The advent of self-driving has decisively expanded the presence of laser-imaging detection and ranging (LiDAR) sensors in the automotive electronics platform. LiDAR works according to the radar principle but uses light pulses emitted by an infrared laser diode.
Maxim Integrated’s new MAX40026 high-speed comparator and MAX40660/MAX40661 high-bandwidth transimpedance amplifiers (TIAs) enable 15 km/h faster autonomous driving at highway speed by doubling the bandwidth and adding 32 channels (for a total of 128 instead of 96) in a LiDAR module of the same size.
Maxim’s Maurizio Gavardoni demonstrates the evaluation board for a four-channel LiDAR receiving system.
It includes optical photodiodes from First Sensor and Maxim’s newly launched TIA and high-speed comparator. (Image: Maxim Integrated)
Along with artificial intelligence, cameras, and radar, sensors are indispensable to assisted and autonomous driving. Because they can provide accurate measurements of objects and detect obstacles on the road — fallen tree limbs, other cars, or even a child who darts out into traffic — LiDAR sensors have helped advance the adoption of advanced driver-assistance systems (ADAS) and are critical to autonomous-vehicle (AV) development. An AV’s perception of the surrounding environment must be extremely precise, which is why experimental robocars are full of sensors. The use of a laser lighting system allows self-driven cars to be operated under low- or no-visibility conditions and even in the absence of road markings.
“LiDAR sensors are playing an increasing role in the fusion of vehicle sensors for their ability to provide accurate distance measurement of objects,” said Maurizio Gavardoni, principal member of the technical staff at Maxim Integrated. “A typical LiDAR sensor sends light pulses that, reflected by objects and detected adequately by photodiodes, allow you to map the surrounding environment.”
LiDAR systems are based on time of flight (ToF), which measures precise timing events (Figure 1). The latest developments have seen several multibeam LiDAR systems, which generate a precise, 3D image of the environment around the vehicle. This information is used to choose the most appropriate driving maneuvers.
Figure 1: Time-of-flight functional diagram (Image: Maxim Integrated)
Figure 2 shows the basic layout of a LiDAR sensor. There are two basic types of LiDAR systems: micropulse LiDAR and high-energy. Micropulse systems have been developed as a result of the ever-increasing computing power available and advances in laser technology. These new systems use very low power, in the order of 1 W, and are entirely safe for most applications. High-energy LiDAR, on the other hand, is common in atmospheric monitoring systems, where the sensors are used to detect atmospheric parameters such as height, stratification, and cloud density.
Figure 2: General layout of a LiDAR sensor with the main electronic parts shown (Image: Maxim Integrated)
“Automotive self-driving systems are evolving from 35 mph to 65 mph and beyond, but faster autonomous self-driving systems are essential,” said Gavardoni. “The challenges in meeting these demands [translate] into high-precision distance measurements of objects, [requiring] more accuracy, more channels to fit in space-constrained platforms, [and compliance with] stringent safety requirements.”
In a LiDAR project, the transimpedance amplifier is the most critical part of an electronic layout. Low noise, high gain, low group delay, and fast recovery from overload make the new Maxim TIAs ideal for distance-measurement applications.
TIA circuits are often used in applications that share the need for circuitry to buffer and scale the output of electro-optical solutions to achieve high speed and high dynamic range. TIA is a current-to-voltage converter, almost exclusively implemented with one or more operational amplifiers (Figure 3).
Figure 3: General layout of a TIA with a reverse polarization photodiode (Image: Wikipedia)
Phototransistors and photodiodes are closely related and convert incident laser light into electrical current. To achieve maximum performance from these devices, designers must pay special attention to interface circuits, wavelengths, and optical-mechanical alignment. The MAX40660/MAX40661 transimpedance amplifiers enable much faster self-driving systems using high resolution. The TIAs reduce current consumption by more than 80% in low-power mode. Maxim’s TIAs support 128 channels with a bandwidth of 490 MHz in the case of the MAX40660 and 2.1-pA/√Hz noise density to provide greater measurement accuracy (Figure 4).
Figure 4: MAX40660 block diagram (Image: Maxim Integrated)
The MAX40026, meanwhile, is a single-supply, high-speed comparator for TOF distance measurement applications. Its low, 10-picosecond propagation delay dispersion contributes to the accurate detection of fixed and moving objects. “Lower dispersion delay and more channels per system enable more precise timing measurement, thus enhancing system resolution and enabling higher driving speed,” said Gavardoni.
The MAX40026 has an input common-mode range of 1.5 V to VDD + 0.1 V, compatible with the output swings of several widely used high-speed TIAs. The low-voltage differential signaling (LVDS) output stage minimizes power dissipation and interfaces directly with many FPGAs and CPUs (Figure 5).
Figure 5: MAX40026 functional diagram (Image: Maxim Integrated)
The size of the new solutions is further reduced, allowing many more channels to be inserted into vehicle platforms with limited space. These integrated circuits meet the automotive industry’s most stringent safety requirements, with AEC-Q100 qualification, improved electrostatic discharge (ESD) performance, and effects and diagnostic analysis (FMEDA), in order to support ISO 26262 certification at the system level.
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