The high gain and single-ended output of the LMH34400 is designed to be used in connection with a time-to-digital converter (TDC) for a time-of-flight (ToF) based receiver in a LIDAR system. The TDC function can be implemented using a stand-alone TDC or using a FPGA. ToF receive circuits that use TDC are significantly less expensive and consume considerably less power when compared to an analog-to-digital converter (ADC) based solution. The output of the LMH34400 presents an analog representation of the returned light pulse. Common practice uses a time-discriminator circuit before the TDC to precisely use a deterministic portion of the returned waveform to stop the TDC. The most straightforward method to accomplish this action is called leading-edge discrimination. This method uses a high-speed comparator with a low propagation delay to stop the TDC when the waveform crosses a chosen incoming light amplitude value.

In many applications, the amplitude of the returned pulse can vary considerably as a result of the difference in target reflectivity or simply the light source spreading out to a target moving to longer distances. For this reason, choose a comparator with low dispersion. If the dispersion is high, then the amplitude variation in the returned signal is converted to a variation in the timing signal presented to the TDC. This behavior is known as a walk error. Figure 7-1 shows the LMH34400 connected to the TLV3601 high-speed comparator. In this configuration, an incoming optical pulse sources current out of the amplifier input pin and delivers a proportional voltage pulse to the comparator input. The amplifier output has 1.0‑V dc with no input current; therefore, set the reference voltage of the comparator to a level greater than 1.0 V. For example, to have the comparator change states when the input is greater than 10 μA, then set the V_REF voltage to 1.0 V + (40 kΩ × 10 μA) = 1.4 V.

Figure 7-1 LMH34400 to Interface to Comparator and TDC