PCB Technical - Several tips to perfectly improve the immunity of the PCB board to power changes

For the converter and the final system, it must be ensured that noise on any given input does not affect performance. Is it spicy? In order to understand the power supply noise and meet the system design requirements, what aspects should we pay attention to on the PCB board?

First select the converter, then select the regulator, LDO, switching regulator, etc. Not all regulators are suitable. You should check the noise and ripple specifications in the regulator's data sheet, as well as the switching frequency (if using a switching regulator). A typical regulator may have 10µVrms noise in a 100kHz bandwidth. Assuming that the noise is white noise, it is equivalent to a noise density of 31.6nVrms/√Hz in the target frequency band.

Check the power supply rejection index of the converter to understand when the performance of the converter will degrade due to power supply noise. In the first Nyquist zone fS/2, the PSRR of most high-speed converters is typically 60dB(1mV/V). If the value is not given in the data sheet, please measure according to the aforementioned method, or ask the manufacturer.

Using a 16-bit ADC with a 2Vp-p full-scale input range, 78dBSNR and 125MSPS sampling rate, its noise floor is 11.26nVrms. Noise from any source must be below this value to prevent it from affecting the converter. In the first Nyquist zone, the converter noise will be 89.02µVrms(11.26nVrms/√Hz)*√(125MHz/2). Although the noise of the regulator (31.6nv/√Hz) is more than twice that of the converter, the converter has a PSRR of 60dB, which will suppress the noise of the switching regulator to 31.6pV/√Hz (31.6nV/√Hz* 1mV/V). This noise is much smaller than the noise floor of the converter, so the noise of the regulator will not degrade the performance of the converter.

Power filtering, grounding and layout are equally important. Adding a 0.1µF capacitor on the ADC power supply pin can make the noise lower than the aforementioned calculated value. Remember that some power pins draw more current or are more sensitive than other power pins. Therefore, decoupling capacitors should be used with caution, but be aware that some power pins may require additional decoupling capacitors. Adding a simple LC filter at the output of the power supply also helps to reduce noise. However, when using a switching regulator, the cascade filter can suppress noise to a lower level. What needs to be remembered is that every increase of one level of gain will increase by approximately 20dB per 10-octave.

One thing to note is that the above analysis is only for a single converter. If the system involves multiple converters or channels, the noise analysis will be different. For example, ultrasound systems use many ADC channels, which are digitally summed to increase the dynamic range. Basically, every time the number of channels is doubled, the noise floor of the converter/system will be reduced by 3dB. For the above example, if two converters are used, the noise floor of the converter will become half (−3dB); if four converters are used, the noise floor will become −6dB. This is so because each converter can be treated as an uncorrelated noise source. Uncorrelated noise sources are independent of each other, so RSS (square root of the sum of squares) calculation can be performed. Eventually, as the number of channels increases and the noise floor of the system decreases, the system will become more sensitive, and the design constraints on the power supply will become more stringent.

It is impossible to eliminate all power supply noise in the application, because no system can be completely immune to power supply noise. Therefore, as a user of ADC, we must actively respond in the power supply design and layout stage.

Here are some useful tips to help you maximize the PCB's immunity to power changes:

Decouple all power rails and bus voltages reaching the system board.

Remember: each increase in gain will increase by approximately 20dB per 10-octave.

If the power supply leads are longer and supply power to specific ICs, devices, and/or areas, they should be decoupled again.

Both high frequency and low frequency must be decoupled.

The power entry point before the decoupling capacitor is grounded often uses series ferrite beads. Do this for every power supply voltage entering the system board, whether it comes from an LDO or a switching regulator.

For the added capacitors, tightly stacked power and ground layers (spacing ≤ 4 mils) should be used, so that the PCB design itself has high-frequency decoupling capabilities.

As with any good circuit board layout, the power supply should be kept away from sensitive analog circuits such as the ADC's front-end stage and clock circuits.

Good circuit division is very important, some components can be placed on the back of the PCB to enhance isolation.

Pay attention to the ground return path, especially on the digital side, to ensure that digital transients do not return to the analog part of the circuit board. In some cases, separate ground planes may also be useful.

Keep the analog and digital reference components on their own level. This conventional approach can enhance isolation from noise and coupling interactions.

Follow the IC manufacturer’s recommendations. If the application note or data sheet does not directly explain, the evaluation board should be studied. These are very good starting tools.