What Is a Floating Measurement?

Floating measurement is measured between two points, neither of which is at ground potential. This sounds a bit like the differential measurement introduced earlier in the differential probe, and in fact, it is true. Floating measurement is a type of differential measurement, and in fact, differential probes can also be used for floating measurements

However, generally speaking, " floating measurement" are related to power system measurements, such as switching power supplies, motor drive units, ballasts, uninterruptible power supplies, etc., where two measurement points are not grounded ( ground potential) , signal" public" potential may be lifted from ground ( fluctuate) to several hundred volts. These measurements typically require suppression of high common-mode signals to evaluate low-level signals on these signals. External ground currents also add noise in the display, making measurements more difficult

Typical floating measurements are shown in Figure 1 . In this motor drive system, the three-phase AC lines are rectified to up to 600 V floatingDC bus. The control circuit with reference ground generates a pulse modulation gate drive signal, which is transmitted to the bridge transistor via an isolated drive device, causing each output to swing the entire bus voltage at a pulse modulation frequency. To accurately measure the voltage from the gate to the transmitter, bus conversion must be suppressed. In addition, the compact design of the motor drive, rapid current conversion, and proximity to the rotary motor all result in a harsh EMI environment

Additionally, connecting the ground wire of the oscilloscope probe to any part of the motor driver circuit will cause a ground short circuit

The probe isolation device only floats the probe, not the oscilloscope. This probe isolation can be achieved by a transformer or optical coupling mechanism, as shown in Figure 2 . In this example, as in the intended state, the oscilloscope remains grounded, and the differential signal is applied to the tip and reference lead of the isolated probe. The isolation device sends the differential signal to the receiver, which generates a signal at the reference ground level, which is proportional to the differential input signal. In this way, the probe isolation device is compatible with almost any instrument

To meet different needs, the industry offers various types of isolation devices, including multi-channel isolation devices that provide two or more channels and multiple independent reference leads. Additionally, applications where isolation devices need to be physically separated from instruments over a long distance ( such as 100 meters or more) > CMRR. Additionally, the maximum operating voltage is also a key indicator of the isolation system. Generally, the maximum operating voltage is 600 V RMS or 850 V (DC+ peak AC)

dangerous

To avoid this direct ground short, some oscilloscope users have used unsafe practices that cause faults in the oscilloscope ground circuit. This allows the oscilloscope's ground line to float the motor drive circuit, enabling differential measurements. Unfortunately, this approach also causes the oscilloscope chassis to float in potential that may endanger or even electrocute the oscilloscope user

A "floating" oscilloscope is not only an unsafe practice, but noise and other effects usually impair measurement results. As shown in Figure 3a, this illustrates a floating oscilloscope measuring the voltage from one gate to the driver of a motor drive. The trace at the bottom of Figure 3a is the low-side gate driver voltage, and the top trace is the high-side voltage. Notice the obvious ringing on both traces. This ringing is caused by a large parasitic capacitance from the oscilloscope chassis to ground.

Figure 3b shows the result of the same measurement, but this time using a properly grounded oscilloscope, measured through a probe isolator. Not only is the ringing eliminated in this measurement, but the measurement is also much safer because the oscilloscope is no longer floating above the ground voltage.

Reprinted from Tektronix