damping resistor value of approximately 100 Ω also serves to tune the probe tip high frequency response. Following each
input damping resistor is a pair of attenuation resistors, Rs and Rp. The attenuation resistor values depend on the specific
probe tip attenuation factor design. There are currently two attenuation factors available as shown in the following table with
approximate attenuation resistor values. The probe tip attenuation factor in the table includes an additional 2X factor due to
attenuation from the 50 Ω tip buffer output impedance and the 50 Ω probe amplifier termination impedance.
Table 1: Probe tip attenuation factors
Attenuation factor
Rs
Rp
Dynamic range
Offset voltage range
4x
25 kΩ
25 kΩ
2.5 V
pp
±4.0 V
10x
60 kΩ
15 kΩ
6.0 V
pp
±10.0 V
The input attenuation resistors serve three major functions for the dual input tip buffer:
• The high resistance of the attenuation resistors provides very light loading on the probe input signals.
• The probe tip attenuation factor expands the linear dynamic range of the probe tip inputs.
• The probe tip attenuation network provides a resistive summing junction for injecting an offset voltage signal to extend
the probe tip operating voltage window.
TekFlex active tips contain a pair of unity gain amplifiers that buffer the DUT differential input signal. These buffer amplifiers
provide an impedance conversion for the input signals from the high impedance input attenuator at the tip input to the 50 Ω
output drive at the tip output. The buffer amplifier 50 Ω output impedance is needed to drive the probe TekFlex connector
and meter-long main cable transmission lines with good high frequency fidelity. The new TekFlex interface to which TekFlex
probe tips are attached includes both a dual high frequency connection for the probe tip signals as well as a low frequency
connection for eight messenger wires that provide power and control for the active probe tips.
The control features of the new TekFlex interface include two analog tip offset voltage signals and a pair of digital
communication signals for accessing the probe tip S-parameter memory.
The probe tip S-parameter memory contains characterization data for use with DSP correction filters that are unique to the
probe tip. The S-parameter memory includes a unique data header used in automatically identifying the probe tip type and
serial number. The A and B signal outputs from the probe tip are connected by high frequency spring contacts in the
TekFlex connector to a pair of delay-matched coaxial cables that carry the input signals through the main cable and into the
comp box. These signals terminate to a pair of 50 Ω termination resistors at the probe amplifier in the comp box. The
termination resistors have an adjustable termination voltage connection for optimal loading of the probe tip output signals.
The TekFlex interface messenger wires are routed as a small gauge ribbon cable, along with the delay-matched coaxial
cables, inside a shielded main cable assembly.
The TDP7700 Series probe amplifier has several cascaded amplifier stages that condition the probe tip differential signal for
precision measurement fidelity. The complex input stage of the probe amplifier selects the desired TriMode input mode.
This input stage of the probe amplifier includes connections for injecting a pair of adjustable probe offset voltages for
precision control of the probe output zero voltage, which is a measure of the output signal with volts at both probe tip inputs.
The next stage in the probe amplifier is the step gain stage. The step gain stage provides several switchable, coarse gain
steps, that extend the sensitivity range of the probe measurement down to the millivolt level with good noise performance.
The step gain stage is automatically switched to its optimum setting by the oscilloscope as the oscilloscope vertical channel
sensitivity is adjusted by the user.
The final stage of the probe amplifier is the variable gain stage. The variable gain stage is used for fine tune adjustment of
the gain of the probe and probe tip to its calibrated value. The calibrated gain setting is determined from calibration
constants characterized during manufacturing testing of the probe and probe tip and includes compensation for probe
temperature variation. Although the signal that passes through the probe amplifier is processed as a differential signal
through the internal amplifier stages, it is routed to the oscilloscope’s TekVPI interface as a single-ended output.
Theory of operation
TDP7700 Series TriMode™ Probes Technical Reference
5
