Oscilloscope probes are an essential part of
a measurement system and PC Instruments provides a wide variety of probes for many
different applications. Important characteristics of probes include the bandwidth (i.e.
risetime), accuracy, loading effects, maximum voltage, and whether or not the probe is a
single-ended probe or a differential probe. The software provided with every oscilloscope
from PC Instruments includes a probe setting that compensates the voltage read-outs for
the attenuation factor of the probe. Listed below are the general categories of probes and
the salient characteristics of each type. For more information, download the .pdf of the complete, detailed data sheet
and specifications.
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Passive - High Accuracy - Active Differential Attenuating -
Active Differential with Gain |
Passive Probes for 1 M Ohm
Oscilloscopes
These are the most popular probes due to their low cost and low loading effects on the
circuit being measured. These probes should only be used with oscilloscopes that have a 1
MOhm input resistance and an input capacitance from 12 to 22 pF; such as the PCI-421, PCI-422, PCI-423, PCI-431, PCI-432, PCI-433, PCI-441, PCI-442,
PCI-443, and the PCI-444 Digital Storage Oscilloscopes. Contact the factory if you
need assistance in choosing a probe or if you need a probe with higher attenuating ratios
than those listed here.
PCI-900: X10, 300 MHz
PCI-901: X1/X10, 100 MHz
PCI-902: X100, 250 MHz
Passive Probe for 50 Ohm Oscilloscopes
A 50 Ohm passive probe with an attenuation factor of X10 is useful for measuring high
frequency signals generated by a relatively low source impedance. A 50 Ohm passive probe
will load the circuit being measured with a 500 Ohm resistance and therefore should not be
used to measure voltages with source impedance higher than a few ohms. Measuring digital
signals and wide bandwidth analog signals from amplifiers and DACs are examples of
situations where a 50 Ohm passive probe is appropriate. The PCI-903 should only be used
with oscilloscopes with a 50 Ohm input impedance such as the PCI-425 and PCI-435
Digital Oscilloscopes or the PCI-426 and PCI-436 Sampling Oscilloscopes.
PCI-903: X10, 1 GHz
High Accuracy Probe useful for Matching and Differential Applications
An attenuating oscilloscope probe for 1M Ohm oscilloscopes is essentially a series
resistor with several additional components (including the compensation capacitor) for
high frequency tuning. Most attenuating oscilloscope probes have a resistance tolerance of
2%. This tolerance, when coupled with an oscilloscope with an input resistance tolerance
of 1%, will add an additional probe gain error of almost 3% to the measurement (in
addition to the gain error due to the oscilloscope). When performing a differential
measurement, or a "Channel 1 minus Channel 2" measurement, this error doubles to
almost 6%. To minimize the measurement gain error, the digital storage oscilloscopes from
PC Instruments have been designed with an input resistance tolerance of 0.1%, and the
company provides the PCI-906 probe with resistance tolerance of 0.2%. This combination
reduces the probe gain error from almost 3% to under 0.3%. The PCI-906 probe should be
used in situations where gain accuracy is important or when a differential measurement is
necessary such as telecom pulse testing (e.g. T1 and E1 mask testing).
PCI-906: X10, 250 MHz, 0.2%
Active Differential Probe with Attenuation
The PCI-904 Active X10/X100 Differential Probe is useful in applications requiring the
measurement of the voltage difference between two points in a circuit, neither of which
are ground. This probe should be used to measure signals from motor controllers, power
electronics, SCR circuits, and other "high side" measurements. Some engineers
choose to "float" the measuring equipment to make a high side measurement, but
this technique is very dangerous and is likely to distort the waveform measurement. When
an oscilloscope is floated, the impedance of the input lead and the reference (i.e. the
alligator clip on the probe) is not matched. In fact, the impedance of the alligator clip
is very unpredictable and will vary with probe placement and the physical layout of the
system. The safe, and accurate, method is to use the PCI-904 Active Differential Probe.
PCI-904: X10/X100, 15 MHz
Active Differential Probe with Gain
The PCI-905 Active Differential Probe has a gain of 10 and is used to measure small
signals from circuits that have a high amount of common mode voltage. Examples of these
situations include light detectors, diode detectors, and power detectors, as well as other
sensors. In these situations, the signal can be very small, and the common mode noise (or
"ground noise") might be larger than the signal. A differential measurement is
necessary to reject the common mode voltage, even if the voltage being measured is
referenced to ground. In this situation, the probe is used to reject "ground
noise" found in the system that might be due to a large amount of interference or a
large ground current that creates voltage noise at the source of the signal.
PCI-905: Gain = 10, 10 MHz
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