The LOG200EVM PCB board layout includes a photosensor footprint (D1) to install a radial photodiode on current input I1 for the logarithmic numerator. See Figure 3-3 for the photosensor input and photosensor biasing connections.

The photodiode IBIAS adaptive biasing current output is accessible through optional jumper resistor R2. The adaptive current output biasing scheme creates a voltage to bias a photodiode with a current that is proportional to the photocurrent. This allows small bias voltages for low photodiode currents, reducing the dark current of the photodiode, and higher reverse-bias voltages as the photocurrent increases, reducing the capacitance of the photodiode.

The IBIAS function produces a current that is approximately 1.1x larger than the numerator current I1. Since the photosensor produces the input numerator current, the remaining 0.1*I1 current flows through the RBIAS resistor R3. The RBIAS resistor R3 needs to be scaled depending on the photosensor biasing requirements, the photosensor current range and the LOG200 supply voltage. The voltage at the IBIAS pin must not exceed (V+) - 1 V at the photosensor maximum current. If the IBIAS function is not used, then leave the IBIAS pin floating by removing resistor jumper R2.

Alternatively, the photosensor cathode can be reverse biased via test point TP1, DBIAS. Verify the resistor R2 is removed when biasing the photosensor through TP1.

Capacitors C1 and C3 can help providing dynamic currents during fast transients and help to improve stability. The value for best bias response depends on the photosensor and application requirements.

Figure 3-3 presents a simplified diagram of the photodiode connections. For a detailed schematic of the photosensor connections, see Figure 4-1.

Figure 2-3 LOG200EVM Photodiode Connections