Active probes: why they are worth buying

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.