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Probe Selection
TDS 620A, 640A, & 644A User Manual
Low Impedance (Z
O
) Probes
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 scope input. Input capacitance is
much lower than high Z passive probes, typically 1 pF, but input resistance is
also lower (500 to 5000 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
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 mechan-
ically much larger than for a passive probe. High voltage probes have the
added advantage of lower input capacitance (typically 2-3 pF).
P6009
P6015A
Figure 3-59: The P6009 and P6015A High Voltage Probes