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Appendix E: Probe Selection
D–2
TDS 500C, TDS 600B, & TDS 700C User Manual
However, their 8 pF to 12 pF (over 60 pF for 1X) capacitive loading can distort
timing and phase measurements. Use high input resistance passive probes for
measurements involving:
H Device characterization (above 15 V, thermal drift applications)
H Maximum amplitude sensitivity using 1X high impedance
H Large voltage range (between 15 and 500 V)
H Qualitative or go/no-go measurements
Low impedance probes measure frequency more accurately than general purpose
probes, but they make less accurate amplitude measurements. They offer a higher
bandwidth to cost ratio.
These probes must be terminated in a 50 W scope input. Input capacitance is
much lower than high Z passive probes, typically 1 pF, but input resistance is
also lower (500 to 5000 W typically). Although that DC loading degrades
amplitude accuracy, the lower input capacitance reduces high frequency loading
to the circuit under test. That makes Z
O
probes ideal for timing and phase
measurements when amplitude accuracy is not a major concern.
Z
O
probes are useful for measurements up to 40 V.
High voltage probes have attenuation factors in the 100X to 1000X range. The
considerations that apply to other passive probes apply to high voltage probes
with a few exceptions. Since the voltage range on high voltage probes varies
from 1 kV to 20 kV (DC + peak AC), the probe head design is mechanically
much larger than for a passive probe. High voltage probes have the added
advantage of lower input capacitance (typically 2-3 pF).
Low Impedance (Z
O
)
Probes
High Voltage Probes