Theory of operation
Figure 18: Simplified diagram of the P7700 Series probe tip input network
The offset voltage affects the probe tip buffer’s measured signal through a resistive
summer configuration that forms the buffer’s input attenuator network. The high
value resistors used in the buffer’s input attenuator result in an interaction between
the input signal and the offset voltage DC level.
Calibrated offset voltage performance requires that both the input signal and the
offset voltage generator in the probe have a source resistance that is very small
compared to the 25 kΩ attenuator resistors. The source resistance of the offset
volta
ge generator in the probe is less than 1 Ω. Typically, when the probe and
tip are connected to a DUT, the DUT signals have a source resistance R
s
much
less than 25 kΩ.
If a probe tip is attached to a probe but not soldered to anything, the inputs will
be open which effectively makes the source resistance look much larger than the
25 kΩ attenuator resistors. As a result, the offset voltage control is no longer
calibrated and will have 2X the calibrated effect on the measured probe output.
This effect can be helpful in troubleshooting connection issues with the tips. If a
pr
obe tip has been soldered to a DUT and adjusting the offset voltage causes the
offset to move 2X the adjustment, it could indicate a broken solder joint that has
left the probe tip input open.
26 P7700 Series TriMode Probes Technical Reference
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