Floating Measurements

Oscilloscope users often have to make measurements where neither point of the measurement is at ground (earth potential). Signal "common" may be elevated to hundreds of volts from ground. In addition, many of these measurements require the rejection of high common mode signals in order to evaluate low-level signals riding on them. Extraneous ground currents can also add hum (noise) to the display. Too often, these problems force users into using dangerous measurement techniques.

Fortunately, there are solutions available which are not only safe but much more accurate than the extremely dangerous and unacceptable procedure of floating your oscilloscope. These fall into general categories: differential probes, isolated input oscilloscopes, isolation transformers and isolation amplifiers. Tektronix provides both differential probes and Isolated Input Oscilloscopes. For safe and accurate operation, an oscilloscope must be referenced to ground. "Floating" a scope by defeating the scope's protective grounding system not only presents a danger to the operator because of high voltage shock hazard on the entire scope, but also stresses the scope's power transformer insulation. Measurement accuracy is also degraded since the capacitance of the scope chassis is now connected to your circuit.

Differential probes and isolated input oscilloscopes enable safe measurements of floating voltages or signals that are not referenced to ground. They are particularly useful for measuring signals in line-connected power electronics circuits such as switching supplies, motor drives, ballasts and uninterruptible power sources.

Even when a measurement is ground referenced a differential measurement solution may be necessary for eliminating ground loops or ground bounce problems.

Differential Probes

Differential Probe systems compare the voltage levels at two different points in the circuit and provide the difference as an output. They will typically have better CMRR performance at higher frequencies than voltage isolators. The P6246/P6247/P6248/P6330 and ADA400A w/1103 and P5200 are probe systems that enable differential measurements to be displayed by Tektronix TDS and most other oscilloscopes. The P6246, P6247, P6248, P6330, P5205, P5210 and ADA400A work with the TEKPROBE interface. The P6246/P6247/P6248/P6330 are well suited for fast, lower amplitude signals, while the P5200/P5205/P5210 handle slower signals with higher voltage amplitudes. The ADA400A Differential Preamplifier provides the capability to display low frequency, very low amplitude differential signals even in high noise environments.

Isolated Input Oscilloscopes

The 100 MHz THS700 series of two channel handheld digital storage oscilloscopes enables fully isolated measurements. With the P5102, 10X, 100 MHz passive voltage probe, fast signals up to 1000 V_RMS CAT II can be measured. And is floatable up to 600 V_RMS CAT II or 300 V_RMS CAT III (reference lead to earth ground).

The 100/200 MHz TPS2000 series two and four isolated channel digital storage oscilloscopes enable fully isolated measurements. With the P5120, 20X, 200 MHz passive voltage probe, signals up to 1000 V_RMS CAT II can be measured. And is floatable up to 600 V_RMS CAT II or 300 V_RMS CAT III (reference lead to earth ground).

WARNING: For safe operation, do not use the P5200 High Voltage Differential Probe with oscilloscopes that have floating inputs (isolated inputs), such as the Tektronix TPS2000 series oscilloscopes and THS700 series oscilloscopes. The P5200 High Voltage Differential Probe requires an oscilloscope or other measurement instrument with grounded inputs.

What is CMRR (Common Mode Rejection Ratio) and Why is it Important?

Differential measurements, simply stated, show you the difference in levels between two test points. Specifically, a floating measurement is a differential measurement that is not referenced to ground. To accurately make this measurement, the measurement system must reject the signal that is common to both test points and display the voltage difference. CMRR defines the ability of a measurement system to accurately resolve difference measurements in the presence of common-mode signals. CMRR stands for Common Mode Rejection Ratio. CMRR serves as a figure of merit for a differential amplifier defined by: CMRR = |Ad / Ac|, where Ad is the voltage gain for the difference signal. Ac is the voltage gain for the common-mode signal. Ideally, Ad should be large, while Ac should equal zero. Since a perfect differential amplifier has a CMRR equal to infinity, the higher the CMRR for the differential measurement system, the closer it is to the ideal. For example, with a system CMRR of 10,000:1, a common-mode input of 5 V will result in an output of 0.5 mV. (5 V/0.5 mV = 10,000:1 CMRR). CMRR is also expressed in dB with the relationship of: CMRR dB = 20 log CMRR. Hence, a CMRR of 10,000:1 would be 80 dB.

CMRR is degraded by an increase in signal frequency, differences in source impedance, inappropriate ground connections and mismatch in probes and scope input amplifiers. Since common mode signals in switching circuits contain significant high frequency components, CMRR at higher frequencies is critical for making accurate measurements. With a CMRR of 10,000:1 (80 dB), if our common-mode signal is 50V, we will be able to distinguish voltage differences > 5 mV. With a CMRR of 100:1 (40 dB), however, we would only be able to resolve differences > 500 mV.

CMRR dB vs Ratio