Oscilloscope Knowledge: Why Use Active Probes?

Active probes offer greater bandwidth and lower input capacitance than passive probes. In this article, we will describe the characteristics of active probes compared to passive probes. We will examine both single-ended and differential probes, and we will also describe the proper use of probe accessories.

Why use active probes?

Passive probes are well suited for measurement applications with bandwidths below 50 MHz. This is because the input capacitance of passive probes is in the 9 or 10 picofarad (pF) range. This loads the device under test. These loading effects increase with increasing frequency. To avoid this loading effect, active probes insert an amplifier between the passive probe's compensated attenuator and the oscilloscope input (Figure 1).

The amplifier buffers the connecting cable, allowing the cable to be terminated into a characteristic impedance of nominally 50 Ω. This isolates the probe from the capacitive loading of the cable and the oscilloscope's input circuitry. The amplifier is designed to minimize input capacitance, nominally 4 pF. The compensated attenuator further reduces this capacitance. To achieve a 10:1 attenuation, the expected input capacitance is approximately 0.4 pF. However, the input protection circuitry and probe tip hardware add additional capacitance.

The Teledyne LeCroy ZS1000 1 GHz single-ended active probe is a typical active probe with 0.9 pF input capacitance and 1 MΩ input resistance.

Figure 1: Simplified schematics of a high impedance passive probe and a single-ended active probe, with the amplifier buffering the connecting cable and oscilloscope input while providing low input capacitance. (Image source: Digi-Key Electronics)

Low input capacitance extends the useful frequency range of an active probe. This can be seen in Figure 2, which compares the input impedance of a 10:1 high impedance passive probe to that of the ZS1000.

Figure 2: Input impedance as a function of frequency for a high impedance passive probe and the ZS1000 single-ended active probe. (Image source: Digi-Key Electronics) The

ZS1000 has an input impedance of 1 MΩ and an input capacitance of 0.9 pF, compared to the passive probe’s 10 MΩ input impedance and 9.5 pF input capacitance. At frequencies above 20 kHz, the ZS1000’s input impedance is much higher, resulting in less signal loading. At 500 MHz, the ZS1000’s input impedance is 354 Ω, compared to the passive probe’s 34 Ω input impedance.

Perhaps the best way to compare is to look at the differences in how the probes respond to fast edges (Figure 3).

Figure 3: Oscilloscope response to fast edges using a 50 Ω direct connect, a passive probe, and a ZS Series active probe. (Image source: Teledyne LeCroy) The

50 Ω direct connect response is used as a reference waveform. The active probe response is almost indistinguishable from the reference waveform. The passive probe response has rounded corners due to the higher input capacitance. Note the measured rise times. The reference waveform has a rise time (parameter readout P1) of 456 picoseconds (ps) and the active probe (P2) has a rise time of 492 picoseconds. The passive probe has a rise time (P3) of 1.8 nanoseconds (ns).

Active probes generally outperform passive probes at the same bandwidth. But it is also important to remember that active probes require power. For this reason, active probes almost always have dedicated connectors for different manufacturers’ oscilloscopes. In the case of the ZS1000 active probe, it has a Teledyne LeCroy ProBus interface for powering the probe from the oscilloscope. The interface allows the probe to be integrated with the oscilloscope so that the probe can be sensed and fully controlled from the oscilloscope's front panel.

Active probes also have a smaller input voltage range than passive probes. This requires special attention to prevent damage to the probe. The ZS1000 probes have an input voltage range of ±8 volts, with a maximum nondestructive voltage of 20 volts. This voltage range is greater than the voltage requirements of any logic level currently in use, making these probes ideal for high-speed logic measurements.

Probe Accessories

The ZS1000 probes come with a variety of accessories (Figure 4). Note that most of the probe tips and ground leads are very small. Smaller physical size means lower capacitance and inductance, which means less loading on the circuit under test. Longer ground leads and micro clips are useful for low-frequency applications, where their added reactance does not affect the measurement.

Figure 4: The ZS1000 1 GHz active probe comes with a host of accessories, including long ground leads for low-frequency signals and a variety of tips that make test points more accessible. (Image source: Teledyne LeCroy) The

standard probe tip is designed for general probing. The pogo pin tip and ground lead provide vertical compliance to ensure effective contact without undue mechanical pressure. The IC tip is insulated except at the very tip to prevent accidental shorting of adjacent IC pins. The curved tip is ideal for probing under adjacent components and for applications where the probe must remain parallel to the board. The square pin adapter carries the signal and ground leads and features a standard 2.54 mm pin spacing. The
ground leads include narrow and wide ground plates. Ground plates offer the advantage of a low inductance ground connection. They are typically used in conjunction with a copper pad. The copper pad has an adhesive backside that adheres to the IC. It can then be soldered directly to the IC ground lead, providing a very low ground inductance connection. The offset ground is designed to connect to the probe ground socket and wrap around the probe. This allows the probe tip and ground to be kept close together while keeping the ground lead very short.

Differential Probes

A differential probe measures the voltage difference between two inputs. While a single-ended probe measures the voltage between a single point and ground, a differential probe measures the voltage between two inputs without ground. This is useful when measurements need to be made on line-side circuits in switch-mode power supplies that are not referenced to ground.

Because a differential probe measures the difference between two inputs, signals common to both inputs, called common-mode signals, are cancelled or have their amplitude significantly reduced. This means that bias levels, noise, and crosstalk that are common to both inputs can be cancelled or at least have their amplitude significantly reduced. A

conceptual block diagram of a differential probe is shown below (Figure 5). The diagram includes a device under test, modeled as a differential source, with common-mode components.

Figure 5: Conceptual diagram of a differential probe and a device under test, where the device under test is modeled as a differential source with a common-mode component. (Image source: Digi-Key Electronics) The

core element of a differential probe is the differential amplifier. The differential amplifier output is the difference between the + and – inputs. In front of the differential amplifier, the circuit looks like two single-ended active probes. As shown, the differential probe inputs are connected to a common differential source, consisting of two differential components, Vp and Vn, and a common-mode source, Vcom. An

ideal differential probe works as follows: The voltage at the upper (+) probe input is Vp + Vcom. The voltage at the lower (-) probe input is – Vn + Vcom. Applying these inputs to the differential amplifier produces an output of Vp + Vn, assuming unity gain. The common-mode signal is now removed.

The degree to which the common-mode signal is attenuated in a differential probe is determined by the common-mode rejection ratio (CMRR). CMRR is the power ratio of the differential gain to the common-mode gain of the differential probe, expressed in decibels (dB). CMRR is usually frequency dependent, decreasing with increasing frequency, and is usually specified at multiple frequencies.

An example of a 1 GHz bandwidth differential probe is the Teledyne LeCroy ZD1000 probe, which has a differential input range of ±8 volts and a CMRR of 60 dB at 60 Hz (Figure 6). This probe is designed for use with Teledyne LeCroy oscilloscopes. It has a differential input resistance of 120 kΩ and a differential input capacitance of less than 1 pF.

Figure 6: ZD1000 differential probe using small IC adapters. These probe tips have insulation on one side to prevent shorting to adjacent IC pins. They also feature low inductance resistor compensation to reduce inductive peaking. (Image source: Teledyne LeCroy)

The ZD1000 also includes several probe tip adapters to meet the needs of many probing applications. Keep in mind that the probing configuration of a differential probe should be symmetrical, using the same adapter for both inputs to achieve the best possible CMRR.

High Voltage Differential Probes

The key advantage of differential probes is that the inputs are not referenced to ground, providing the ability to attenuate common-mode signals. These features can also be very useful when testing switch-mode power devices, where the line side is not referenced to ground. High voltage differential probes, such as the Teledyne LeCroy HVD3106, are suitable for such applications (Figure 7).

Figure 7: The Teledyne LeCroy HVD3106 probe and related accessories are designed for safe high-voltage probing in accordance with the IEC/EN 61010-31:2015 standard. (Image source: Teledyne LeCroy)

The probe has a maximum differential voltage of 1500 volts. This wide voltage range is achieved by using a 500:1 attenuation in front of the differential amplifier. The probe has a CMRR of 85 dB at 60 Hz. In addition, the physical configuration of the probe and its accessories is designed to carefully probe high voltages with safety levels in accordance with the IEC/EN 61010-31:2015 standard.

Conclusion

Active probes offer the advantages of increased bandwidth and reduced probe loading. The value of differential probes is increased ground isolation and reduced common-mode signals. And the proprietary interface fully integrates these probes into the oscilloscope user interface, making installation and operation easier.