Offset voltage
The offset voltage 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 of the probe input. The A and B probe
inputs both have an independent offset voltage control when used in A mode or B mode respectively. The following figure
shows a simplified diagram of a TekFlex probe tip input network.
Figure 8: Simplified diagram of the TekFlex 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 on-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 zero volts. If a noticeable zero volt offset is present under the above
conditions, an oscilloscope SPC and an autozero operation should be performed.
In A or B mode, the single-ended offset can be controlled through the probe menu. In C or D mode, both the differential or
common-mode offsets can be controlled through the probe menu. The single-ended offsets are related to the differential/
common-mode offsets by the following relationships:
• Differential = (A - B)
• Common = (A + B)/2
• A = (Differential/2)
• B = Common - (Differential/2)
The offset range limits apply to each side of the single-ended inputs. Due to this, the common-mode and differential offset
ranges are limited to the area shown in the diamond-shaped graph below. When the maximum differential offset is applied,
the common-mode offset is restricted to 0V. Similarly, with the maximum common-mode offset applied, the differential offset
is constrained to 0V.
Theory of operation
TDP7700 Series TriMode™ Probes Technical Reference
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