![]() ![]() do not measure or display) any common mode signal. Differential probes, because they are optimized to acquire only the differential signal between two test points, always reject (i.e. One of them is to use differential probes as opposed to a standard passive probe. There are several strategies for avoiding the hazard. Similarly, it arises in switching power supplies and the dc bus in a variable-frequency motor drive. This problematic situation arises when the user attempts to measure floating voltages in a three-phase Y configuration or in either of the two ungrounded three-phase Delta electrical conductors. This fault current flows not when the probe tip contacts a floating voltage (that is OK) but when the ground return lead contacts a floating voltage, regardless of whether the probe tip is touching anything and regardless of whether the oscilloscope is powered up. There will also be fault current through the power cord’s green grounding conductor, then the branch circuit’s bare grounding conductor to the neutral bar in the entrance panel at the electrical service. There will be a powerful fault current flowing through the ground return lead into the oscilloscope. This cannot be done using a single-ended probe. Both are referenced to but float above the electrical system ground potential. Let us say you want to measure and display in a standard bench-type oscilloscope the voltage between two terminals. Of course, differential probes are also active devices because they contain semiconductors, but the overall design, configuration with respect to the oscilloscope and ultimate purpose are entirely different. In this way, an active probe with ☒.5-V dynamic range can read zero to 5 Vdc. The user may introduce an offset voltage to augment the limited dynamic range by adjusting the center point. In addition to 1 MΩ impedance as seen by the circuit under test, the capacitive reactance is often less than 1 pF. To limit electromagnetic interference, the ground return lead may be screened. They can be damaged by probing above rated voltage and also by electrostatic discharge. Their dynamic range is lower, typically three to eight volts. When you plug the active probe cable connector into the analog channel input port, the oscilloscope detects the active probe type and characteristics and, depending on the manufacturer, it may permit the user to refine measurement and display.Īctive probes differ from passive probes in other respects. Besides amplification, there is signal filtering and also the capability for varying amounts of automation between probe and oscilloscope. In the active probe body, there are additional components. Drawing virtually no current it has a little discernable effect on the circuit that is probed. The device has high input impedance because of insulated gate technology. But at 600 MHz plus, it may be necessary, depending on the amount of accuracy demanded by the application, to bring out the far more expensive active probe.Īctive probes succeed in practically eliminating reactive loading of the circuit under investigation by means of a small solid-state amplifier in the probe body adjacent to the tip. Passive probes, in general, are satisfactory for debugging and troubleshooting many circuits. (Even a partial turn at high frequency is, in effect, a fractional-turn coil.) By the same token, an active probe can tolerate a longer ground return lead where necessary. Inductive loading, also heavier in the passive probe, can be partially mitigated by using a shorter ground return lead and keeping it as straight as possible. The 10:1 passive probe offers less bandwidth and imposes heavier capacitive loading than an active probe. At dc to the mid-megahertz range, impedance consists primarily of resistance, but as frequency rises, capacitive and inductive reactance play an increasingly prominent role in signal attenuation. A mismatch gives rise to reflections, collisions and loss of data. Impedance matching from the probe tip to channel input port is critical. It is appropriate when the frequency of the signal under investigation is less than 600 MHz. ![]() By far the most used oscilloscope probe is the passive 10:1 attenuation probe.
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