity of 0.9mA/mWatt of optical power
at 1550nm. This would provide an output of
current of 90µA with an input power of 100µW.
The calibration control corrects for variations in
the responsivity of various photo diodes.
PIN = 20(VO -4) dBm
VO
Equation relating optical power
input PIN to output voltage VO.
Example:
VO = 2.750 Volts
PIN = 20(2.750 - 4) = -25 dBm
Figure 1
Optics
1 2 3 4 5
9 8 7 6
10
0.100"
0.650"
0.55"
0.050"
™
™
Page 1
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