The Common-Mode Rejection Ratio of Differential Oscilloscope Probes

Key Takeaways

•  A differential probe displays the difference between two voltage inputs.

• Differential amplifiers are placed between oscilloscopes and differential probes as a signal conditioning preamplifier.

• Typical values of differential probe common-mode rejection ratios are good at low frequencies and deteriorate with an increase in frequency. 

Oscilloscopes are used to graphically view electric signals and how they vary over time

Oscilloscopes are a key component of any electronic engineer’s workbench. Oscilloscopes are used to graphically view electric signals and how they vary over time. One method used by engineers to confirm the complete functionality of a circuit is to compare expected signals with signals viewed in oscilloscopes at various test points. At these test points, oscilloscope probes are connected to get signals to be displayed on the oscilloscope screen. Test points can be a voltage point and a ground referring to single-ended signaling or differential signaling with two voltage points, where neither one is ground. Due to the inadequacy of common-mode noise rejection in single-ended measurements, low-level signals are processed differentially. The high common-mode rejection ratio (CMRR) of differential probes allow them to achieve noise rejection, especially at higher frequencies. Let’s explore how high CMRR is achieved in differential oscilloscope probes.

Differential Oscilloscope Probes

When both leads of the oscilloscope probe are not connected to the circuit ground, such a probe is called a differential probe. In differential probes, the two leads are connected to differential inputs, otherwise known as floating inputs. As no ground is connected to the differential probe, there are no grounding issues associated with it. The differential probe displays the difference between the two voltage inputs, and they appear as floating relative to the scope’s ground. The probe’s output voltage--difference voltage--allows it to drive the ground-referenced inputs of the oscilloscopes.

Differential probes acquire the difference voltage between the two test points where the probe ends are connected. As they are optimized for difference voltage, common-mode signals in the two inputs are always rejected by differential probes. The differential probes employ a differential amplifier inside the probe body to amplify the difference voltage. From the differential amplifier of the differential probe, only one analog signal path extends to the oscilloscope channel. This reduces the need for signal path matching. Compared to earlier designs, circuit miniaturization has made differential amplifier circuits so compact that they fit well inside the probe body.

Let’s look at how the differential amplifier in the differential probes helps in common-mode noise rejection. 

Differential Amplifiers and Common-Mode Noise Rejection in Differential Oscilloscope Probes 

Differential amplifiers are placed between oscilloscopes and differential probes as signal conditioning preamplifiers. A differential amplifier helps to achieve common-mode noise rejection, which means the issue of common-mode signals in the two input voltages of the probes reaching the oscilloscope channels is eliminated.

Common-mode voltages are voltages that appear common to both sides of the differential probe inputs. There are several sources from which common-mode noise can get mixed with differential probe voltage inputs. Some of those sources are:

• Radiated signals coupling to the differential probe voltage inputs.
• An offset from the signal caused by driver circuits.
• The difference in ground reference between the differential probe inputs. 

Irrespective of the common-mode noise source, it is necessary to remove common-mode noise from oscilloscope inputs to precisely measure signals. 

An ideal differential amplifier eliminates all common-mode signals present in differential probe pairs. When measuring low-level signals using differential probes, it is important to know how efficient the differential amplifier is in common-mode noise rejection. The measure of the ability of the differential amplifier to eliminate common-mode voltages is called the common-mode rejection ratio or CMRR.

The Common-Mode Rejection Ratio of Differential Amplifiers in Differential Oscilloscope Probes 

The common-mode rejection ratio is the qualifier that specifies how much attenuation the differential amplifier can exhibit for signals that are common to both inputs of the differential amplifier.

As the differential amplifier gives difference voltages as output, an ideal differential amplifier eliminates common-mode voltages. However, the practical differential amplification is not perfect, and hence the error is specified in terms of CMRR. A real differential amplifier gives some small output voltage for common voltages, and the CMRR for such can be expressed as:

The CMRR of differential probes is expressed in dB. The CMRR values of differential amplifiers are dependent on the frequency of the input signals. Typical values of the differential probe CMRR are good at low frequencies and deteriorate with an increase in frequency. The new, high-performing differential amplifiers maintain high CMRR and provide the best noise-immunity throughout the widest differential voltage range.

To improve the measurement capabilities of oscilloscopes in noisy and high common-mode environments, high common-mode rejection ratio differential probes are required. When a differential probe offers features such as high CMRR, low probe noise, and high offset capability, it eases the measurement of low-level signals in high common-mode environments using an oscilloscope.

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