The difference between passive probes and active probes

When you buy a mid-range or low-end oscilloscope, it usually comes with a high-impedance passive probe per oscilloscope channel. Passive probes are more rugged and less expensive than active probes. They provide wide dynamic range and bandwidths in excess of 500 MHz when the oscilloscope input is terminated with 1 MΩ impedance.

You may face a dilemma as to whether you need a probe other than a standard passive probe. Considering that a standard passive probe is a general-purpose tool designed for probing connections over a wide range, if you want to measure high-speed signals with fast rise/fall times, an active probe with sufficient bandwidth is recommended. Similarly, differential or current signal measurements are also best made with an active differential or current probe. Therefore, consider an active probe that is specialized or performs better over a small range.

Single-ended, differential or multimode

Oscilloscopes measure signals with reference to a common reference point, ground. The signal measured with an oscilloscope and a standard single-ended probe is equal to the potential difference between a specific point on the device under test and ground.

If you need to measure the voltages on different components in a circuit that are not connected to ground, you need to make differential measurements. For this, you need to use a differential probe. Because the differential amplifier inside a differential probe amplifies the signal difference between the two probe points, it suppresses the voltage common to both points and extracts the potential difference between the two points to the oscilloscope's input.

Differential probes are a common tool for high-bandwidth oscilloscope users, but it is difficult for such probes to display the individual components that make up a differential signal. In order to measure the single-ended and common-mode components of a differential signal, users are in urgent need of an oscilloscope probe with high-speed data signal measurement capabilities.

bandwidth

The bandwidth of an oscilloscope and probe is defined as the frequency response at which the amplitude decreases by 3dB. Most oscilloscopes or probes with bandwidth specifications of 1 GHz or less typically have what is called a Gaussian response, with a slow roll-off starting at about one-third the -3dB frequency. Oscilloscopes with bandwidth specifications exceeding 1 GHz typically have a flat frequency or brick-wall response with a steeper roll-off near the -3dB point.

As a rule of thumb, the probe (or oscilloscope) bandwidth should be 3 to 5 times greater than or equal to the bandwidth of the fast signal on your system to avoid attenuation of the signal frequency components. Therefore, you must first determine what the fast rise or fall time is for your specific signal and deduce the signal bandwidth from that. If you are referencing a specific communications standard, you should be able to find the fast rise/fall time specifications for that standard. Once you have determined the fast rise/fall time, use the following formula to determine the bandwidth of your signal based on the threshold you are using.

Probe loading

When you connect an oscilloscope probe to a circuit, the probe becomes part of the circuit being measured, and the electrical characteristics of the probe affect the response of the entire measurement system and the operation of the device being measured. The probe introduces resistive, capacitive, and inductive loads to the device being measured.

Of these three sources of probe loading, capacitive loading is the most troublesome, affecting bandwidth, rise or fall time, and delay. Capacitive loading often changes the shape of the waveform being measured.

Here is a diagram of a DUT and a simplified electrical model of a probe connected to the DUT. Ideally, Vinput (the voltage at the probe input) should be the same as Vsource (the voltage at the DUT before probing). In reality, as the probe is connected to the DUT, the impedance of the DUT and the probe determines the voltage at the probe input. This is a simple resistor divider circuit.

Probe noise

Many engineers are concerned about the effect of the inherent noise of the probe/oscilloscope on their measurements. There are many factors that affect the noise figure of a measurement. However, one important characteristic to consider is the signal-to-noise ratio. Generally, a probe with a lower attenuation ratio will result in a higher signal-to-noise ratio and lower noise, but it will also result in lower input resistance, lower dynamic range, and lower common-mode range. There are some trade-offs. A simple way to evaluate the amount of probe noise is to check the attenuation ratio and the noise level of the probe as specified in the probe data sheet or manual.

Probe accessories

A measurement system is only as good as its weakest link. The bandwidth of an oscilloscope or probe is always a key specification, but there is more to a measurement system than just an oscilloscope and a probe.

In reality, the oscilloscope is often not the weak link in the measurement system. The measurement system also includes probes, cables, connectors, and fixtures. Each of these elements has the potential to introduce greater bandwidth loss than the oscilloscope. Unlike probes and probe accessories, cables and connectors generally have very low loss.

Probe response correction

Probe response correction has become increasingly common for high bandwidth active probes. A common calibration method for probes is a DC adjustment, which in turn causes the DC gain and offset of the probe to be adjusted. This method has been used in a large number of passive and active probes to properly compensate for the DC gain and offset factors.

As oscilloscope performance increased to the multi-GHz level, DC correction methods were no longer adequate for characterizing and correcting high frequency responses. AC correction refers to a frequency-dependent correction scheme designed to adjust the AC response characteristics of a probe to match those of an ideal probe, with a flat frequency response up to its rated bandwidth (-3dB point). Over the course of the evolution of probes, probe tip accessories, and oscilloscopes, manufacturers have accurately measured the S-parameters of multiple devices, averaged their characteristics, and created correction filters that represent common probe and oscilloscope systems. This form of correction provides a significant improvement in the accuracy of the nominal correction without any additional inconvenience to the oscilloscope user.