Understanding, Selecting, and Using Passive Oscilloscope Probes

Editor’s Note: Probes connect the input of the oscilloscope to test points on the device under test (DUT). There are many types, including: high impedance passive, low capacitance, single-ended active, differential active, high voltage, and current probes. This is the first in a series of three articles on probe selection and application, and will focus on passive probes. Part 2 and part 3 will address active probes and current probes, respectively.

Passive probes offer a good way of connecting an oscilloscope to the device or circuit under test. They are low cost, reliable, and offer reasonable signal integrity when used knowledgably. This article will explore passive probes from the theory of operation through basic adjustments and use. The characteristics of passive probes that affect proper measurement are presented with an eye toward understanding the most effective application of these devices.

Oscilloscopes commonly offer either a 50 W or 1 MW input termination. The 50 W termination is generally used with matching coaxial cables to connect to circuit elements with 50 W sourcing. This results in a high-quality interconnection with minimum signal distortion. Use the 1 MW input termination to connect to circuits with a higher source impedance. This connection can be made in a couple of ways, directly using a cable or an X1 probe, or by using a high impedance probe (Figure 1).

Figure 1: Simplified circuit comparing a direct connection (a) and the use of a high impedance probe (b) to connect a signal to an oscilloscope’s 1 MW input. (Image source: DigiKey)

The 1 MW input also includes a shunt capacitance of from 15 to 25 pF. With a direct connection, the unmatched cable adds an additional 10 to 30 picofarads (pF) per foot of cable length. In a typical situation with a three-foot-long cable, the load at the probe end of the cable is 1 MW with a parallel capacitance of approximately 90 pF (Figure 1a). For low-frequency measurements, the capacitive loading is negligible. For instance, at 1 kHz the capacitive reactance is about 1.8 MW. However, for higher frequency signals, the effects can be very bad. At 100 MHz, the capacitive reactance is reduced to about 18 W, which will attenuate the signal significantly.

The effects of the input capacitance of the oscilloscope and that of the connecting cable can be reduced as shown in the high impedance probe (Figure 1b). This probe is basically a compensated attenuator. The input resistor, nominally 9 MW, forms a 10:1 attenuator with the 1 MW input termination of the scope. The capacitors C_in and C_comp are used to compensate the attenuator and form an all pass network. Compensation is ideal when the RC products of C_in and R_in equal those of R_o and the sum of the cable capacitance C_in and C_comp. C_comp is used to adjust compensation. The capacitance at the input is dominated by C_in, which is about one tenth that of the sum of the other capacitors in the circuit. For this, example it will be about 10 pF.

High impedance passive probes

Almost every major oscilloscope supplier includes a set of high impedance probes with their instruments. Teledyne LeCroy’s HDO4104A four channel, 1 GHz oscilloscope comes with four PP018-1 probes. These are 10:1 high impedance passive probes with a bandwidth of 500 MHz and an input capacitance of 10 pF. The probes are designed to handle input voltages of at least 350 Vrms.

Figure 2: The PP018 high impedance probe has a bandwidth of 500 MHz and an input capacitance of 10 pF. It is shown here along with supplied accessories. (Image source: Teledyne LeCroy)

Most passive probes use an attenuation sense pin that tells the oscilloscope to scale the waveforms automatically, requiring no user input.

Low frequency compensation of a high impedance probe

High impedance probes are matched to the channel they are connected to by the process of low frequency compensation. For this process, all oscilloscopes provide a low-frequency square wave, typically 1 kHz, usually called the CAL output. To make use of this feature, first connect the probe to the desired channel and then connect the probe tip to the CAL output. Trigger the scope and view the selected channel trace on the screen. Use the adjustment tool to vary the compensation adjustment in the probe connector box to obtain a square corner on the square wave trace as shown in the center trace (Figure 3).

Figure 3: Low frequency compensate the probe by adjusting the compensation adjustment to obtain a square corner on the CAL square wave, as shown in the center trace. (Image source: DigiKey)

Compensation should be done whenever the probe is connected to a different channel, and especially before any critical measurement. Many high impedance probes also include a high frequency compensation adjustment. This adjustment does not commonly have to be performed. The probe manual will supply details of this test.

Smart probing

Applying high impedance probes correctly requires attention to fundamentals so that they don’t distort the measured waveform. For example, how will the input capacitance of the probe affect the measurement?

To find out, compute the capacitive reactance (1/2πfC_in) for the probe at the highest frequency component of the signal. Does the circuit under test support that load? If it does go ahead. If not, look for a different probing solution, such as an active probe (Part 2 of this series). A good rule of thumb is to restrict the use of high impedance probes to signals under 25 MHz. The probe manual will commonly provide a graph of the probe input impedance versus frequency to help assess the usefulness of the probe at any given frequency.

Probe accessories can also lead to problems, again mostly at high frequencies. A case in point is the ground lead inductance. The ground lead shown in Figure 2 is about 4.3 inches (11 cm) long. It has significant inductance. When the probe is connected, any voltage across that inductance will be in series with the signal. It pays to keep the ground path length as short as possible. To that end, there are a number of accessories included with the probe. The probe tip ground and the BNC adaptor are included for this purpose. Figure 4 compares the results of using different ground accessories to measure a step signal with a 3 nanosecond (ns) rise time.

Figure 4: Showing the effects of ground lead inductance on a signal: it pays to keep the ground lead as short as possible as it presents an inductance in series with the signal. (Image source: DigiKey)

The yellow trace in Figure 4 is the signal coming from the generator measured using the 50 W input termination, it serves as a reference to signal quality. The red trace shows the result of using the 11 cm ground lead. High-frequency components from the signal developed across the lead inductance have caused the observed overshoot. The probe tip ground and BNC adaptor have about the same response with a much smaller overshoot because the ground path length is much shorter with respectively lower series inductance.

As mentioned, this effect only occurs where the signal has significant high-frequency content. If the same measurement were made with a sine wave, the difference would not be at all apparent. These are effects to keep in mind when using a probe.

Probe accessories and their purpose

Table 1 lists the accessories supplied with the PP018-1-ND probe and their purpose.

Table 1: Probe accessories supplied with the PP018 probes and their purpose (refer to Figure 2). (Information source: DigiKey).

Selecting alternative probes for an oscilloscope

Sometimes, the measurement application may require a different oscilloscope probe. For instance, a test of a power supply requires both a direct connection for ripple measurement, and an x10 high impedance probe for measuring the voltage rails. Having to change between two probes is time consuming, but DigiKey lists several x1/x10 switchable probes. This means no need to change probes, but how is an appropriate substitute specified?

The first step is to determine the bandwidth required for the measurement. In this example a probe bandwidth of under 100 MHz would do. Check the maximum voltage rating of the probe to insure that it meets the measurement requirements. Finally, make sure that the oscilloscope’s input capacitance is within the compensation range of the probe x10 specification.

A good probe to go with the 15 pF input capacitance of the HDO 4104A oscilloscope is the SP300B x1/x10 switchable probe with a 300 MHz bandwidth, 300 volts maximum input, and a compensation range of 10 to 35 pF.

Conclusion

High impedance passive probes, applied with fundamental knowledge of test issues and techniques and some experience, are a good general purpose tool for connecting an oscilloscope to a test circuit. Keep in mind that they are not the only solution to probing issues, but they are a cost-effective starting point.