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

3.2 Photodiode Adaptive Biasing Current Output

The LOG200 includes an IBIAS current source to bias a photodiode with a reverse voltage proportional to the photocurrent. The adaptive biasing circuit produces a small reverse bias voltage across the photosensor during low photodiode current measurements, reducing the photodiode's dark current and improving the measurement's accuracy. During the measurement of high photodiode currents, a higher reverse-bias voltage is developed across the photodiode, reducing the photodiode's effective capacitance, increasing the circuit bandwidth, and providing a faster transient response.

Figure 3-1 shows a block diagram with the adaptive biasing circuit and photodiode connections on the LOG200EVM. Internal transistor Q3 is used to measure the input current, and the IBIAS circuit produces a current output of approximately 1.14 times (typ) the input current of the I1 pin. The IBIAS pin connects to the photodiode cathode, and the RBIAS resistor connects in parallel to the photodiode cathode and VCM.

IBIAS Photodiode Adaptive Biasing Current OutputFigure 3-1 IBIAS Photodiode Adaptive Biasing Current Output

When using the IBIAS circuit, 1.0 times the input current flows through the photodiode, and the remaining 0.143 times of the input current flows through RBIAS. This configuration establishes a bias voltage across the RBIAS resistance. The photodiode anode is connected to the I1 input and held at a constant VCM voltage. The cathode voltage effectively rises by 0.143 × RBIAS × I1, thus providing a current-dependent reverse bias voltage for the photodiode. Select the proper RBIAS resistor for the designed for photosensor reverse bias voltage.

Scale the RBIAS resistor to allow a 1V headroom for the positive (VS+) supply while considering the maximum current of the photodiode. Also, a CCOMP capacitor from IBIAS to GND needs to be placed for stability; a suggested value is 22pF.

For example, the circuit of Figure 3-1, consider the case where the LOG200 is to support a photodiode application with an IPD current in the range of 20nA to 2.5mA, with VCM set to +2.5V while a unipolar 5V supply powers the LOG200.

Equation 1 provides the maximum possible bias voltage across RBIAS:

Equation 1. V R B I A S _ M A X   =   ( V S + )   -   1 V   -   V C M

Equation 2 provides the maximum possible IBIAS current:

Equation 2. I R B I A S _ M A X   =   I B I A S R A T I O _ M A X   -   1   × I P D _ M A X    

Equation 3 solves for the maximum possible RBIAS resistance:

Equation 3. R B I A S _ M A X   =   V R B I A S _ M A X   I R B I A S _ M A X    

Equation 4 calculates the maximum RBIAS resistor allowing a 10% margin for error:

Equation 4. R B I A S   =   90 %   × R B I A S _ M A X

Table 3-1 shows the calculation results as a function of IPD_MAX

Table 3-1 RBIAS Resistor Calculations

I

PD_Max
(mA)		 IBIAS_Ratio_Max @ 1mA
(A/A)		 IBIAS_Max
(mA)		 VBIAS_Max
(V)		 RBIAS_Max
(kΩ)		 RBIAS
(kΩ)
2.5 1.195 0.488 1.5 3.077 2.770

Figure 3-1 shows the typical VBIAS voltage across RBIAS when IPD = 2.5mA.

Example of RBIAS Calculation Figure 3-2 Example of RBIAS Calculation