In Figure 104, the photodiodes are connected to the multiplexer input in photovoltaic mode. Depending on the application requirements, either photovoltaic mode or photoconductive mode can be used. The multiplexer in the ADS816x is used as a current multiplexer in this example. One common amplifier for all photodiodes reduces cost, complexity, PCB area, and power consumption. This common amplifier also simplifies system calibration because the gain and offset error are the same for all channels. Finally, the low leakage current of the multiplexer is ideal for photodiode applications.
The OPA320 is used as a transimpedance amplifier that can also drive the ADC inputs. In order to set the output voltage of the OPA320 to 0.1 V in dark conditions, an equivalent bias voltage (VB) is applied at the noninverting terminal. Equation 12 shows that this bias voltage is derived using a resistive voltage divider on the REFby2 output (2.048V).
Equation 13 shows that the feedback resistor for the transimpedance amplifier can be selected by designing for a 4-V output for a 90-µA input.
Equation 14 computes the value of the feedback capacitance to limit the bandwidth of the transimpedance circuit to 1 MHz.
Transimpedance amplifiers can have potential stability concerns. Stability is a function of the feedback capacitance, the capacitance on the inverting input of the amplifier, and the amplifier gain bandwidth. In this case the capacitance on the inverting amplifier input (CIN, as calculated by Equation 15 and Equation 16) includes the photodiode junction capacitance (CJ), the multiplexer capacitance (CMUX), the trace capacitance, and the op amp input differential (CD) and common-mode (CCM2) capacitances. Equation 17 and Equation 18 compute the minimum gain bandwidth of the amplifier for stability for a given CIN. The minimum required gain bandwidth is 10.9 MHz and the gain bandwidth for the OPA320 is 20 MHz, so the stability test passes.