Operator’s Manual
EAR99 technology subject to restrictions on copyrights page.
7
Now, consider the advantages of the HVFO
design in a circuit where the HVFO tip wires
are connected only across V
S
(left).
The V
CM
has no impact on the loading of the
circuit, since the R
LEAD
>> R
SENSE
.
The small C
LEAD
requires minimal charging
current, since the voltage V
S
is very low
(volts, not hundreds of volts or kilovolts).
The tip attenuation is very low, so the signal
is not being highly attenuated and then
highly amplified by oscilloscope, which
would result in higher noise.
The CMRR can easily be made very high,
since there is no requirement to identically
match a differential lead pair, and the high
CMRR performance is inherent in the fiber optic isolation from ground.
In the example above, the HVFO will perform better than a conventional, high-attenuation HV
differential probe when some or all of the following are true:
•
Common mode voltages (V
CM
) are higher –170V
DC
or more.
•
DUT impedance is lower.
•
Signal rise times are faster (within the limitation of the probe bandwidth).
•
Switched energy is higher.
It is unlikely that the HVFO108 will show much improvement on a very low voltage DC bus (~20 V)
floating signal unless the rise time is very fast. However, with typical 120/240 V
AC
supplied
equipment (340-500 V
DC
bus voltage), 600 V
AC
class equipment (1000 V
DC
bus voltage), or
≥
5 k V
AC
class equipment, the advantages of the HVFO will be obvious, in most cases.
