One more important spec to pay attention to when using a comparator in the receiver path is input overdrive dispersion. Input Overdrive refers to how much the input signal to the comparator exceeds its threshold or reference voltage value, and dispersion is defined as the amount of propagation delay variance over a certain range of input overdrives.

Below is a curve showcasing the propagation delay response of a comparator with varying input overdrives. As seen below, larger input overdrives result in a faster response time –however this faster response tends to saturate at input overdrive > = 100 mV.

There is not a simple solution to be able to deskew the different propagation delay times according to different input overdrives, so it is important to select a comparator with very small input overdrive dispersion so that the system can calculate an accurate distance to an object regardless of the amplitude of the reflected pulse.

Input overdrive dispersion is also tied to the rated distance the ToF system can calculate. Selecting a comparator with small input overdrive dispersion will result in a system that can more accurately compute distances for a wide range of target objects. For example, an object at the same distance as another object may have better reflective properties. This would result in a reflected pulse that has a larger amplitude than the other object. Selecting a comparator with small input overdrive dispersion means the system can measure the distance between these two objects with little to no variance.

A great comparator option for ToF systems is Texas Instruments’ newest high speed comparator with LVDS outputs. The device has just 800 ps of propagation delay and a high toggle frequency of 1.5 GHz (3.0 Gbps). It’s narrow pulse width detection capabilities of 600 ps, and input overdrive dispersion of just 450ps make it a great option as a receiver in optical ToF systems The device has favorable specs compared to similar comparators in the same package and pinout options, and can be easily used as a drop in replacement for ToF systems already using comparators in their receiver path.

In comparison to similar parts, the TLV3604 boast both overdrive dispersion specs and minimum pulse width specs that are at least 2 times better than similar pin-to-pin comparators, while operating with less power consumption. This means that the part would be able to both measure distances to objects further and measure it more accurately as well.