PICO PicoConnect 900 Guia Del Usuario página 8

PicoConnect 900 Series Probes
4 About the PicoConnect passive probes
A high-impedance test probe might initially seem to be the best way of making a
measurement without disturbing the circuit under test. In practice, however, at very
high frequencies, probe tip capacitance causes the impedance to drop below the
probe's nominal high resistance. Low-impedance test probes usually achieve the
lowest capacitance at the probe tip and, in doing so, achieve a more consistent, flat
loading and response across the signal's bandwidth. They can also drive the cable and
measurement instrument directly without a buffering amplifier.
Typical 10 MΩ 10 pF
oscilloscope probe
PicoConnect 900 passive
oscilloscope probes (220 – 960 Ω)
Above: Comparison of test node loading for PicoConnect probes and typical oscilloscope probes
For high signal frequencies, the low-impedance probe becomes plausible because the
test node also has low impedance. The probe point effectively lies on a low-impedance
(Z ) transmission line, generally terminated at both ends. To be low-invasive, the
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probe loading impedance need only be high compared with Z
of only 5 x Z is typically acceptable. PicoConnect probes are very compact to minimize
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their capacitive and inductive parasitics, and to ease probing and connection to today's
fine-geometry systems. It follows that they have a low power-dissipation rating. The
PicoConnect low-impedance microwave probe is therefore suited to the measurement
of signals at low test node impedance and low RMS voltage. Peak voltage rating is
higher since only passive circuitry is used.
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Frequency
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Test probe loading
vs frequency
Typical 1 MΩ 0.9 pF
active oscilloscope probe
/2. A probe impedance
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User's Guide
DO274-4