A Tektronix differential probe is connected to a nine-volt battery.
Hi and welcome to our 50th Test and Measurement Video. Right now we’ll connect a differential probe to the amazing Tektronix MDO3104 oscilloscope and see some applications for it.
One of the primary advantages of the differential probe is that it permits the user to safely measure voltages where both sides of the circuit consist of voltages that are referenced to ground and float above it. This situation is encountered when probing the DC bus inside an AC motor variable frequency drive (VFD). Not only are positive and negative sides both referenced to ground, but the voltages are substantial. In a 480-volt unit, the DC voltage is about 678 volts, due to the output characteristics of the full-wave rectifier. Specifically, it is 1.414 times the RMS supply voltage.
This measurement is frequently made on a malfunctioning VFD system, as when the motor is overheating or cutting out, in order to see if there is excessive AC ripple.
The reading can be safely made with a hand-held, battery-operated oscilloscope where the leads are fully isolated from ground, but if a grounded bench oscilloscope is being used, the differential probe setup is required.
Similarly, displaying three-phase, line-to-line voltages where none of the legs is grounded is a problem for a bench-type oscilloscope, and here again the differential probe eliminates any possibility of sparks and smoke.
Another important application of the differential probe is that it eliminates any common-mode voltage. That is implicit in the word “differential”. When voltage is applied to the differential probe plus and minus leads, this is known as differential voltage, and its waveform appears in the display when the differential probe is connected to an active analog input channel.
When voltage is applied to either the plus or minus lead of the differential probe and the other side is connected to ground, this is known as a common-mode signal. The differential probe rejects common-mode voltage as long as it does not exceed the common-mode range.
In theory, a differential probe, which is actually a variety of differential amplifier and shares its characteristics, accesses all differential voltages and rejects all common-mode voltages. In reality, however, the amount that a differential probe or for that matter any differential amplifier rejects common-mode voltage is known as the common-mode rejection ratio. This CMRR is equal to the differential mode gain divided by the common-mode gain. Common-mode gain in turn is equal to the output voltage divided by the input voltage when both inputs are driven only by the common-mode signal. Common-mode rejection ratios are best when both input
attenuators are precisely matched. That in part is why differential probes are expensive.
The Tektronix differential probe comes with an assortment of probe tips, permitting the user to access signals from a variety of sources.
After the oscilloscope has booted up and the differential probe is plugged into the active analog channel input, the green Status LED on the probe output should come on, signifying that the TMDPO200 differential probe is able to communicate with the oscilloscope.
Pressing Menu on the differential probe connector causes a number of alternatives to appear in a menu bar to the right of the display. At the top, the user can choose between 75.0-volt and 750-volt ranges, so this value should be set based on the signal that is to be accessed. For circuit boards with small signal voltages, the 75.0-volt level is appropriate. Viewing a 480-volt AC motor variable frequency drive DC bus, the 750-volt range should be chosen. Notice that as the two different ranges are selected, the attenuation level changes: 250X for the 750-volt range and 25X for the 75.0-volt range.
Generally, the other choices should be left as defaults. Be sure the termination is 1 megohm. The 50-ohm termination may allow excessive current to flow in the circuit under investigation.
For most measurements, Full Bandwidth is best. If there is excessive noise, usually with a high-frequency component, the bandwidth can be temporarily set to 20 MHz or 5 MHz in order to get a clear waveform. Don’t forget to set it back when you are finished with this measurement.
While we’re in this menu bar, it’s a good opportunity to see what coupling and invert do. The default coupling is DC. When we press AC coupling, the DC component is removed, as when a capacitor is inserted in series with the signal. In this way, AC ripple in the output of a rectifier is shown
Invert may be turned on or off to reverse the oscilloscope polarity.
Thanks for watching. New videos are added periodically, so check back frequently.
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