™ DN135 DATA SHEET CURRENT TO VOLTAGE CONVERTER FOR PIGTAILED PHOTODIODE FIBER OPTIC POWER MONITORS 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 +5V OPTICAL POWER IN PIN Pigtailed Photodiode -5V 9 2 PIN = 20(VO -4) dBm 4 Calibration Adjustment Example: 1.25 Volts Ref. 10KΩ 6.81KΩ VO = 2.750 Volts PIN = 20(2.750 - 4) = -25 dBm INPUT POWER dBm Figure 1 OUTPUT VOLTAGE INPUT CURRENT OUTPUT VOLTAGE Volts Amperes Volts 1 mW 0 4.000 1 mA 4.000 100 µ W 10 µ W -10 3.500 100 µA 3.500 -20 3.000 10 µA 3.000 -30 2.500 1 µA 2.500 -40 2.000 100 nA 2.000 1 µW 100 nW Output VO Therm Optics 0.55" DN135 XXXX 1 2 3 4 5 0.050" 0.100" 1 2 3 4 5 6 7 8 9 10 FUNCTION FACTORY CALIBRATION IIN CURRENT INPUT GROUND VREF 1.25 VOLT REFERENCE GROUND +5 VOLTS VC CALIBRATION INPUT -5 VOLTS VO VOLTAGE OUTPUT FACTORY CALIBRATION DN135 7 Equation relating optical power input PIN to output voltage VO. 8 6 3, 5 Watts 0.650" PIN # FEATURES • • • • 10 9 8 7 6 10 nW -50 1.500 10 nA 1.500 1 nW -60 1.000 1 nA 1.000 100 pW -70 0.500 100 pA 0.500 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. ™ Page 1 EQUATION RELATING THE OUTPUT VOLTAGE OF THE DN135 TO THE INPUT CURRENT. Log (IIN) + 4 VO = + 4VC + ß (IIN) 2 or Log (IIN) VO = + 5.5 + ß (IIN) 2 IIN When VC = 0.875 V IIN is in Amperes ß =0.010 ± .002 at 25°C. The effect of ß on the output voltage is very small when the input current is less than 100µA. VDD VEE +5V 3 5 8 Anti-Log Element Input VO 2 ∑ VC 9 Output X4 7 3.56KΩ 0.875V 10KΩ +5V Calibration Adjustment VREF 1.25 Volts FUNCTIONAL BLOCK DIAGRAM OT THE DN135 1.25V REF. 4 OUTPUT VOLTAGE VS. INPUT CURRENT FOR THE DN135 OUTPUT VOLTAGE vs. INPUT CURRENT/OPTICAL POWER 1 Output 1 Output 1 Output Voltage Voltage Tol. Voltage Tol. Optical Input Power (dBm) T= 25°C T= 25°C T= -20°C to +75°C Volts Volts Volts 4.0 3.5 1 mA 4.000 +0.012 +0.024 100 µA 3.500 ±0.002 ±0.010 10 µA 3.000 ±0.000 ±0.010 1 µA 2.500 ±0.001 ±0.010 100 nA 2.000 ±0.002 ±0.010 10 nA 1.500 ±0.002 ±0.010 1 nA 1.000 ±0.002 ±0.020 100 pA 0.500 ±0.005 ±0.150 1. This is the output voltage after the DN135 has been calibrated to produce an output voltage of 3.000 Volts with an input current of 10µA at an ambient temperature of 25°C. -70 -60 -50 -40 -30 -20 -10 0 3.0 Output Voltage (Volts) Amperes GND. -5V 6 6.81KΩ Input Current GND. 2.5 2.0 1.5 1.0 0.5 -10 0 -9 10 5 2 -70 -8 10 5 -7 10 2 -60 5 -6 10 2 -50 5 -5 10 2 -40 5 2 -30 -4 10 5 -3 10 2 -20 5 10 2 -10 5 0 2 Input Current (Amperes) ™ Last Updated 6/25/01 1004 Mallory Way • Carson City, NV 89701 • Phone: 702-882-7721 • Fax: 702-882-7675 • E-mail: [email protected] • http://www.thermoptics.com Page 2