8.2.1.1 Design Requirements

For this application, the design requirements are as follows:

• VS+ = 5 V, VS– = –5 V, VCM = GND, REFA = GND

• IREF (1 µA) connected to I2

• Input current range: 10 nA ≤ I1 ≤ 100 µA

• Photodiode: GP8195-12

– VR = 5 V

– 20 pA dark current (typical)

– 1-pF typical capacitance (1.5

‑pF maximum)

– Spectral response range from λ = 0.9 μm to λ = 1.7 μm

For bench testing of the system, the following configuration is used:

• Laser diode: LPS-1310-FC

– λ = 1.31 μm

– Threshold current 5 mA to 20 mA

– Current control mode used

• Laser controller: THOR CLD1010

• Variable attenuator: VOA50-FC-SM 50-dB in-line

• External modulation: Agilent 33250A 80

‑MHz waveform generator

8.2.1.2 Detailed Design Procedure

The G8195-12 photodiode was used with a fixed reverse bias voltage of 5 V. The cathode was connected to

the VS+ 5-V supply, and the anode to the I1 pin. GND is used for the VCM potential. The IBIAS feature and

REF165 voltage reference were not needed; therefore, the IBIAS and REF165 pins are left floating. The auxiliary

amplifier was not needed; therefore, the auxiliary amplifier was placed in a buffer configuration and used to

buffer the REF25 reference voltage.

GND was used for the REFA input of the logarithmic difference amplifier. The circuit output follows the

expression

VLOGOUT=250mV×log10 I11µA

(8)

such that the expected output for a 100-nA input is –500 mV, the expected output for a 10-µA input is 250 mV,

and so on.

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