IEEE Power Electronics Magazine - March 2018 - 39

differential amplifier-probe pair or
differential probe contains a difprobe. In addition, the maximum CM
ferential amplifier that is physically
There are practical
voltage is limited only by the breakpositioned near the probe tips,
design and perfordown of the fiber-optic isolation barwith two matched high-voltage
rier and can be extremely high.
attenuators between the differenmance implications
tial amplifier and tips/leads. The
for a differential
two attenuators must be precisely
Input Impedance
matched to achieve a high CMRR.
All probes add a load to the test ciramplifier-probe pair
In contrast to the differential amplificuit, potentially altering the characcombination versus
er-probe pair combination, the tips/
teristics of the specific waveform. A
leads are part of the overall probe
probe with high input impedance
an active, differential
design and are typically shorter
adds less load and draws less current
probe.
than those of the probes connected
from the circuit. However, at higher
to a discrete differential amplifier.
dc bus voltages, the probe loading
As a result, the frequency response
could draw significant amounts of
of both the lead and tip is more consistent and has less
current from the circuit when the signal being measured is
impact on CMRR performance.
at a low voltage compared with the "floating" dc bus voltIn contrast, a high-voltage, fiber-optic isolated probe
age. This is because both the positive and negative leads
contains an input attenuator that affects only the difference
are tied to ground through high attenuation, as shown in
between the two inputs. Therefore, the attenuation value is
Figure 3. A probe with fiber-optic isolation has essentially
much lower. Furthermore, one of these inputs is connected
infinite impedance to ground and presents only a small
directly to the amplifier's reference or floating ground
impedance between the two inputs (Figure 4). This impedpotential. Because the amplifier's reference has the same
ance could be smaller than that presented by a convenpotential as the CM voltage of the signal being measured,
tional high-attenuation, high-voltage differential probe.
the probe attenuation is lower, and there is no CM voltage
Because the signal being measured is at a low voltage, the
across the attenuator.
net probe loading on the circuit will be less for the fiberThe CMRR performance of a fiber-optic isolated
optic probe.
probe is a more complex mechanism and depends on the
Figure 5 depicts an example of how a probe's input
parasitic capacitance to the device under test (DUT) as
impedance affects the measurement. The yellow waveform
well as the unbalanced input circuit of the probe. But, in
is an upper-side gate drive signal measured with a highgeneral, the CMRR of a high-voltage, fiber-optic isolated
quality, high-voltage differential probe. The magenta trace
probe surpasses that of a high-voltage, high-attenuation
is the same signal measured with a high-voltage, fiber-optic

■■A

RSENSE (kΩ)

RSENSE (kΩ)
dc + 5 V

dc + 5 V
VS
dc

RLEAD (MΩ)
CLEAD (pF)

ILOAD

VCM

ILOAD

To Circuit

ILOAD

To Circuit

dc
RLEAD (MΩ)
CLEAD (pF)

50-200x
Attenuation

2-40x
Attenuation
HVFO Amplifier/
Modulating
Transmitter

RLEAD (MΩ)
CLEAD (pF)
VCM

Conventional
High Attenuation
HVD Probe
HVFO
Signal
Demodulating ~1-5 V
Receiver

Signal ~1-5 V
0V

FIG 3 A measurement system with a conventional probe or
amplifier. HVD: high-voltage differential.

FIG 4 A measurement system with a high-voltage, optically
isolated probe. HVFO: high-voltage fiber optically.

March 2018

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2018