How to verify the reliability of current probe?

　　Current probes are essential accessories for oscilloscopes to measure current, but the prices of different brands often vary greatly. What kind of current probe is reliable? Here we share the complete process of current probe reliability verification and present all the measured results. You can also use this method to verify the current probe you use.

　　This article conducts a comprehensive performance parameter verification of the ZCP0030-50 current probe. The main test parameters include DC accuracy, rise time, square wave response, noise, and actual measurement of the switching power supply switch tube current waveform. In order to make the actual measurement results more intuitive, we choose T company's TCP312 current probe as the actual measurement comparison, and provide pictures and test results in all tests to ensure that the test process is traceable.

　　1. DC accuracy verification

　　The nominal DC accuracy of ZCP0030-50 is 1%rdg±1mV, and the measured accuracy is shown in Table 1.

　　1. Test conditions

　　The test instruments used for the accuracy test are FLUKE5720 calibrator and 6.5-digit multimeter DMM6000. Test steps: the calibrator FLUKE5720 outputs different measured current values, and the multimeter reads the response voltage value of the current probe.

　　Figure 2.1 DC accuracy test environment

　　2. Accuracy data

　　Table 2.1 DC accuracy test data

　　3. Verify the conclusion

　　The data in the table above show that the DC accuracy of ZCP0030-50 within the measuring range is within 1%rdg±1mV.

　　2. Rise time verification

　　The bandwidth of ZCP0030-50 is 50MHz, corresponding to a rise time of ≤7ns. The probe rise time refers to the time required for the measurement system response waveform to rise from 10% to 90% of the steady-state value when testing a pulse. It is a critical parameter. The faster the rise time, the faster the probe responds, and the faster the current waveform can be measured, and the better the performance of restoring the current waveform.

　　1. Test conditions

　　The test instruments used to test the rise time of the current probe are the signal generator Keysight 33600 and the oscilloscope ZDS 2022. The signal generator generates a square wave with a rise time of 5ns, and the oscilloscope displays the rise waveform and time after passing through the current probe.

　　2. Waveform data

　　Figure 3.1 Rise time of ZCP0030-50

　　As shown in Figure 3.1, the rise time measured by the oscilloscope is 8.5ns. This total time is the rise time of the signal generator itself plus the rise time of the probe. Therefore, the rise time of the probe actually needs to subtract the rise time of the signal generator itself from the total time. The specific calculation formula is as follows.

　　3. Verify the conclusion

　　The measured results are within the theoretical calculation range, the rise time is normal, and the measured bandwidth is above 50MHz.

　　3. Square wave response verification

　　Square wave response is mainly used to test the amplitude-frequency response characteristics of current probes. If the current probe can fully reproduce the square wave waveform without distortion, it means that the amplitude-frequency response of the probe is flat and good.

　　1. Test conditions

　　When testing the square wave response of the current probe, the test instruments used are the signal generator Keysight33600 and the oscilloscope ZDS2022. The signal generator generates a square wave with a peak-to-peak current of 100mA and frequencies of 1KHz, 100KHz, and 1MHz, and the oscilloscope displays the corresponding square wave waveform.

　　2. Waveform data

　　As shown in Figure 3, when testing the 1KHz current square wave, the square wave response of the probe is very good, the waveform is basically not distorted, the waveform is relatively flat, and there is no tilt.

　　Figure 3 1KHz current square wave response

　　As shown in FIG4 , when testing a 100KHz current square wave, the square wave response of the probe is very good, the waveform is basically not distorted, the waveform is relatively flat, and there is no tilt.

　　Figure 4 100KHz current square wave response

　　As shown in Figure 3, when testing a 1MHz current square wave, the waveform of the overall square wave response of the probe is basically not distorted, the waveform is relatively flat, and there is no tilt.

　　Figure 5 100KHz current square wave response

　　As shown in FIG5 , when testing a 1 MHz current square wave, the waveform of the overall square wave response of the probe is basically not distorted, the waveform is relatively flat, and there is no tilt.

　　3. Verify the conclusion

　　The measured results are within the theoretical calculation range, and the amplitude-frequency response is flat.

　　4. Noise Evaluation

　　Noise is an important parameter of current probe. The size of noise directly affects the resolution of the probe. The smaller the noise, the higher the resolution.

　　1. Test conditions

　　The noise test mainly uses the oscilloscope ZDS2022 to observe the test and record the corresponding test values.

　　2. Accuracy data

　　As shown in FIG6 , the peak-to-peak value of the actual noise of the tested current probe is less than 10 mA, and the AC effective value is less than 1 mArms.

　　Figure 6 Noise test

　　3. Verify the conclusion

　　The measured results are within the theoretical calculation range and the noise is well controlled.

　　5. Switching power supply waveform measurement

　　The oscilloscope current probe is mainly used with an oscilloscope to observe the current waveform on the circuit, such as the current waveform on the MOS tube, which is convenient for engineers to debug the circuit and find circuit problems. By measuring the current waveform on the switching tube of the switching power supply, ZCP0030-50 and TCP312 of T company are compared and tested to further verify the performance.

　　Figure 7 Connection diagram of current probe for testing switch current

　　1. Test conditions

　　The main test instruments used in the current waveform comparison test are: ZDS2022 oscilloscope, electronic load, switching power supply, current probe ZCP0030-50 and TCP312.

　　The test steps are as follows:

　　(1) After degaussing and zeroing the current probes ZCP0030-50 and TCP312, clamp the two probes to the wires drawn between the DS poles of the MOS tube, and connect the BNC outputs to Channel 1 and Channel 2 of the oscilloscope respectively. See Figure 8 for details.

　　Figure 8 Instrument connection for switch tube current waveform comparison test

　　(2) Connect the output of the switching power supply to the electronic load and power the switching power supply. The electronic load can be set to no-load or full-load state, and observe the different current waveforms on the MOS tube under two different loads.

　　2. Comparative testing

　　As shown in Figure 9, when the switching power supply is fully loaded, the current probe ZCP0030-50 and TCP312 can both fully measure the current waveform on the MOS tube. Since the bandwidth of TCP312 is 120MHz, the peak-to-peak value difference between the two is about 4%. As shown in Figure 10, when the switching power supply is unloaded, the current waveform measured by the current probe.

　　Figure 9 Current waveform between DS electrodes of MOS tube at full load

　　Figure 10 Current waveform between DS electrodes of MOS tube when no load

　　The switching frequency of the switching power supply is not very high, about 100KHz, but the rise time of the instantaneous change is at the nanosecond level, so a high-bandwidth current probe is required to fully reproduce the details of the current waveform. When the switching power supply is fully loaded, the current waveform is enlarged to observe the waveform details where the current changes rapidly. The waveform details displayed by the two probes are almost the same, as shown in Figure 11. When the switching power supply is unloaded, it is shown in Figure 12.

　　Figure 11 Details of the current waveform between the DS electrodes of the MOS tube at full load

　　Figure 12 Details of the current waveform between the DS electrodes of the MOS tube when no load

　　3. Verify the conclusion

　　For high-frequency noise, the slight difference in the measured results comes from the difference in bandwidth, but for signals with a switching frequency of only a few hundred kHz, the difference in bandwidth has little effect on the experimental results. Here, it is recommended to select a 50M bandwidth.

　　VI. Conclusion

　　Generally speaking, comprehensive measurements of DC accuracy, rise time, square wave response, noise, and current waveform of the switching tube of the switching power supply can relatively completely verify the overall test results.