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Why active probes are worth buying

Posted: 07 Jun 2012 ?? ?Print Version ?Bookmark and Share

Keywords:oscilloscope? probe? active? passive?

When you purchase a low- to mid-range oscilloscope, it usually comes standard with a high-impedance passive probe per oscilloscope channel. Compared to active probes, passive probes are more rugged and less expensive. They offer a wide dynamic range and bandwidth as high as >500MHz when connected to 1 M? input of the oscilloscope. Although active probes are more expensive than passive ones, they offer a superior level of performance that may be essential in certain circumstances. In fact, they are usually the probe of choice when users need high bandwidth, high signal fidelity performance.

Probe loading concerns
Clearly, the primary benefit of an active probe over a passive one is higher bandwidth. However, there are other important characteristics of the probes that need to be considered, especially when you're measuring high-speed signals. The primary consideration should be probe loading.

The issue with probe loading is that when you attach a probe to the target system in order to make a measurement, the probe becomes a part of the circuit, and it introduces loading to the circuit. This causes the scope to make a different measurement, and the deviation depends on how much the probe loads the circuit. Therefore, the less loading there is, the fewer adverse effects a probe has on the signal, or the less that it distorts or changes the target signal. In general, active probes provide less loading effects at high bandwidth ranges than passive probes. However, it is impossible to totally eliminate the loading effect of a probe, regardless of whether it's an active or a passive one.

The example below is a comparison between the input impedance characteristics of a general purpose passive probe with 10 M?//4 pF and an active probe with 1 M?//1 pF. Input impedance is used to describe the loading effects of a probe. At DC and low frequency ranges, the probe's resistive component is the main factor that loads down the circuit under test. However, as the frequency goes up, the capacitance of the probe tip in parallel with the DC resistance starts to reduce the input impedance of the probe, resulting in greater loading and a more adverse effect to the target. Although this 10:1 passive probe comes with higher input impedance (10 M?) at low frequency ranges, input loading characteristics of the active probe are usually better at high frequencies because of lower input capacitance.

A probe's data sheet usually shows the input resistance and capacitance as a single number, so be sure to look for the input impedance characteristics plot of the probe from the probe manufacturer as well. Because the input impedance is not a constant number and drops over signal frequency, you may want to choose a probe that gives you the highest input impedance possible at your typical target frequency.

Figure 1: Input impedance vs. frequency characteristics of a typical active probe and a passive probe; an active probe provides higher input impedance at higher frequencies.

Differential probe benefits
To reduce power consumptions, today's designs are using smaller voltage signals. Since these small voltage levels are susceptible to noise and electromagnetic interference, designers are frequently choosing to use differential signals. The best way to make a measurement on small differential signals is to use a differential active probe. Also, a high-voltage differential probe is a tool of choice when it comes to measuring high-voltage floating signals commonly found in power supplies or motor drives.

A differential probe uses a differential amplifier to subtract two input signals, resulting in one differential signal for measurement by one channel of the scope. This provides a significantly high common mode rejection (CMRR) performance as compared to a single-ended active probe or passive probe. Also, differential probes provide better signal integrity due to very low impedance grounding and higher input impedance. Since the effective ground plane between the signal connections in differential probes is more ideal than most of the ground connections in single-ended probes, differential probes can make better and more repeatable measurements on single-ended signals than single-ended probes can.

Z0 passive probe
One type of passive probe is a low-impedance resistor divider probe, also known as a 50 ? passive probe or Z0 passive probe. At the cost of resistive loading, this probe offers a deceivingly very low input capacitance (~2 pF or less) and high bandwidth (>1.5GHz). The probe tip typically contains a resistor, either 450 ? or 4,950 ?. The low-impedance resistor divider probe provides either 500 ? or 5 k? input resistance to give 10:1 or 100:1 attenuation with the 50 ? input of the scope.

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