HVFO108 High-Voltage Fiber Optically Isolated Probe
8
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Effect of Tip Design on Current Paths
The HVFO108 three-wire tip provides
optimal measurement results by
separating the signal return currents from
the currents associated with the probe
stray parasitic capacitance. This reduces
the noise measured in the system, and
improves the CMRR performance of the
probe.
The image at left shows how the tip
construction affects the current paths.
The blue Signal wire conducts the Signal
current, which follows a return path
through the green Reference wire.
The green Reference wire is connected to
the Signal coaxial outer conductor close to
the Signal connection. This ensures that
the Signal and Return currents are equal and opposite through the common-mode choke
(i.e., I
RETURN
= -I
SIGNAL
).
The black Shield wire connects to the same in-circuit location as the Reference wire, and then
electrically connects to the tip at the internal shield of the transmitter. The current flowing
through the Shield wire will drive the reference voltage for the single-ended amplifier, including
the effects of any parasitic capacitance.
The HVFO108, by design, is not a differential probe, so the Signal and Reference wires are
imbalanced in impedance. By using separate Reference (signal return) and Shield wires to carry
currents, the probe segregates these currents to overcome any impact of impedance imbalance
between the Signal and Reference wires. If only the Signal and Reference wires were connected
(and not the Shield wire), the probe response at DC and high frequency would be unaffected, but
the CMRR would be degraded. If only the Signal and Shield wires were connected (and not the
Reference wire), the probe response at DC would be unaffected, but the high-frequency
response would be degraded, and CMRR may also be degraded.
CAUTION:
The Reference and the Shield wires are electrically connected inside the tip.
Failure to connect them to the same measurement potential will damage the DUT and/or
the HVFO108.
The common-mode capacitance (C
PAR
in the drawing) is ~7 pF. This value is highly dependent
