Uncovering the secrets of testing and measurement - Test every Monday [Week 6]

2. When using or selecting a probe, what aspects should be paid attention to in order to optimize the measurement accuracy of the power supply?

3. There is a item PSRR (power supply rejection ratio) in the switching power supply test. What does it mean? How to measure it? (The answer to this question is not in the technical article above)

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1. When performing power supply testing, what methods can be used to maximize the dynamic range of the oscilloscope?

(Answer provided by Agilent's Du Jiwei) There are several aspects to consider when optimizing the dynamic range of an oscilloscope, but the choice or use of probes and cables should also be considered.

o Use high-resolution acquisition to improve measurement accuracy. The advantage is that the vertical resolution of the oscilloscope is improved. This method can be used even for single acquisition, which is better than the average method. The disadvantage is that the effective bandwidth of the actual oscilloscope will decrease, but for power supply testing, the oscilloscope does not need to have an excessively high bandwidth, so as long as the bandwidth is not less than 20MHz, it is acceptable. When using the Agilent 9000 series oscilloscope, when using the high-resolution mode, you can directly set the required number of vertical resolution bits. The following table shows the relationship between the sampling rate, vertical resolution, and effective bandwidth of the 9000 series oscilloscope in the high-resolution mode.

 
o Use AC coupling. If the oscilloscope itself supports AC coupling, you can directly set it to AC coupling. If the oscilloscope does not support AC coupling, you can solder an appropriate coupling capacitor at the location where the object under test leads out of the measurement point to isolate the AC component in the measured signal.

o Bandwidth limit: Usually 20MHz bandwidth limit is used, so the oscilloscope bandwidth limit can be set to 20MHz, or a low-bandwidth probe can be used to limit the measurement bandwidth; The indicators of the chips provided by some manufacturers are not given in the case of <=20MHz. At this time, it is recommended to use the oscilloscope's arbitrary bandwidth limit function, such as the low-pass filter in the math function, which can generally be set to any value. The advanced math option of Agilent's 3000X series oscilloscope supports this function, and the 9000A series is standard with this function.

2. When using or selecting a probe, what aspects should be paid attention to in order to optimize the measurement accuracy of the power supply?

 (Answer provided by Agilent's Du Jiwei)

o Regarding the connection with the object under test, if single-ended measurement is possible, it is recommended to use a cable or a 1:1 probe. The advantage of doing so is that you can optimize the vertical sensitivity of the oscilloscope and maximize the dynamic range of the oscilloscope. For example, you can use the N2870A 1:1 35MHz or 10070C 20MHz 1:1 passive probe.

o In some applications, it is not allowed to measure the measured signal and the oscilloscope on the same ground. In this case, it is recommended to use a differential probe for floating measurement. When selecting a differential probe, try to choose one with a relatively small attenuation ratio, such as Agilent's 2:1 N2750A series differential probe. If a 5:1 or 10:1 attenuation ratio is used, the measurement accuracy will be reduced exponentially.

o The ground wire of the probe should be as short as possible, or the magnetic flux area formed by the signal point should be as small as possible. For example, the ground wire can be wrapped around the probe body. It is best to use a very short spring ground.

3. There is a item PSRR (power supply rejection ratio) in the switching power supply test. What does it mean? How to measure it? (The answer to this question is not in the technical article above)

(Answer provided by Agilent's Du Jiwei)

o Power supply rejection ratio, for a specific definition, please refer to Wikipedia, etc. Basically, a signal source and an oscilloscope are needed for measurement, but engineers often do not measure because it is not easy to find two instruments or they are afraid of trouble. Agilent DSOX3000A series oscilloscopes have a built-in arbitrary waveform generator that can generate any signal as the input of the power supply, and use the oscilloscope to measure the output, thereby automatically giving the PSRR value. Moreover, it is an automatic measurement software, and the user does not need to calculate it by himself. This makes the measurement of PSRR feasible and simple. Moreover, the arbitrary waveform generator option of DSOX3000A is free of charge.