Probing is critical for making quality oscilloscope measurements, and often a probe is the first link in your oscilloscope measurement chain. If probe performance is not adequate for your application, you will see distorted or misleading signals on your oscilloscope. Selecting the right probe for your application is the first step toward making reliable measurements. How you use the probe also affects your ability to make accurate measurements and obtain useful measurement results. In this application note, you will find eight useful hints for selecting the right probe for your application and for making your scope probing better. The following probing tips will help you avoid the most common probing pitfalls.
Table of contents
Hint #1 – Passive or active probe?
Hint #2 – Probe loading check with two probes
Hint #3 – Compensate probe before use
Hint #4 – High sensitivity, wide dynamic range current measurement
Hint #5 – Make safe floating measurements with a differential probe
Hint #6 – Check the common-mode rejection
Hint #7 – Check the probe coupling
Hint #8 – Damp the resonance
Hint 1: Passive or active probe?
For general-purpose mid-to-low-frequency (less than 600-MHz) measurements, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer a wide dynamic range (greater than 300 V) and high input resistance to match a scope’s input impedance. However, they impose heavier capacitive loading and offer lower bandwidths than low-impedance (z0) passive probes or active probes. All in all, high-impedance passive probes are a great choice for general-purpose debugging and troubleshooting on most analog or digital circuits.
For high-frequency applications (greater than 600 MHz) that demand precision across a broad frequency range, active probes are the way to go. They cost more than passive probes and their input voltage is limited, but because of their significantly lower capacitive loading, they give you more accurate insight into fast signals.
A passive probe loads the signal down with its input resistance, inductance, and capacitance (green trace). You probably expect that your oscilloscope probe will not affect the signals in your device under test (DUT). However, in this case, the passive probe does have an effect on the DUT. The probed signal’s rise time becomes 4 ns instead of the expected 600 psec, partly due to the probe’s input impedance, but also due to its limited 500-MHz bandwidth in measuring a 583-MHz signal (0.35/600 psec = 583 MHz). The inductive and capacitive effects of the passive probe also cause overshoot and ripping effects in the probe output (purple trace). Some designers are not concerned about this amount of measurement error. For others, this amount of measurement error is unacceptable.
Hint 2: Probe loading check with two probes
Before probing a circuit, connect your probe tip to a point on your circuit and then connect your second probe to the same point. Ideally, you should see no change in your signal. If you see a change, it is caused by the probe loading.
In an ideal world, a scope probe would be a non-intrusive (having infinite input resistance, zero capacitance, and inductance) wire attached to the circuit of interest and it would provide an exact replica of the signal being measured. But in the real world, the probe becomes part of the measurement and it introduces loading to the circuit.
To check the probe loading effect, first, connect one probe to the circuit under test or a known step signal and the other end to the scope’s input. Watch the trace on the scope screen, save the trace and recall it on the screen so that the trace remains on the screen for comparison. Then, using another probe of the same kind, connect to the same point and see how the original trace changes over the double probing.
You may need to make adjustments to your probing or consider using a probe with lower loading to make a better measurement.
