Product Insights: Differential Probes

Introduction

A differential probe is an active device used to take measurements of certain circuit elements of electronic devices. They are most often used in conjunction with an oscilloscope. Knowing the basic operation of differential probes is a must-have skill for any compliance engineering professional involved in making accurate, repeatable, and safe measurements. Key features and applications of differential probes are summarized in this article.

Why Are Differential Probes Needed?

Differential probes are essential due to the limitations inherent in single-ended measurements, which we initially learn to perform using standard passive oscilloscope probes.

In single-ended, passive-probe measurements, voltage is measured between the probe tip and ground, where a highly inductive ground lead is used to connect to ground. This ground lead is also connected to the scope’s electrical ground, which is also connected to all measurement channels on the scope and to the electrical ground of the laboratory. This means the point on the circuit under measurement connected to this ground lead is also tied to electrical ground. Undesirable effects from this situation include possible flow of excessive current, incorrect measurement results, damage to circuit or scope, and possibly harm to personnel performing the measurement via electric shock.

Purpose and Operation of Differential Probes

A differential measurement is required when measuring two points in a circuit where neither is ground. A differential probe measures the voltage difference between two points in a circuit. Examples include:

Ungrounded Measurements

Differential probes are essential when making scope measurements that are not ground-referenced. For instance:

• Measuring switch mode power supplies, inverters, and motor drivers where ground-referenced measurements are undesirable.
• Voltage between gate and source of the switching element and the working voltage across the isolation barrier of a switch-mode power supply, where high voltages are present.

Differential Signal Measurements

These probes are used to measure differential signals, common in:

• Audio equipment: Differential audio signals tolerate common mode noise.
• High-speed serial communication lines (e.g., PCIe) where noise immunity matters.
• USB, CAN, and other differentially encoded communication signals where increased immunity to noise is required and information is transmitted by the difference in voltage between two signals.

Other

• Ground of the system that is noisy or unstable (ground bounce).

Differential Measurements with Passive Probes

In a pinch, it is possible to perform some differential-type measurements with two passive probes and a scope. However, there are several drawbacks to this approach.

• Use of scope’s math function required. To obtain the differential measurement, the output of one channel is subtracted from the output of another (Channel 2 minus Channel 1).
• Two scope channels per differential signal are required.
• Requires match set of probes with same:
  • Type
  • Model
  • Manufacturer
  • Cable lengths
  • Ground lengths
  • Ground connections

Differential Measurements with a Differential Probe

Unlike a single-ended probe, which compares a point to ground, a differential probe does not have a highly inductive ground lead. It directly measures the voltage difference between any two points. The two inputs connected are located anywhere on the circuit. One point does not have to be ground.

Inside the differential probe, there is a differential amplifier that preconditions the signal for the oscilloscope, ensuring a usable input. A selectable attenuation inside the probe body produces an output voltage that is the difference between the voltage at both measurement points.

Advantages and Use Cases of Differential Probes

Differential probes focus on the difference between signals, not their individual magnitudes. Common mode (CM) noise (same signal on both inputs) is something we want to avoid in compliance engineering, especially when trying to capture an accurate measurement, as CM noise affects both signals equally, but the voltage difference remains unchanged when using a differential probe. A differential probe’s ability to not respond to CM noise is called CM noise tolerance.

CM Noise Tolerance:

• Differential signals have a tolerance for CM noise.
• CM noise occurs on both lines of a differential signal.
• The differential probe’s amplifier rejects CM noise, ensuring accurate measurements.
• The amount of CM noise the differential probe can reject is called the Common Mode Rejection Ratio (CMRR). Ideally, CMRR is infinite, but it is usually specified between 30 and 80 dB for various practical reasons.

Pro Tip: Differential probes are supplied with relatively long probe leads, which makes them susceptible to CM noise pickup. Lower noise is achieved by twisting the lead wires together, but at the cost of higher capacitive loading. In most situations, higher capacitive loading is not an issue, so make sure to twist the leads together!

Other advantages of differential probes include:

• Only one scope channel is required.
• It is possible to make “single-ended” measurements in addition to differential measurements.
• They are safer to use.

Disadvantages of differential over single-ended probes are few. The basic disadvantages include:

• Cost more.
• Are larger.
• Since they are active devices, they require separate power.

Summary

In summary, differential probes allow precise measurements in scenarios where ground referencing is not feasible, making them a valuable tool for compliance engineers and technicians.

References and Further Reading

1. Ott, Electromagnetic Compatibility Engineering, Wiley, 2009.
2. Smith, D.C., High Frequency Measurements and Noise in Electronic Circuits, 3^rd Edition, Springer, 1993.