Active probes: why they are worth buying
Jae-yong Chang, Agilent Technologies - May 22, 2012
When you buy 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 >500 MHz when
connected to 1 MΩ 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 matters
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 MΩ//4 pF and an active probe with 1 MΩ//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 MΩ) 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 Ω 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 (>1.5 GHz). The probe tip typically contains a
resistor, either 450 Ω or 4,950 Ω. The low-impedance resistor divider probe provides either 500 Ω or
5 kΩ input resistance to give 10:1 or 100:1 attenuation with the 50 Ω input of the scope.
The total input impedance at DC or low frequency range is only 500 Ω (10:1) or 5,000 Ω (100:1)
when the probe is terminated into the 50 Ω input of the scope.
For many designers, this probe is often selected as a low-cost alternative to a higher priced active
probe. When you use this probe, however, you should be very careful with the resistive loading
effect because it may alter the measured amplitude of the signal as well as the bias point.
Many open collector or open drain outputs of ICs require the use of an external pull-up or pull-down
resistor to keep the digital output in a defined logic state. To measure the amplitude of a signal with
relatively high source impedance accurately, it is important to use a probe with high input
impedance. Here in the example (see Figure 2), the 100:1 resistor divider probe with 5,000 Ω input
resistance and the active probe with 1 MΩ input resistance are measuring a 5 V I2C serial bus with a
pull-up of 10 kΩ.
Figure 2. To measure the amplitude of the signal having relatively high source impedance
accurately, it is important to note that you use a probe with high input impedance.
The amplitude of the data line signal measured with the resistor divider probe is decreased to 1.65 V
due to the resistive loading of the low impedance probe, while the output measured with an active
probe with 1 MΩ input impedance measures the amplitude correctly at ~5 V (see Figure 3). Notice
that the measurement is somewhat cleaner with the active probe (see Figure 4). This resistor divider
probe is only useful to look at a 50 Ω transmission line or signal with low source impedance (usually
≤50 Ω) to avoid heavy resistive loading.
Figure 3. The amplitude of the signal measured with the 100:1 resistor divider probe is
decreased to 1.65 V due to the resistive loading of the low impedance probe.
Figure 4. The output measured with an active probe with 1 MΩ input impedance makes
the amplitude measurement correctly.
Conclusion
There are some key benefits and trade-offs between passive and active probes, and it is important to
keep these in mind when you choose active probes over standard passive probes with your
oscilloscope. Generally speaking, a passive probe is a safe choice for general purpose probing and
troubleshooting, while for high-frequency applications with lower probe loading, active probes
provide much more accurate insights into measuring fast signals.
About the Author
Jae-yong Chang is the product manager and planner for Agilent’s oscilloscope product line in the
Oscilloscope Products Division based in Colorado Springs, Colorado. He joined HP Korea in 1990 as
a R&D design engineer, and has held various positions in R&D and marketing in HP and Agilent
Technologies. He received his BA and MS degree in Physics from Sogang University, Seoul, Korea.