Choosing the most appropriate probe for the application is the first step to making reliable measurements. How the probe is used also affects the oscilloscope user’s ability to gain useful measurement results and make accurate measurements. To help users select the perfect probe for their application and make probing better, this article will discuss some of the useful probing tips.
Users should first determine whether they need a passive or an active probe. Passive high-impedance resistor divider probes are ideal for general-purpose mid-to-low-frequency (below 600 MHz) measurements. However, although these inexpensive and rugged tools provide high input resistance, they offer lower bandwidths than low-impedance (z0) active probes or passive probes, while imposing heavier capacitive loading.
On the other hand, active probes are suitable for high-frequency applications (more than 600 MHz) that require precision in a broad frequency range. Despite their limited input voltage, these expensive probes deliver accurate insight of fast signals due to their significantly lower capacitive loading.
Prior to probing a circuit, users should then check the probe loading effect using two oscilloscope probes. This can be achieved by connecting the probe tip to a point in the circuit and attaching the second point to that same point. Ideally, no change would be visible on the signal. Any change observed is caused by the probe loading.
Next, users should check the probe compensation before connecting the probe to an oscilloscope input as it may have been previously adjusted to match a different input. Passive probes generally integrate compensation RC divider networks. Probe compensation refers to the process of adjusting the RC divider to maintain the probe’s attenuation ratio over its rated bandwidth.
It makes sense to employ that feature if the oscilloscope can automatically compensate for the probe’s performance. Otherwise, users should adjust the variable capacitance of the probe using manual compensation.
Oscilloscope users should carefully evaluate the noise characteristics of their device prior to making measurements.
With the advent of mobile phones and various battery-powered devices, engineers nowadays need higher-sensitivity current measurement that will ensure that the current consumption of their device is kept within acceptable limits. The simplest way to make current measurement without breaking the circuit is by using an oscilloscope with a clamp-on current probe. However, this process becomes quite tricky when the current level drop to low milliampere range or below.
The inherent noise of the oscilloscope turns into a real issue as the current level declines, revealing one undesirable characteristic of the oscilloscope — vertical noise. When measuring low-level signals, the noise of the measurement system may undermine the accuracy of the actual signal measurement. Given that oscilloscopes are broadband measurement devices, the higher their bandwidth, the higher their vertical noise will also be. It has to be noted that the oscilloscope’s acquisition memory can affect the noise floor when making low-level measurements.
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