Table 2: Rise time and overshoot degradation versus lead length
Signal wire lead length
10 - 90 Rise time
20 - 80 Rise time
Effective bandwidth
0.25 mm
30.5 ps
21 ps
20 GHz
1.90 mm
27.9 ps
19.3 ps
20 GHz
3.05 mm
30.1 ps
20.9 ps
20 GHz
5.08 mm
34 ps
23.6
17 GHz
7.62 mm
42.8 ps
29.2
14 GHz
Using offset voltage to extend TekFlex® solder-in tip input voltage range
The single-ended linear dynamic range of the TekFlex solder-in tip inputs is specified to be 2.5 V
p-p
, which is a range from
-1.25 V to +1.25 V with zero volt offset. The dynamic range of buffers is limited by the input attenuation factor, which is 2X
for the solder-in probe tips as shown in the following simplified figure. A 2X attenuation factor was selected for the probe tips
to optimize dynamic range and noise, since a higher attenuation factor would have increased probe noise. Although the
dynamic range of the probe tip buffer cannot be extended, it is possible to extend the range over which the tip dynamic
range window can be moved by adjusting the probe offset voltage. The offset voltage range of the TekFlex solder-in tips is
-4 V to +4 V, which is adjusted using the Probe Setup screen of the oscilloscope or the offset knobs on the oscilloscope
front panel. Using the offset voltage controls, it is possible to make measurements within any 2.5 V
p-p
window between
-5.25 V and +5.25 V. As an example, by setting the offset voltage to +3.0 V, it is possible to measure an HDMI signal, which
has a signal swing between about +2.8 V and +3.3 V.
Figure 21: 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 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
voltage 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.
Theory of operation
TDP7700 Series TriMode™ Probes Technical Reference
17
