In order to prevent demodulation, the analog circuit needs to be linear and stable when it is in an interference environment, especially the feedback loop, which needs to be in a linear and stable state over a wide frequency range. This often requires buffering the capacitive load while using a A small series resistor (approximately 500) is connected in series with an integrated feedback capacitor of approximately 5 PF. This is due to the fact that the immunity of most analog devices is caused by RF demodulation. Each pin of the op amp is sensitive to RF interference. This is independent of the feedback line used. All semiconductors demodulate the RF, but the problem on the analog circuit is more serious.
For stability and linearity testing, a small but steeply rising (<1 ns) square wave signal is injected at the input (which can also be fed through the capacitor to the output and power terminals). The fundamental frequency of the square wave must be expected in the circuit. In the frequency band, the circuit output uses 100MHz (at least) oscilloscopes and probes for impact and ringing checks, as well as audio or instrumentation circuits. For higher speed analog circuits, select a wider oscilloscope and pay attention to the use. Probe tips.
An overshoot of more than 50% of the signal height indicates that the circuit is unstable, and the overshoot should be effectively attenuated. Any long-term ringing of the signal (more than two cycles) or sudden oscillation indicates that the stability is not good.
After obtaining a stable and linear circuit, all the wires may need to be filtered. The digital circuit part of the same product always senses the noise to the internal connection, and the external connection is subjected to the external electromagnetic environment.
Never attempt to use active circuitry to filter and reject RF bandwidth to meet EMC requirements. Only passive filters (preferably RC type) can be used. In an op amp circuit, the integral feedback method is effective only in the frequency range where its open-loop gain is much larger than the closed-loop gain, but at higher frequencies it cannot control the frequency response.
Circuits with high input and output impedance should be avoided. The comparator must have hysteresis (positive feedback) to prevent the output from malfunctioning due to noise and interference, and to prevent oscillation near the switching point. Do not use an output conversion comparator that is much faster than it actually needs to keep dv/dt in a low state.
For high-frequency analog signals (such as radio frequency signals), transmission line technology is necessary, depending on its length and the highest frequency of communication, even for low-frequency signals, if the transmission line technology for internal connections, the immunity will be improved.
Some circuits in analog integrated circuits are extremely sensitive to high field strengths, which can be shielded with a small metal shell (if heat is allowed) and the shielded box is soldered to the PCB ground plane.
Like digital circuits, analog devices also need to provide high quality RF bypass (decoupling) for the power supply, but also require low frequency power supply bypass because the power supply noise rejection (PSRR) of the analog device is very weak for frequencies above 1 kHz. RC or LC filtering is required for each analog supply pin of each op amp, comparator or data converter. These supply filter corner frequencies and transition band slopes should compensate for the corner frequency and slope of the device PSRR. To obtain the desired PSRR within the frequency band of interest.
RF design is rarely involved in general EMC design guidelines because RF designers are generally familiar with most continuous EMC phenomena, but it should be noted that the LO and IF frequencies generally have large leakages, so Need to focus on shielding and filtering.
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