Tips | How to measure power supply ripple?
1. Ripple measurement point requirements
PDN is a network. The ripples are different in different places on a power plane on the motherboard. It is recommended to choose the farthest place with the largest load and the harshest environment.
If a power network supplies power to multiple power-hungry ICs at the same time, the power pins of these large ICs need to be tested for ripple levels.
2. Oscilloscope requirements
There are many capacitors and parasitic parameters on the power supply traces, forming a similar low-pass filter. The frequency components contained in the power supply are not high, up to tens of megahertz.
The switching frequency of the DC-DC power supply is only a few megahertz. If you just want to measure the ripple level, without considering the noise coupled to the power line by the high-frequency signal on the motherboard, you only need to choose a low-bandwidth oscilloscope.
There is a bandwidth suppression function on the oscilloscope, which can suppress the bandwidth of the oscilloscope to 20MHz. When measuring ripple, this function must be turned on.
The larger the vertical scale of the instrument (the smaller the voltage represented by each grid), the higher the sensitivity of the instrument and the better the noise performance. When measuring, the signal should fill the screen to obtain the highest instrument resolution.
In addition, you can also use the oscilloscope's high-resolution mode, such as High Res.
3. Probe requirements
Low bandwidth probes can be used. The low-end probes purchased on Taobao and electronic markets only have a bandwidth of 20MHz, which is completely sufficient.
But remember to choose a probe with X1 gear. Because if you use a 10X probe, the signal is attenuated 10 times in the probe and then amplified 10 times after entering the oscilloscope. For small signals, after attenuation by 10 times, this part of the signal is likely to be hidden in the noise floor. The following is a professional explanation of Tektronix oscilloscopes:
The signal-to-noise ratio of the oscilloscope input is calculated as follows:
For example, if we choose a 10mV (this is a 10mV noise, but we treat it as a signal at this time) input, the random noise is 200uV (this indicator is the random noise in the oscilloscope product technical data, which is the noise of the internal circuit of the oscilloscope. Generally Expressed in Vrms), then the SNR of the 10x probe is:
The SNR of the 1X attenuated probe is:
Therefore, the 1X probe can obtain a better signal-to-noise ratio and can restore small signals more accurately. Therefore, you must use a 1X probe when measuring small signals, and pay attention to the bandwidth of the 1X probe.
Some people also suggest connecting a 0.1uF capacitor in parallel between the positive and negative electrodes of the probe to filter out high-frequency signals. In fact, this is also a way to limit the bandwidth, and the limited bandwidth is very low. I don’t think it is special. necessary.
4. Probe grounding requirements
The probes all have a ground clamp, which forms a loop with the positive electrode of the probe. If a high-frequency signal on the board passes through, a voltage larger than the noise signal will be formed on the oscilloscope. Therefore, when measuring small signals such as ripple, Be sure to keep this loop as small as possible.
The best way to shorten the ground path is to use a grounding ring, which can be purchased on Taobao.
5. Professional testing methods
Use a 50Ω coaxial wire with one end interrupted and weld it to the load end capacitor. The shorter the welding wire, the better. Connect the other end to the oscilloscope, and set the oscilloscope to 50Ω input, 1X.
Because the internal resistance of the power supply is very low, a few ohms or even lower, it is enough to use the 50Ω of the oscilloscope as the load.
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