How to Solve Signal Integrity Problems in PCB Design and Layout

Power noise

Common sources of interference in circuit applications come from the power supply, which is usually introduced through the power device's power supply pins. For example, the timing diagram of the A/D converter output. The sampling speed of the A/D converter is 40 ksps and 4096 samples are taken.

In this example, there is no bypass capacitor 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.

An in-depth study of the circuit shows that the noise source seen on the timing diagram comes from the switching power supply. Bypass capacitors and choke rings have been added to the circuit. A 10mF capacitor was added to the power supply, and three 0.1mF capacitors were placed beside the power supply pins as close to the active components as possible. As can be seen on the resulting new timing diagram, a stable dc output is generated and the histogram can verify this. The data shows that these changes in the circuit eliminate noise sources from the circuit signal path.

Interference-prone external clock

Other system noise sources may come from digital switches in clock sources or circuits. If this noise is related to the conversion process, it will not appear as interference in the conversion process. However, if this noise is not related to the conversion process, this noise can be easily found using FFT (Fast Fourier Transform) analysis.

An example of clock signal interference can be found in the FFT diagram. This figure uses a circuit and adds a bypass capacitor. The excitation seen in the FFT shown in Figure 4 is generated by the 19.84-MHz clock signal on the board. In this example, almost no consideration was given to the coupling between the traces when designing and wiring the PCB. The result of ignoring this detail can be seen in the FFT diagram.

This problem can be solved by modifying the layout of the PCB design to route the high-impedance analog traces away from the digital switch traces, or to add an anti-aliasing filter before the A/D converter in the analog signal path. The random coupling between traces is somewhat more difficult to find, in which case time domain analysis may be more effective.

Improper use of amplifier

Return to the circuit and apply a 1 kHz ac signal to the non-inverting input of the instrumentation amplifier. This signal is not a pressure sensing feature, but this example can be used to illustrate the effects of the device in the analog signal path.

The FFT plot shows the circuit performance after applying the above conditions. Note that the fundamental wave looks distorted and many harmonics have the same distortion. Distortion is caused by a slight over-excitation of the amplifier. The solution to this problem is to reduce the amplifier gain.

Conclusion

Addressing signal integrity issues can take a lot of time, especially when engineers don't have the tools to solve tough problems. There are three best analysis tools in the "slam box": frequency domain analysis tool (FFT), time domain analysis tool (oscilloscope photo) and DC analysis tool (histogram). Engineers can use these tools to identify power supply noise, external clock sources, and overdrive amplifier distortion.