Page 19 - EngineerIT September 2021

MEASUREMENT

How to effectively design and optimise

the TIA interfaces of LIDAR systems

By Noe Quintero, Analog Design Engineer and Tony Pirc, Automotive Application Engineer

Introduction For multichannel applications, a multiplexer is used to reduce
In this article, we will address the effects of various input coupling the number of ADCs. When photons strike the photodetector,
options for transimpedance amplifiers (TIAs) and shed light on it generates a current that is converted to a voltage by the TIA.
easily overlooked consequences for each case. The intent is to This voltage is then quantised to a digital value by an ADC.
help engineers effectively design and optimise the TIA interfaces Another popular choice for a quantiser is with a comparator
of a LIDAR system. We will highlight the design challenges when and time-to-digits converter (TDC), as shown in Figure 2.
coupling a high gain optical detector to the TIA input. These TDC systems have an order of magnitude less cost and
LIDAR input is not a trivial topic, and there are going to be power, while trading off lower system performance. Also, the
many moving parts in this process. The signal chain is going use of a multiplexer is typically not used with TDC but can be
to be different depending on which design you choose, so it’s implemented to reduce the number of TDCs and comparators.
important to consider all of your options before choosing the one There are three dominant types of detectors. Photodiodes
that is best for your project. are detectors that convert photons to electrons, but they
offer no optical gain and are not a popular choice for these
LIDAR and you applications. A popular detector in LIDAR systems is the
Time of flight LIDAR receiver signal chains have a slew of design avalanche photodiode (APD). APDs are photodiodes that
trade-offs that can potentially compromise the performance are reverse biased up to breakdown of the junctions with the
of your imaging system. The interface between the TIA and benefit of gaining optical gain. The third type of detectors are
its detector is part of this challenge. This is especially true for single photon avalanche diodes (SPADs). A SPAD is reverse
transimpedance amplifiers with multichannel switching. biased with an excess bias voltage that is between the
breakdown voltage and the second, higher breakdown voltage
LIDAR receiver building blocks associated with the SPAD’s guard ring. At this bias, a single
A time of flight LIDAR system is composed of a transmitting charge carrier injected into the depletion layer can trigger a
signal chain and a receiving signal chain. The transmitting self-sustaining avalanche, resulting in thousands of virtual gain
portion sends a pulse of photons at an object, and the receiving at the detector. It would seem the SPADs would be the natural
portion measures the amplitude and nature of that pulse. The choice due to their sensitivity.
amount of time it takes for the light to travel from transmission However, LIDAR systems must contend with many real-
to detection tells you how far the light has travelled. At the world consequences and too much gain will saturate the
most fundamental level, the receiving signal chain consists of a receive chain too easily. Also, the extra gain comes with extra
photodetector, a TIA and an analogue-to-digital converter (ADC), noise, called the excessive noise factor (ENF). The ENF is
as seen in Figure 1. exponentially correlated to the bias, and too much gain can

Figure 1: LIDAR receiver signal chain quantised by an ADC. Figure 2: LIDAR receiver signal chain quantised by a TDC.

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