Analysis of the analog front-end circuit AD8283 that completely changes the design of automotive radar

In the United States, the traffic accident death rate has been declining in recent years, and in 2010 it was 3% lower than in 2009, reaching a new low since 1949. The U.S. Department of Transportation believes that the decline in accident death rates is due to the widespread application of automobile safety technologies, especially the active safety technologies that have gradually emerged in recent years, such as the front-end collision warning system and lane change assistance system, which is also the development direction of automobile safety today.

In the above-mentioned active safety applications, radar technology has become a key factor. In recent years, automotive radar technology has continued to become a hot electronic technology that the technology media pays attention to. Although the application of this technology in automobiles can be traced back to at least more than a decade ago, to this day, radar technology is still very limited in terms of application breadth and depth. It is still a "patent" of some mid-to-high-end cars and is more focused on parking assistance applications. In fact, radar technology has great potential in automotive safety applications: including lane change assistance, side collision avoidance detection, adaptive cruise control, driver blind spot detection, brake assist/collision buffer system, intersection traffic alarm, and so on.

Figure 1: Radar technology is widely used in automotive active safety

Analog front-end circuits that revolutionize radar design

However, most current automotive radar systems have low integration, occupy a large space, and are expensive, limiting radar technology that can provide important safety protection for more drivers and passengers to high-end vehicles. The AD8283 launched by Analog Devices integrates the automotive radar receiver analog front end (AFE) into a small chip, greatly improving the integration level and reducing the size and cost of automotive radar. The low-cost, high-performance AD8283 is expected to apply radar active safety technology to more mid- and low-end vehicles.

Figure 2: AD8283 greatly reduces the size of traditional radar analog front-end circuits

The AD8283 has three major advantages for radar system designers and OEMs. The first is size. It allows many discrete devices to be integrated in a 10mmx10mm package, which is very important because radar sensor modules must fit into small areas in the car, such as behind the shock absorber, which are not designed to accommodate such electronic devices. The second is ease of use. The AD8283 provides users with flexible user-programmable settings (the programmable functions will be detailed below) so that users can improve performance under different working conditions, develop systems more easily, and accelerate the launch of new systems. The third is cost. This product is priced lower than discrete devices, and the cost can be reduced by more than 50% compared with discrete device solutions. Therefore, OEMs will be able to provide radar systems for more applications on more models, which will bring safer driving experience to more drivers.

The six-channel AD8283 allows radar systems to receive and decode a greater number of transmitted signals for target identification and classification. This allows for more positioning time, thereby improving the radar's ability to resolve the approximate size of the target. The new analog front-end chip also has the advantage of low power consumption, with each channel consuming only 170mW. The new device has passed AEC-Q100 certification and can operate stably in the automotive application temperature range of -40°C to 105°C.

Signal chain analysis

AD8283 integrates a low noise amplifier (LNA), a programmable gain amplifier (PGA), an anti-aliasing filter (AAF), and an ADC. For traditional discrete solutions, it is very challenging to make the circuit composed of all these devices meet the key performance indicators of the radar system. However, the advantages of the single-chip integration of this chip have solved these problems that have always been a headache for engineers. These performance indicators include LNA noise, PGA gain range, AAF cutoff characteristics, and ADC sampling rate and resolution. The following will analyze in detail the internal circuit structure of AD8283 and how to achieve these performance requirements.

The LNA is located at the front end of the signal path, so good noise performance depends on an ultra-low noise LNA, which can effectively reduce the noise input to the subsequent PGA and AAF circuits. To achieve good input impedance matching, the AD8283 provides an optional input impedance of 200Ω or 200kΩ, which the design engineer can program through the SPI port. The low-value feedback resistor and current drive capability of the output stage enable the LNA to obtain an output reference noise of less than 3.5nV/√Hz at a channel gain of 34dB. The use of a fully differential topology and negative feedback minimizes second-order distortion. The differential signal achieves a smaller signal swing at each output, further reducing third-order distortion. To achieve the best noise performance, it is recommended to pay attention to matching the impedance of the positive and negative input terminals, which can effectively suppress common-mode noise.

The programmable amplifier integrated in the AD8283 achieves great flexibility in application. Design engineers can easily implement amplifier programming settings through the SPI interface: the gain can be programmed in the range of 16dB to 34dB in 6dB steps through the SPI port; at maximum gain, the voltage noise referred to the input is less than 3.5nV/rtHz. The flexible amplifier settings of the AD8283 are critical for automotive applications because the solution can automatically adjust the characteristics of the radar sensor according to the length of the distance, such as preventing the system from overloading and avoiding the return signal from having a very high amplitude that must be attenuated.

The anti-aliasing filter integrated in the AD8283 implements band-stop filtering before the signal enters the ADC, thereby avoiding signal aliasing. The filter combination uses pole and zero filtering to implement a third-order elliptic filter, allowing signals outside the cutoff frequency to roll off quickly. The filter uses on-chip fine-tuning to correct the capacitor value to set the desired cutoff frequency. This fine-tuning method reduces the cutoff frequency variation caused by standard IC process tolerances of resistors and capacitors. The chip's default cutoff frequency of the -3dB low-pass filter is 1/3 or 1/4 of the ADC sampling clock frequency, which can be adjusted to 0.8, 0.9, 1, 1.1, 1.2, or 1.3 times the frequency through the SPI interface. Usually the fine-tuning function is turned off to avoid changing the capacitor setting at critical moments. This function can be turned on or off through the SPI interface. The filter fine-tuning initialization must be completed after the system is powered on and the filter cutoff frequency or ADC sampling rate is reprogrammed. It is recommended that the fine-tuning action can be performed intermittently when the system is idle to compensate for temperature drift.

ADC technology is critical for all sensor applications. AD8283 takes advantage of ADI's industry-leading technology in this field and integrates a 12-bit, 80MSPS (million samples per second) ADC, which can fully meet the requirements of most automotive radars, with a signal-to-noise ratio (SNR) of 67dB and a spurious-free dynamic range (SFDR) of 68dB. Under the condition of fully meeting the application requirements, AD8283 only integrates a single ADC circuit inside and integrates a multiplexer circuit in front of the ADC, thus avoiding the use of an independent ADC for each channel, thereby effectively reducing the cost of the chip. After each ADC sampling is completed, the multiplexer automatically switches the scan between the activated channels. The on-hold time of each channel is one clock cycle, and the switching is synchronized with the ADC sampling to ensure that the switching time and channel setup time will not conflict with the ADC sampling.

Automotive Radar Design Recommendations Based on AD8283

The main application of AD8283 is high-speed ramp frequency modulation continuous wave radar (HSR-FMCW), which is also a technology widely used in current automotive radars. Figure 3 is a simplified system block diagram of FMCW radar. Automotive radar systems based on AD8283 usually require the use of key circuits such as DSP and PLL, for which ADI provides key supporting components.

FMCW radar systems require very high RF performance, and the current method that highly relies on the linearity of voltage-controlled oscillators (VCOs) is very complex, lacks flexibility, and is costly, so there are many design challenges. Last year, ADI launched the industry-leading ADF4158 PLL device, which provides a good choice for this. The ADF4158 is a feature-rich 6.1GHz programmable device that can meet the needs of various FMCW radar system design engineers with simple configuration.

The FMCW radar system implemented using the ADF4158 has a highly linear ramp characteristic that is completely independent of the VCO linearity, thereby improving radar resolution and reducing cost and complexity associated with system calibration. For those cost-sensitive designs, the cost advantage of the ADF4158 is also attractive, with the ADF4158 based on reliable BiCMOS process technology costing only one-third of similar devices.

Similarly, for DSP matching selection, design engineers can choose ADI's very mature and low-cost ADSP BF531, which has been widely used in the industry. ADI can provide low-priced matching development kits and a large number of application code libraries, which can greatly reduce system design costs and speed up product launch. Therefore, the combination with the cost-effective AD8283 will be able to achieve a more cost-competitive automotive radar system, which will further help manufacturers implement radar active safety technology in a wider range of mid- and low-end vehicles.

Figure 4: In addition to the AD8283, ADI also offers PLL and DSP devices that combine cost and performance advantages

Summary:

As passive and active safety systems begin to merge with other electronic systems in the car, such as communications and advanced driver assistance, cars are becoming more autonomous and intelligent. With the application of radar detection technology, electronic systems that react much faster than the driver will be able to take control, reducing the frequency and severity of accidents. In the field of advanced driver assistance systems, systems using radar technology are becoming more and more common. Adaptive cruise control, blind spot detection, cross traffic warning, etc. are all systems that are already in use today to remind or help the driver control the car. In some cases, these systems can even intervene and control the car to avoid dangerous situations.

As a leader in MEMS, RF, amplifiers, converters and DSP, ADI can provide leading technology for realizing such systems. The challenge facing manufacturers today is how to make products with the smallest size, highest reliability, robustness under higher and lower temperature working conditions, and easy configuration at the lowest cost. ADI's industry-leading technologies such as AD8283 and ADF4158 help automakers overcome these challenges with low cost, smaller size and shorter development time.