Theory of operation
Offset voltage
The offset volt
age control sums an adjustable DC voltage with the probe signal
input. It is commonly used to nullify an input DC bias voltage to center the input
signal swing within the linear dynamic range o f the probe input. The A and B
probe inputs both have an independent offset voltage control. The following
figure shows a simplified diagram of a P7700 Series probe tip input network.
Figure 10: 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 input attenuator result in an interaction between the
input signal and the offset voltage DC level.
Calibrated offset voltage performance requires 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 (R
s
)of the offset voltage
generator in the probe is less than 1 Ω and measured DUT signals typically have
R
s
<< 25 kΩ. If a probe tip is attached to a probe TekFlex connector with its probe
tip
inputs open, the input signal source resistance is much larger than the 25 kΩ
attenuator resistors. As a result, the offset voltage control is no longer calibrated
and will have twice the calibrated effect on the measured probe output.
The offset voltage control, accessible from the attached oscilloscope front-panel
control and the o n-screen user interface, allows the probe dynamic range to be
effectively moved up and down within the limits of the offset voltage range and
the operating voltage window. When the offset voltage is set to zero volts and the
input signal is zero volts (inputs shorted to ground, not open), the displayed signal
should be ze ro volts. If a noticeable zero volt offset is present under the above
conditions, a probe compensation operation should be performed.
Offset voltages can be automatically generated by the probe and can be selected
using the Auto Offset button in the Offset section o f the Probe Setup screen. You
canalsoenterspecific offset values directly in the offset fields.
18 P7700 Series TriMode Probes Technical Reference
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