HVFO108 High-Voltage Fiber Optically Isolated Probe
6
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Probe Design Compared to a Differential Probe
When properly used in its intended application, the HVFO108 will achieve much better results
than a conventional, high-attenuation HV differential probe due to the dramatically different
probe design.
Consider the case where a conventional,
high- attenuation HV differential probe is
connected in-circuit to measure a small
signal floating on a HV DC bus (left).
The differential leads are connected across
a sense resistor at a voltage V
S
that is
floating above ground by the DC bus
common-mode (V
CM
) voltage. The probe
must use high attenuation to reduce the
measured voltage (V
CM
+V
S
) to a low enough
level to use safely with an oscilloscope.
The two input leads are connected across
the low voltage V
S
, but due to the probe
construction, these leads are actually
measuring V
CM
and (V
CM
+V
S
), and the
difference between them is V
S
.
The high V
CM
value (typically 500 to 1000 V
DC
) results in large current flows (based on the
proportion of R
LEAD
to R
SENSE
) when R
LEAD
is not >> R
SENSE
. These large current flows may load the
circuit and inhibit proper DUT operation.
The input lead capacitance C
LEAD
has a very high voltage (either V
CM
or V
CM
+V
S
), which will result
in a large capacitive charging current that will further load the DUT and likely affect the fidelity
of the measured signal. These limitations are inherent in this design.
The high attenuation requires that the oscilloscope be operated in a very high sensitivity (small
V/div) setting, which results in higher gain
and more noise on the signal―
often enough to make
the measurement nearly unusable.
Since the probe is connected to ground, the common mode rejection ratio (CMRR, p.32) of the
differential amplifier in the probe must be very good or else signal noise and interference will
not be rejected and will add to measurement noise. High enough CMRR is difficult and costly to
accomplish, since it requires very precise matching of the + and – probe input leads and
excellent amplifier design.