2 How ToF Measurements are Calculated

Optical ToF systems consist of a light transmitter and receiver. The emitter is typically an LED or a laser diode that sends short analog light pulses which reflect off a target. When this pulse is sent, this is typically denoted as the “Start Event”. The reflected light from the target is returned to the receiver, generally a photodiode combined with a TIA to convert the photonic energy into electrical current. Since the speed at which light travels through a given medium is constant, the distance to the target can be calculated by measuring the time delay between when the initial light pulse is sent, to when it is reflected and received back.

Using light waves allows for reliable detection of fast-moving objects, and performs well in ambient light conditions. Likewise, measurements can be calibrated for temperature, humidity, and air pressure. Once the reflected light has returned back to the receiver, this is denoted as the “Stop Event”. The time difference between the start and stop event is used to determine the distance to the object; the propagation delays of all devices within the system are then de-skewed out of this time difference. What is left is the time it took for the light pulse to have a full round trip cycle from when the initial pulse is sent and received back. Dividing this value by two gives the time it takes for the light to reach just the target object. The equation for distance can then be defined as:

where

d is distance,

c is the speed of light (3x10^8),

t is the time difference between the start and stop event.

An example of a ToF system is shown in Figure 2-2 . The system uses a laser diode as the stimulus to the target object. Once the laser driver reaches the target and gets reflected back, transimpedance amplifiers are then used to convert the photodiode current into a voltage. From here, either a high-speed comparator or an ADC is used to capture data and send downstream to another device such as an FPGA or a high end DSP for computation of the ToF measurement.

Although many of the components upstream from a comparator or ADC in the receiver path of the ToF system are the same, the way in which it measures distance can be quite different. With a comparator, one of the inputs is tied to a reference voltage. The light pulse expected in the reflection from the target object is then measured and compared to this reference voltage. If the reflected light pulse crosses the reference threshold, the distance to that object can be measured. The comparator will output high denoting the “Stop Event”, and downstream devices can compute the distance to the target from this trigger.

An ADC meanwhile, samples the analog signal at a high frequency and digitizes the signal to bits. These bits contain amplitude and timing information to send downstream to a processing device where not only distance can be calculated, but other useful measurements such as color or material type can be determined as well, based off the reflectivity or amplitude of the reflected pulse.

There are several benefits of choosing either a comparator or an ADC to capture ToF data, but choosing the correct device comes from the system needs and exactly what data needs to be captured. If more than just distance is required to be calculated, than an ADC in the receiver path would be an appropriate choice. However, if just a distance measurement to the target object is needed, than a comparator would be a more practical and cost effective solution.

Although using a comparator in the receiver path limits the system to only measuring the distance to an object, using a comparator greatly simplifies the signal chain, as there is no need for a high end signal processing device in the backend. This greatly improves the robustness of the system as it is typically much easier to integrate a comparator into the receiver path, rather than an ADC which requires more complex devices to be tested and integrated into the system. A Simple TDC or an FPGA and MCU combination is all that is needed downstream to calculate the ToF distance measurement. As mentioned previously, distance can simply be calculated with the equation

This results in cost savings in multiple ways: a smaller solution size, a lower cost receiver chip, and lower cost in external components and downstream processing devices.