Floating and Differential Voltage Challenges and Solutions in Oscilloscope Measurements

Fundamentally, an oscilloscope is an instrument that measures voltage (on the Y-axis) versus time (on the X-axis), and all voltage measurements are made between two points. When designing an oscilloscope, most designers expect one of these two points to be firmly grounded. As a result, the oscilloscope's probe reference leads are electrically connected to the housing of the BNC input, then to the chassis, and ultimately to ground through the power supply lead.

However, this design creates a problem often encountered by oscilloscope users: many voltage measurements are not referenced to ground. Differential and floating voltages are ubiquitous in real-world applications, and this creates a significant problem for the user.

When a voltage is not directly referenced to ground, we call it a “floating voltage”. Similarly, when the voltage is measured between two points, neither of which is grounded, we call this “differential voltage”. In addition, signals are usually transmitted differentially over two lines, one line carrying the opposite signal of the other, which allows the receiver to effectively suppress any common mode noise.

For example, the differential signal of a CAN bus is the difference between the CAN high level and the CAN low level, and this differential transmission method provides excellent suppression of common mode noise. In addition to this, switched-mode power supplies, three-phase power supplies, current-sensing resistors, and biosignals can all experience problems with floating or differential signals. If the ground lead is incorrectly introduced in such signal measurements, it may lead to signal distortion in the less severe cases, and may even damage the device under test in the more severe cases.

There are several possible solutions to the problem of floating measurements. However, it is not advisable to disconnect the ground lead directly at the power plug, as this will leave the chassis and all BNC inputs unprotected from ground and increase the risk of electric shock.

A safer and commonly used method is to use two probes for pseudo-differential measurements. Specifically, two probes are used to measure two separate points, then invert the signal from one channel and mathematically subtract the signal from one channel from the other. In this way, we can display only the differential signal while ensuring the safety of the measurement. However, there are some limitations to this method, such as the need for two channels and a math channel to produce a trace, and any imbalance between the probes can lead to skew between the channels and a low common mode rejection ratio (CMRR).

Therefore, a more elegant solution for measurement scenarios that require high accuracy and safety is to use an active differential voltage probe. This probe offers the advantages of true differential measurements, high CMRR, and the need for only one oscilloscope channel per measurement. It can effectively solve the problem of floating and differential voltage measurement and improve the accuracy and safety of the measurement.