Nanjing PCB design power supply noise
The common interference source in the circuit comes from the power supply, and this interference signal is usually introduced through the power supply pin of the active device. For example, the timing diagram of A/D converter output in is shown in. The sampling speed of A/D converter is 40ksps, and 4096 samples are taken.
In this example, there are no bypass capacitors on the instrumentation amplifier, reference voltage source and A/D converter. In addition, the input of the circuit is based on a low-noise, 2.5V DC voltage source.
The in-depth study of the circuit shows that the noise source seen in the timing diagram comes from the switching power supply. Bypass capacitor and choke ring are added to the circuit. A 10mF capacitor is added to the power supply, and three 0.1mF capacitors are placed next to the power supply pins as close as possible to the active components. It can be seen from the generated new timing chart that a stable DC output is generated, which can be verified by the histogram. The data shows that these changes of the circuit eliminate the noise source from the signal path of the circuit.
Nanjing PCB Design Disturbs the External Clock
Other sources of system noise may come from clock sources or digital switches in circuits. If this noise is related to the conversion process, it will not appear as interference in the conversion process. However, if this noise has nothing to do with the conversion process, it can be easily found by FFT (Fast Fourier Transform) analysis.
Digital noise coupled to analog traces is sometimes misunderstood as broadband noise. FFT can easily identify the frequency of this so-called “noise”, so the noise source can be identified.
Slight overexcitation of the amplifier will distort the signal. Through the converted FFT diagram, the distortion of the signal can be found quickly.
See the FFT diagram shown in fig. 4 for an example of clock interference. In this figure, the circuit shown in Figure 1 is used, and a bypass capacitor is added. The excitation seen in the FFT diagram is generated by the 19.84MHz clock signal on the circuit board. In this case, the coupling between traces is hardly considered in wiring, and the result of ignoring this detail can be seen in the FFT diagram.
This problem can be solved by modifying the wiring to keep the high impedance analog wiring away from the digital switch wiring; Or in the analog signal path, an anti-aliasing filter is added before the A/D converter. To some extent, the random coupling between lines is more difficult to find. In this case, time domain analysis may be more effective. PCB design skills and test characteristics
Improper use of amplifier
Apply a 1kHz AC signal to the non-inverting input of the instrumentation amplifier. This signal is not a characteristic of pressure sensing, but this example can be used to illustrate the influence of devices in the analog signal path.
The FFT shows the circuit performance after the above conditions are applied. Note that the fundamental wave appears distorted, and many harmonics have the same distortion. The distortion is caused by slightly overexciting the amplifier. The solution to this problem is to reduce the amplifier gain.
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Solving signal integrity problems can take a lot of time, especially when engineers don’t have the tools to solve tough problems. There are three excellent analysis tools in the “Trick Box”: frequency domain analysis tools (FFT), time domain analysis tools (oscilloscope photos) and DC analysis tools (histogram). Engineers can use these tools to identify power supply noise, external clock source and distortion of overdrive amplifier.