How to reduce the DC deviation of EMI in the automotive control system

“In the scientific and technological fields such as medical equipment, automotive instrumentation and industrial control, when the equipment design involves strain gauges, sensor interfaces, and current monitoring, it is usually necessary to use precision analog front-end amplifiers to extract and amplify very weak real signals and suppress common Unwanted signals such as mode voltage and noise. First, designers will focus on ensuring that accuracy parameters such as device-level noise, offset, gain, and temperature stability meet application requirements.

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In the scientific and technological fields such as medical equipment, automotive instrumentation and industrial control, when the equipment design involves strain gauges, sensor interfaces, and current monitoring, it is usually necessary to use precision analog front-end amplifiers to extract and amplify very weak real signals and suppress common Unwanted signals such as mode voltage and noise. First, designers will focus on ensuring that accuracy parameters such as device-level noise, offset, gain, and temperature stability meet application requirements.

Then, the designer selects a front-end analog device that meets the requirements of the total error budget based on the above-mentioned characteristics. However, there is a problem that is often overlooked in such applications, that is, high-frequency interference caused by external signals, which is commonly referred to as “electromagnetic interference (EMI).” EMI can occur in many ways, mainly affected by the final application. For example, an instrumentation amplifier may be used in a control board that interfaces with a DC motor, and the current loop of the motor includes power leads, brushes, commutators, and coils. It usually emits high-frequency signals like an antenna. A small voltage that interferes with the input of the instrumentation amplifier.

Another example is current detection in automotive solenoid valve control. The solenoid valve is powered by the vehicle battery through long wires, which are like antennas. A series shunt resistor is connected in the wire path, and then the voltage on the resistor is measured by a current detection amplifier. There may be high-frequency common-mode signals in the line, and the input of the amplifier is susceptible to such external signals. Once affected by external high-frequency interference, it may cause the accuracy of the analog device to decrease, and may even be unable to control the solenoid valve circuit. The performance of this state in the amplifier is that the output accuracy of the amplifier exceeds the error budget and the tolerance in the data sheet, and may even reach the limit in some cases, causing the control loop to shut down.

How does EMI cause a large DC deviation? It may be one of the following situations: According to the design, many instrumentation amplifiers can show excellent common-mode rejection performance in the frequency range of up to tens of kilohertz. However, when an unshielded amplifier is exposed to tens or hundreds of “MHz” of RF radiation, problems may arise. At this time, the input stage of the amplifier may appear asymmetrical rectification, resulting in DC offset. After further amplification, it will be very obvious, and the gain of the amplifier may even reach the upper limit of its output or some external circuits.

Examples of how high-frequency signals affect analog devices

This example will introduce a typical high-end current detection application in detail. Figure 1 shows a common configuration used to monitor solenoid valves or other inductive loads in an automotive application environment.

Figure 1. High-side current monitoring

We used two current-sense amplifier configurations with similar designs to study the effects of high-frequency interference. The functions and pinout of the two devices are exactly the same; however, one of them has a built-in EMI filter circuit, while the other does not.

Figure 2. Current sensor output (no built-in EMI filter, forward power = 12 dBm, 100 mV/div, DC output peaks at 3 MHz)

Figure 2 shows the deviation of the DC output of the current sensor from its ideal value when the input changes in a wide frequency range. It can be seen from the figure that in the frequency range of 1 MHz to 20 MHz, the deviation is the most significant (>0.1 V), and the DC error reaches the maximum value (1 V) at 3 MHz. Occupies a large proportion of the output voltage range.

Figure 3 shows the test results of the same experiment and configuration when another pin-compatible current sensor is used. The current sensor has the same circuit architecture and similar DC specifications as the previous example, but has a built-in input EMI filter circuit. Note that the voltage range has been expanded by a factor of 20.
　

Figure 3. Current sensor output (built-in EMI filter, forward power = 12 dBm, 5 mV/frequency division, DC output reaches its peak value at 100 MHz)

In this case, the error is only about 3 mV at 40 MHz, and the peak error (when greater than 100 MHz) is less than 30 mV, the performance is improved by 35 times. This clearly shows that the built-in EMI filter circuit helps to significantly improve the protection performance of the current sensor and protect it from the high-frequency signal at the input. In practical applications, although the severity of EMI is not clear, if a current sensor with built-in EMI filtering function is used, the control loop will actually remain within its tolerance range.

Both devices are tested under exactly the same conditions. The only difference is that the AD8208 (see “Appendix”) is equipped with an internal low-pass RF input filter on both the input pin and the power supply pin. It may seem trivial to add such components on the chip, but since the application is usually controlled by PWM, the current sense amplifier must be able to withstand a continuous switching common-mode voltage of up to 45 V in this case. Therefore, to maintain accurate high gain and common-mode rejection performance, the input filter must be strictly matched.

Why and how to ensure EMI compatibility during design and testing

Automotive applications are particularly sensitive to EMI events, and the latter is unavoidable in a noisy electrical environment composed of central batteries, bundled wiring harnesses, various inductive loads, antennas, and external interference related to the car. Since airbag configuration, cruise control, brakes and suspension and other key functions are controlled by Electronic equipment, it is necessary to ensure EMI compatibility and never allow false alarms or false triggers due to external interference. Earlier, EMI compatibility testing was the last test in automotive applications. If there is an error, the designer must find a solution in haste, and this often involves changing the circuit board layout, adding additional filters, or even replacing the device.

This uncertainty greatly increases design costs and causes a lot of trouble for engineers. For a long time, the automotive industry has been taking practical measures to improve EMI compatibility. Since equipment must comply with EMI standards, automotive OEMs now require semiconductor manufacturers (such as ADI) to perform EMI testing at the device level before they consider using their devices. Now that this process has become widespread, all IC manufacturers use standard specifications to test the EMI compatibility of their devices.

If you want to know the standard EMI test requirements of various types of integrated circuits, please purchase from the International Electrotechnical Commission (IEC) to obtain relevant documents. You can learn about EMI and EMC through documents such as IEC 62132 and IEC 61967, which describe in great detail how to use industry-recognized standards to test specific integrated circuits. The above-mentioned various tests are carried out according to the instructions of these guidelines.

Specifically, these tests are all done using the “direct power injection method”, which is a method of coupling RF signals to specific device pins through capacitors. According to the type of IC to be tested, each input of the device is tested for different RF signal power levels and frequency ranges. Figure 4 shows a schematic diagram of the principle of performing a direct power injection test on a specific pin.
　　

Figure 4. Direct power injection

These standards contain a large amount of necessary information on circuit configuration, layout methods, and monitoring techniques to help correctly understand the success of device testing. A more complete schematic diagram of the IEC standard is shown in Figure 5.
　

Figure 5. Schematic diagram of EMI tolerance test

Summarize

The EMI compatibility of integrated circuits is the key to the success of electronic design. This article only starts from whether the amplifier has built-in EMI filters, and introduces the significant differences in the DC performance of two very similar amplifiers in the RF environment when performing DC measurements. In automotive applications, EMI is a very important aspect when considering safety and reliability. Nowadays, when designing and testing devices for critical applications, IC manufacturers (such as ADI) are paying more and more attention to EMI tolerance considerations. The IEC standards specify useful and relevant guidelines in great detail. For the automotive application market, current detection devices such as AD8207, AD8208 and AD8209 have all passed EMI testing. New devices such as the lithium-ion battery safety monitor AD8280 and the digital programmable sensor signal amplifier AD8556 have been specially designed and tested to meet EMI related requirements.

appendix

More details of AD8208: AD8203 (Figure A) is a single-supply difference amplifier, very suitable for amplifying and low-pass filtering small differential voltages under large common-mode voltages. When using a +5 V single power supply, the input common-mode voltage range is -2 V to +45 V. This amplifier provides enhanced input overvoltage and ESD protection, and built-in EMI filtering.
　

Figure A. AD8208 differential amplifier

AD8208 has excellent AC and DC performance, and has passed relevant certifications, and is suitable for automotive applications that require stable and reliable precision devices to improve system control. Typical offset and gain drift are less than 5 ?V/°C and 10 ppm/°C, respectively. The device is available in SOIC and MSOP packages, with a minimum common-mode rejection ratio (CMR) of 80 dB from DC to 10 kHz.

In addition, an externally available 100kΩ resistor is provided, which can be used for low-pass filtering and to establish gains other than 20.