HP 54750A, User Manual, Page: 439 / 478

How the Instrument W orks
System Architecture
Because of the physical geometry of this type of probe and because the
divider does not have to be capacitively compensated, this type of probe
has the lowest capacitive loading of any probe . This low capacitance
and its inherent wide bandwidth make it best suited for wide bandwidth
measurements or those measurements where timing is the most critical
parameter .
The disadvantage of this type of probe is its relatively heavy resistive loading.
Not all circuits can drive 500 or 1 k. Even for measurements in a relatively
low impedance circuit, the amplitude errors can be signicant. Changes in
bias levels or operating current in the circuit under test might aect the
circuit's behavior .
This type of probe is the best choice for minimum disturbance probing of ECL
circuits and 50 transmission lines . The 1 k divider probes are also usually
suitable for high-speed CMOS circuits . If you are interested in troubleshooting
CMOS , consult the data sheet for the particular CMOS part to make sure that
it can drive a 1 k load and to determine what the voltage error would be .
Active probes An active probe , like the HP 54701A, has a buer amplier at the tip . Refer
to Figure 13-11. This buer amplier drives a 50 cable terminated in 50
at the analyzer input. A ctive probes oer the best overall combination of
resistive loading, capacitive loading, and bandwidth, even though an active
probe does not have the highest resistance , highest bandwidth, or lowest
capacitance available . The disadvantages of active probes , besides their
higher cost, are the larger size of the tip and a somewhat limited input
dynamic range . Previous active probe designs were more susceptible to
damage , particularly to ESD , and required careful handling. The HP 54701A
is designed to withstand 200 V peak ac and 12 kV of ESD , so it functions
reliably in adverse conditions .
13-19