Teledyne LeCroy AP033 - Manual (Page 16)

AP033 Active Differential Probe
10 922260-00 Rev A
Probe Input Loading
Attaching any probe to a test circuit will add some loading. In most applications, the high
impedance of the AP033 Active Differential Probe inputs imparts an insignificant load to the test
circuit. However at very high frequencies, the capacitive reactance of the probe’s input capacitance
may load the circuit enough to affect measurement accuracy. The equivalent model of the probe
input circuits is shown below:
Figure 2, AP033 Equivalent Input Model
Grounding the Probe
The single lead along with one of the larger (0.8 mm) mini clips can be used to ground the probe to
the test circuit. Insert the pin end of the lead into the receptacle marked:
CAUTION: Do not use the attenuator encoding receptacle (unmarked socket near the – input)
to ground the probe. Connection to the encoding receptacle will not provide adequate grounding
and may result in an incorrect effective gain indication.
In many cases it is not necessary to ground the probe to the circuit under test. However, if the test
circuit is isolated from earth ground, it is usually necessary to connect the probe ground to a point
in the circuit. Grounding test circuits that are referenced to earth ground may improve the fidelity
of high frequency components in the waveforms. The potential for improvement with grounding
will vary depending on the common mode source impedance. However, connecting the probe
ground to a circuit that is referenced to earth ground can create a ground loop that may add noise
to low amplitude signals. The rejection of high frequency common mode signals is improved when
the probe head is ungrounded.