The circuit in Figure 7-6 shows a photodiode anode connected to the LMH34400 that is followed by a comparator. To create the 10‑ns, 25‑µA input‑current pulses, choose a photodiode with low input capacitance and minimal biasing requirements that allows for easy optical coupling through fiber. Given the 40‑kΩ gain from the LMH34400, the expected output voltage with a 25‑µA input current is a 1‑V peak signal. With input‑pulse edge rates of 1 ns, the LMH34400 is expected to produce slower pulse edges because the input slew rate is higher than the capabilities of the device. To increase the edge rates of the LMH34400 output signal, the TLV3601 comparator is added after the LMH34400 because the TLV3601 has a rise and fall time of 750 ps.

For a simple time-of-flight application, the output of the comparator can be connected to a time-to-digital converter (TDC) to perform a simple distance calculation. Using this method, the distance is calculated by measuring the time between outgoing and incoming pulse edges and multiplying by two. In the signal chain, the LMH34400 provides the initial signal amplification and conversion to a voltage. Then, the TLV3601 further increases the output amplitude, as well as provides a clean, fast edge to a following time-to-digital converter.

In the circuit design, the interface between the LMH34400 and the TLV3601 does not require any additional biasing or level shifting, because the LMH34400 default output bias interfaces easily with the comparator. Additionally, this 1‑V bias level allows for setting the threshold voltage to half of the supply voltage, which keeps the threshold voltage as close as possible to the center of the comparator bias range. However, this threshold can be set at any value greater than 1 V to adjust for the dynamic-range requirements of the application. In this realization, the output of the TLV3601 is connected to a 50‑Ω series output resistance to properly interface with 50‑Ω terminated test equipment.