SBOA601 January   2025 LOG200

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Critical Photodiode Specifications
  6. 3Interfacing the LOG200 With the Photosensor
    1. 3.1 Photodiode Connections
    2. 3.2 Photodiode Adaptive Biasing Current Output
  7. 4Optical Bench for Current Sensing Measurements
    1. 4.1 Transient Response with Photosensor
  8. 5Optical Power Measurements with the LOG200
  9. 6Error Sources and Uncalibrated Error Analysis
  10. 7Auxiliary Op Amp Circuits
    1. 7.1 Single-Ended to Differential Conversion Circuit
    2. 7.2 Sallen-Key Low-Pass Filter
  11. 8Summary
  12. 9References

4 Optical Bench for Current Sensing Measurements

The LOG200EVM is configured on the bench, with the G8195-12 photodiode installed,as shown in Figure 3-1.

The REF25 (2.5V reference) sets the VCM potential. The IREF current (1µA) feeds I2 via resistor R16. RBIAS (R3 on the LOG200EVM) is set to 1.65kΩ, with a compensation capacitor of CCOMP (C1) of 22pF. The difference amplifier reference input (REF) connects to the REF165 (1.65V Reference).

Figure 4-1 shows the current sense bench fixture.

LOG200 Optical Current Sense TestFigure 4-1 LOG200 Optical Current Sense Test

Table 6-3 lists the bench equipment used during these measurements.

Table 4-1 LOG200 Optical Bench Equipment
DeviceManufacturerPart #Description / Comments
Function GeneratorAgilentAgilent 3350030MHz signal function waveform generator for laser modulation
Laser ControllerThorlabsCLD1010Set to current control Mode
Driven with Function Gen
Laser DiodeThorlabsLPS-1310-FCλ = 1.31μm
Threshold current 5mA to 20mA
Variable optical attenuatorThorlabsVOA50-FC-SM1310/1550nm
50dB, FC/PC Connectors
Evaluation BoardTexas InstrumentsLOG200EVMLOG200 eval board with photodiode
Photodiode

Hamamatsu

G8195-12λ = 1.31μm
1pF typical cap
Response range, λ = 0.9μm to λ = 1.7μm
Responsivity: 0.9A/W typ, λ = 1.55μm

The LOG200EVM jumpers are set as shown on Figure 4-2 and Table 4-2.

LOG200EVM Jumper SettingsFigure 4-2 LOG200EVM Jumper Settings
Table 4-2 LOG200 Jumper Description
JumperFunctionBench PositionDescription
J7Out_ASMA J5 referenceShunt 2-3Shunt 2-3: SMA connector J3 referred to GND
Shunt 1-2: SMA connector J3 referred to REFA
J8VCM SelectShunt 3-4Shunt 1-2: VCM input connected to GND
Shunt 3-4: Connects VCM input to 2.5V reference
J9VS– connectionShunt 1-2Open: Bipolar supply configuration
Shunt 1-2: Connects VS– to GND for unipolar supply configuration
J11REF A SelectShunt 5-6Shunt 1-2: Sets log voltage REFA to external voltage through J10
Shunt 3-4: Sets REFA to REF25
Shunt 5-6: Sets REFA to REF165
Shunt 7-8: Sets REFA to GND
J15Secondary op-amp inputShunt 1-2Shunt 1-2: Connects Aux AMP+IN to OUTA
Shunt 3-4: Connects Aux AMP -IN to OUTA