EMI Filtering Reduces Errors in Precision Analog Applications

In technological fields such as medical devices, automotive instrumentation, and industrial control, precision analog front-end amplifiers are often required when the device design involves strain gages, sensor interfaces, and current monitoring, in order to extract and amplify very weak real signals and suppress common Unwanted signals such as mode voltage and noise. First, the designer will focus on ensuring that accuracy parameters such as device-level noise, offset, gain, and temperature stability meet the application requirements.

The designer then selects a front-end analog device that meets the total error budget based on these characteristics. However, an often overlooked issue in such applications is high-frequency interference caused by external signals, commonly referred to as “electromagnetic interference (EMI).” EMI can occur in a number of ways, mainly affected by the end application. For example, an instrumentation amplifier might be used in a control board that interfaces with a DC motor, and the motor’s current loop, which includes power leads, brushes, commutators, and coils, often acts like an antenna, emitting high-frequency signals, which may cause Tiny voltages that interfere with the input of the instrumentation amplifier.

Another example is current sensing in automotive solenoid valve control. The solenoid valve is powered by the vehicle battery through long wires that act like an antenna. A series shunt resistor is connected in the wire path, and the voltage across this resistor is measured by a current sense amplifier. There may be high-frequency common-mode signals on the line, and the input of the amplifier is susceptible to such external signals. Once affected by external high-frequency interference, the accuracy of the analog device may be degraded, and it may even be impossible to control the solenoid valve circuit. This condition manifests itself in amplifiers where the amplifier output accuracy exceeds the error budget and data sheet tolerances, and may even reach limits in some cases, causing the control loop to shut down.

How does EMI cause large DC offsets? It may be one of the following: By design, many instrumentation amplifiers can exhibit excellent common-mode rejection in the frequency range up to tens of kilohertz. Problems can arise, however, when unshielded amplifiers are exposed to tens or hundreds of “megahertz” of RF radiation. At this time, asymmetric rectification may occur at the input stage of the amplifier, resulting in DC offset. After further amplification, it will be very obvious, plus the gain of the amplifier, even reaching the upper limit of its output or some external circuits.

Example of how high frequency signals affect analog devices

This example will detail a typical high-side current sensing application. Figure 1 shows a common configuration for monitoring solenoid valves or other inductive loads in an automotive application environment.

EMI Filtering Reduces Errors in Precision Analog Applications

Figure 1. High-Side Current Monitoring

We investigated the effect of high frequency interference using two current sense amplifier configurations with similar designs. The functions and pinout of the two devices are identical; however, one has a built-in EMI filter circuit, while the other does not.

EMI Filtering Reduces Errors in Precision Analog Applications

Figure 2. Current sensor output (without built-in EMI filter, forward power = 12 dBm, 100 mV/divide, peak DC output at 3 MHz)

Figure 2 shows the deviation of the DC output of the current sensor from its ideal value when the input varies over a wide frequency range. As can be seen from the graph, the deviation is most significant (>0.1 V) in the frequency range from 1 MHz to 20 MHz, and the DC error reaches its maximum value (1 V) at 3 MHz, which is at 0 V to 5 V of the amplifier. occupies a large percentage of the output voltage range.

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

EMI Filtering Reduces Errors in Precision Analog Applications

Figure 3. Current sensor output (built-in EMI filter, forward power = 12 dBm, 5 mV/divide, peak DC output at >100 MHz)

In this case, the error is only around 3 mV at 40 MHz, and the peak error (greater than 100 MHz) is less than 30 mV, a performance improvement of 35 times. This clearly shows that the built-in EMI filter circuit helps to significantly improve the protection of the current sensor from high frequency signals present at the input. In practice, although the severity of the EMI is not known, the control loop will actually stay within its tolerance if a current sensor with built-in EMI filtering is used.

Both devices were tested under exactly the same conditions. The only difference is that the AD8208 (see “Appendix”) has an internal low-pass RF input filter on both the input pins and the power supply pins. Adding such a part on a chip may seem trivial, but since applications are typically PWM controlled, the current-sense amplifier must be able to withstand continuous switching common-mode voltages up to 45 V in this case. Therefore, to maintain accurate high gain and common-mode rejection, the input filters must be closely matched.

Why and How to Ensure EMI Compatibility When Designing and Testing

Automotive applications are particularly sensitive to EMI events, which cannot be avoided in a noisy electrical environment consisting of central batteries, bundled wiring harnesses, various inductive loads, antennas, and external interference related to the vehicle. Since many key functions such as airbag deployment, cruise control, brakes and suspension are controlled by Electronic equipment, EMI compatibility must be guaranteed, and false alarms or false triggers due to external interference must not be tolerated. Earlier, EMI compliance testing was the last test in automotive applications. If something goes wrong, designers must find a solution in a hurry, which often involves changing the board layout, adding additional filters, or even replacing components.

This uncertainty greatly increases design costs and creates a lot of trouble for engineers. The automotive industry has been taking concrete steps to improve EMI compliance for a long time. Because devices must comply with EMI standards, automotive OEMs now require semiconductor manufacturers such as Analog Devices to perform EMI testing at the device level before considering their devices. Now that this process is widespread, all IC manufacturers use standard specifications to test devices for EMI compliance.

For standard EMI testing requirements for various types of integrated circuits, please purchase documents from the International Electrotechnical Commission (IEC). EMI and EMC can be learned through documents such as IEC 62132 and IEC 61967, which describe in great detail how to test specific integrated circuits using industry-accepted standards. The various tests described above were performed in accordance with these guidelines.

Specifically, these tests are done using “direct power injection,” a method of coupling an RF signal to a specific device pin through a capacitor. Test each input of the device for different RF signal power levels and frequency ranges, depending on the type of IC being tested. Figure 4 shows a schematic diagram of the principle of performing a direct power injection test on a specific pin.

EMI Filtering Reduces Errors in Precision Analog Applications

Figure 4. Direct Power Injection

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

EMI Filtering Reduces Errors in Precision Analog Applications

Figure 5. Schematic of EMI Tolerance Test

Summarize

The EMI compatibility of integrated circuits is the key to the success of electronic designs. This article shows how the DC performance in RF environments differs significantly when two very similar amplifiers perform DC measurements, starting with whether or not the amplifiers have built-in EMI filters. In automotive applications, EMI is a very important aspect when considering safety and reliability. Today, IC manufacturers, such as Analog Devices, are increasingly focusing on EMI tolerance considerations when designing and testing devices for critical applications. The IEC standards describe useful relevant guidelines in great detail. For the automotive market, current sensing devices such as the AD8207, AD8208 and AD8209 are EMI tested. New devices such as the AD8280 lithium-ion battery safety monitor and the AD8556 digitally programmable sensor signal amplifier have been designed and tested to meet EMI requirements.

appendix

More details on the AD8208: The AD8203 (Figure A) is a single-supply difference amplifier ideal for amplifying and low-pass filtering small differential voltages in the presence of large common-mode voltages. The input common-mode voltage range is -2 V to +45 V when operating from a single +5 V supply. The amplifier provides enhanced input overvoltage and ESD protection, and built-in EMI filtering.

Figure A. AD8208 Difference Amplifier

The AD8208 has excellent ac and dc performance and is certified for automotive applications that require stable and reliable precision devices to improve system control. Offset and gain drift are typically less than 5 µV/°C and 10 ppm/°C, respectively. Available in SOIC and MSOP packages, the device features a minimum common-mode rejection ratio (CMR) of 80 dB from DC to 10 kHz.

There is also an externally available 100kΩ resistor that can be used for low pass filtering and to establish gains other than 20.


The Links:   G150XTN060 PM400DVA060

In technological fields such as medical devices, automotive instrumentation, and industrial control, precision analog front-end amplifiers are often required when the device design involves strain gages, sensor interfaces, and current monitoring, in order to extract and amplify very weak real signals and suppress common Unwanted signals such as mode voltage and noise. First, the designer…