3 Optimizing for Different Photodiode Currents

The signal chain can be modified for the desired current range and transimpedance gain, using the equations explained in Section 2. To modify the signal chain to measure values lower than 50 pA, R_f should be increased. The increment in R_f would result in a decrease the maximum measurable current supported by the signal chain with the same ADC. Table 3-1 mentions different measurement ranges supported by the signal chain with different feedback resistance for a 10-bit ADC at 3-V reference. The capacitance values in the Table 3-1 are for maintaining an upper cutoff frequency equal to 1 kHz.

Using a higher R_f with the same ADC enables to achieve a better resolution for detecting minimum change in the input current. For instance, if required range is 50 pA to 5 nA, measuring using R_f = 300 MΩ ensures that every 10 pA change is detectable. Also for the same current value, a larger R_f results in better SNR because increasing R_f reduces the input referred current noise. The graph in Figure 3-1 shows the increasing trend of SNR for 50 pA input signal with increasing feedback resistance.