Table of contents
Explanation of technical terms
1. Select the converter first.
2. Check the power supply rejection index of the converter.
3. Use a 2 V pp full-scale input range
overview
For the converter and the final system. It must be ensured that noise on any given input does not affect performance. Then, in order to understand the power supply noise and meet the system design requirements. What aspects should we pay attention to?
overall architecture process
It is generally divided into 5 steps to describe and share
Explanation of technical terms
Converter: A converter is a device that converts one signal into another . A signal is the form or carrier in which information exists. In automatic instrumentation equipment and automatic control systems , one signal is often converted into another signal compared with a standard or reference quantity in order to connect the two types of instruments. Therefore, the converter is often two instruments (or device) as an intermediate link.
technical details
1. Select the converter first.
Then select regulators, LDOs, switching regulators, etc. Not all regulators are suitable. The noise and ripple specifications in the regulator data sheet should be checked. and switching frequency (if using a switching regulator). A typical regulator may have 10 uv rms noise in a 100 kHz bandwidth. Assuming this noise is white, it corresponds to a noise density of 31.6 nv rmslHz in the frequency band of interest.
2. Check the power supply rejection index of the converter.
Know when converter performance degrades due to power supply noise. In the first Nyquist zone, fS/2, the PSRR of most high speed converters is typically 60 dB (1 mV/V). If the value is not given in the data sheet, please measure according to the above method. Or ask the manufacturer.
3. Use a 2 V pp full-scale input range
A 16-bit ADC with 78 dB SNR and 125 MSPS sampling rate has a noise floor of 11.26 nv rms. Noise from any source must stay below this value to prevent it from affecting the converter. In the first Nyquist zone, the converter noise will be 89.02 uv rms (11.26 nv rms/., Hz) × (125 MHz/2). Although the noise of the regulator (31.6 nvl.Hz) is more than twice that of the converter. But the converter has a PSRR of 60d, which will suppress the noise of the switching regulator to 31.6 pV/Hz (31.6 nVI, Hz × 1 mV/V). This noise is much smaller than the converter's noise floor, so the regulator's noise does not degrade the converter's performance.
4. Power filter.
Grounding and layout are equally important. Adding a 0.1 uF capacitor to the ADC supply pin keeps the noise below the previously calculated value. Keep in mind that some power pins draw more current, or are more sensitive than others. Decoupling capacitors should therefore be used with caution, but be aware that some supply pins may require additional decoupling capacitors. Adding a simple LC filter at the output of the power supply also helps reduce noise. However, when using a switching regulator, a cascaded filter suppresses the noise to a much lower level. Something to keep in mind is that each additional step of gain adds about 20 dB per decade.
5. Need to pay attention
The above analysis is only for a single converter. Noise analysis is different if the system involves multiple converters or channels. For example, ultrasound systems employ many ADC channels that are summed digitally to increase dynamic range. Basically. Each time the number of channels doubles. The converter/system noise floor is then reduced by 3 dB. For the above example. If you use two converters. The noise floor of the converters will be halved (-3 dB) if four converters are used. The noise floor will change to -6 dB. This is so because each converter can be treated as an uncorrelated noise source. Uncorrelated noise sources are independent of each other, so RSS (root sum of squares) calculations can be performed. Finally, as the number of channels increases. The noise floor of the system is reduced. Systems will become more sensitive, with tighter design constraints on power supplies. It is impossible to eliminate all power supply noise in an application. Because it is impossible for any system to be completely immune to power supply noise. therefore. As users of ADCs, we must take active measures during the power supply design and layout stages.
summary
提示：这里可以添加总结
For example:
1. Decouple all power rails and bus voltages to the system board.
2. Remember: Each additional step of gain increases approximately 20 dB per decade.
3. If the power lead is long and powers a specific IC, device and/or area. should be decoupled again.
4. Both high frequency and low frequency should be decoupled.
5. The power supply entry point before the decoupling capacitor to ground often uses a series ferrite bead. Do this for every supply voltage that goes into the system board, whether it's from an LDO or from a switching regulator.
6. For the added capacitors, closely stacked power and ground plane spacing (mils) should be used, so that the PCB design itself has high-frequency decoupling capabilities.
7. As with any good board layout, keep power supplies away from sensitive analog circuits. Such as ADC front-end stage and clock circuit, etc.
8. Good circuit division is critical, some components can be placed on the back of the PCB to enhance isolation.
9. Pay attention to the ground return path, especially on the digital side. Make sure that digital transients do not make their way back into the analog portion of the board. In some cases, a split ground plane may also be useful.
10. Keep analog and digital reference components on their own levels. This general practice increases isolation from noise and coupling interactions.
11. Follow the IC manufacturer's recommendations. If the application note or data sheet does not directly explain. evaluation boards should be investigated. These are great tools to start with.