Tektronix IsoVu User Manual
Introduction
This white paper describes the optically
isolated measurement system
architecture trademarked IsoVu™.
IsoVu offers complete galvanic isolation
and is the industry’s first measurement
solution capable of accurately resolving
high bandwidth, low voltage differential
signals in the presence of large
common mode voltages. The stand out
feature of IsoVu™ is its best in class
common mode rejection across the
entire bandwidth.
Figure 1: IsoVu Measurement System
Measurement Pain Points and Sources of Error
Voltage is the difference of electrical potential between two
points in a circuit, and any voltage measurement between two
points is considered a differential voltage measurement. Even
when using a passive probe with a ground clip it is considered
differential because it’s measuring the difference in potential
between the probe tip and ground. However, the “ground”
potential at the oscilloscope’s input BNC connector is not
necessarily the same as the “ground” in the circuit being
measured. In other words, "ground is not ground”.
Figure 2: Differential Measurement
IsoVu™ Optically Isolated
DC - 1 GHz Measurement System
Offers >120 dB CMRR with 2kV
Common Mode Range
The target audience for this paper includes anyone making the following measurements:
Differential measurements in the following conditions:
o Complete galvanic isolation is required
o High common mode voltage
o High frequency common mode interference
Measurements in high EMI environments
EMI compliance testing
ESD testing
Remote measurements up to 10 meters away from the device under test without loss in
measurement performance
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IsoVu™ Optically Isolated DC - 1 GHz Measurement System Offers
>120 dB CMRR with 2kV Common Mode Range
The objective of the differential measurement is to get a perfect representation of V
Figure 2. Unfortunately, the perfect representation of V
is not possible due to some of the following
A-B
causes:
Common mode voltage or common mode interference
Radiated emissions
Ground loops
Measurement system inaccuracy
o Insufficient bandwidth
o Insufficient common mode rejection capability
o Insufficient sensitivity
The Common Mode Problem
Users making differential measurements with a traditional differential probe often fail to realize the tradeoffs and limitations when measuring in an environment where common mode voltage or common mode
interference is present. The capabilities of today’s differential probes derate over frequency for the
following specifications:
Common Mode Voltage (CMV)
Common Mode Rejection Ratio (CMRR)
Common Mode Loading
as shown in
A-B
Voltage Derating over Frequency
All differential probes have a common mode voltage rating with some probes specifying a common mode
voltage range of thousands of volts. However, the listed specification is generally true only at DC and low
frequencies. The probe’s common mode voltage capability is derated as the frequency of the signal
increases, which severely limits the common mode voltage capability at higher frequencies.
An example of this derating is the common mode voltage plot of Keysight’s N2890A 100 MHz high
voltage differential probe shown in Figure 3. Although the voltage rating of the probe is
1 kV
of 20 V
probe such as this one with a 1 kV
misunderstanding can result in measurement inaccuracy and damaged equipment.
at low frequencies, the probe’s capabilities start to roll off at 2 MHz and this probe is only capable
rms
at 100 MHz. This is a limitation that is rarely understood and users often fail to realize that a
rms
rating is only capable of 20 V
rms
at maximum bandwidth. This
rms
Figure 3: Common Mode Voltage Derating Plot for Keysight N2890A Differential Probe
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IsoVu™ Optically Isolated DC - 1 GHz Measurement System Offers
>120 dB CMRR with 2kV Common Mode Range
CMRR Derating over Frequency
Common-mode rejection ratio (CMRR) is a differential probe’s ability to reject any signal that is common
to both test points in a differential measurement (VA – VB). CMRR is a key figure of merit for differential
probes and amplifiers, and it is defined by:
CMRR = | A
where:
A
= the voltage gain for the difference signal
Diff
ACM = the voltage gain for common-mode signal
Ideally, A
at least -80 dB (10,000:1) is considered quite good. An amplifier that has long been considered best in
class is the LeCroy DA1855A. In Figure 4, the DA1855A’s CMRR exceeds the -80 dB level at low
frequencies up to a few MHz. However, the CMRR capability of this amplifier quickly derates and is only
capable of a mere -20 dB or 10:1 at 100 MHz. What this means is that a common-mode input signal of
10 volts at 100 MHz will induce a 1 V error signal in the differential measurement. It should be noted that
the plot in Figure 4 is for the amplifier only. When using “matched” probes with the amplifier the
performance is further degraded.
/ ACM |
Diff
should be large and ACM would be zero, resulting in an infinite CMRR. In practice, a CMRR of
Diff
Figure 4: CMRR Plot for LeCroy DA1855A Differential Amplifier
Common Mode Loading Derating over Frequency
Although the common mode DC input impedance of a typical probe can be very high, as the signal
frequency increases the common mode impedance is dominated by the common mode capacitance to
ground. At higher frequencies, the capacitive loading becomes a matter of increasing concern by
distorting the waveform and increasing the load on the DUT. As shown in Figure 5, the Keysight N2819A
probe has a large common mode input impedance at DC and low frequencies but the impedance drops to
40 Ohms at 1 GHz.
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