Electromagnetic interference (EMI) filter to reduce the error in precision analog applications - OF: Henri Sino

Filter 18.432M

In the fields of medical equipment, automotive instrumentation, and industrial control, when equipment design involves strain gauges, sensor interfaces, and current monitoring, precision analog front-end amplifiers are often required 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.

The designer then selects the front-end analog device that meets the total error budget based on the above characteristics. However, there is a frequently overlooked problem in such applications, namely high frequency interference caused by external signals, also known as "electromagnetic interference (EMI)." EMI can occur in a variety of ways, primarily 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 contains power leads, brushes, commutators, and coils, which typically emit high-frequency signals like an antenna, and may A small voltage that interferes with the input of the instrumentation amplifier.

Another example is current sensing in automotive solenoid valve control. Solenoid valves are powered by the vehicle's battery through long wires that act like antennas. A series shunt resistor is connected to the wire path, and then the voltage across the resistor is measured by a current sense amplifier. A high frequency common mode signal may be present in 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 the solenoid valve circuit may not be controlled. This state of performance in the amplifier is that the amplifier output accuracy exceeds the error budget and the tolerances in the data sheet, and in some cases may reach the limit, causing the control loop to turn off.

How does EMI cause large DC offsets? It may be the case that many instrumentation amplifiers are designed to exhibit excellent common mode rejection over a frequency range of up to tens of kilohertz. However, when an unshielded amplifier is exposed to tens or hundreds of "megahertz" RF radiation, problems can occur. At this point, the input stage of the amplifier may be asymmetrical rectified, resulting in a dc offset. After further amplification, it will be very noticeable, plus the gain of the amplifier, even reaching the upper limit of its output or part of the external circuit.

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 automotive applications.

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