Figure 7-2 shows the LMH34400 test circuit used to evaluate various bandwidth without using an optical input signal. The voltage source is dc biased close to the input bias voltage of the LMH34400 (approximately 2.5 V). The LMH34400 internal design is optimized to only source current out of the input pin (pin 1). When testing the LMH34400 with a network analyzer or other ac source, control the dc bias such that the sum of the input ac and dc components does not result in a sourcing current into the amplifier input.

In the configuration shown in Figure 7-2, there is a 50‑Ω series resistor that helps with any reflection into the observing instrument. The instrument can be any 50‑Ω impedance input device such as a vector network analyzer (VNA) or oscilloscope. This setup creates a voltage divider on the output and reduces the TIA amplitude by a factor of two. Make sure to consider this factor when interpreting the measured results.

The bandwidth of a transimpedance amplifier strongly depends on the capacitance of the photodiode (C_PD) that is connected to the input pin of the amplifier. The larger the capacitance, the lower the closed-loop bandwidth. Figure 7-3 shows when the C_PD that is connected to the LMH34400 is between 0 pF and 10 pF.

While bandwidth is inversely proportional to the photodiode capacitance, the input-referred current noise and photodiode capacitance are directly proportional. To measure the output noise, use the same circuit in Figure 7-2 with a simple modification. In this case, remove all components on the input pin except C_PD. Figure 7-4 shows the impact of the input-referred noise density as the C_PD is varied from 0 pF to 10 pF. As the capacitance increases, the amplitude and breadth of the high-frequency noise increases significantly.

Figure 7-5 shows the impact of an increasing photodiode capacitance on these two parameters in one plot. In this plot, the integrated input-referred noise is calculated over a fixed range of dc to 250 MHz. Both the small-signal bandwidth and integrated input-referred noise trend toward poorer performance as the capacitance increases. For the highest level of performance, minimize the photodiode capacitance. As the photodiode capacitance is proportional to the photodiode light-capturing area, the final value chosen is a compromise of several system variables and differs between applications.