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Figure 8
Chapter 2: Probing Considerations
Resistive Loading Effects
Bias Changes
Probes with low input resistance can cause bias changes in the device under
test. A good example of this effect can be seen when probing ECL circuits.
Figure 8 represents a typical ECL node with a 60-Ω bias resistor to -2 V. Ip
represents current that flows from ground into the circuit when the probe is
connected. The table shows the current that flows in each device at both the
high (-0.8 V) and low (-1.75 V) states, with and without a 500-Ω probe
connected.
Probing ECL Circuits
Note that in the high state there is little difference in current flow with or
without the probe connected. However, in the low state the output stage is
closer to cutoff. Connecting the probe sources current into the output node,
which reduces the current sourced from the gate output. The output current
drops from 4.2 mA to 0.7 mA. The low output current can cause problems
with switching noise margins. The output gate will have difficulty reaching
the low threshold, so ac performance will suffer because the falling edge
degrades. If a larger bias resistor had been used to keep the current levels
lower, when a 500-Ω probe is attached the output gate could go into cutoff
before it reaches the low threshold.
Recommendation
Be careful not to use a probe just because it has the highest input resistance
available. High-resistance probes usually come with trade-offs in other
important parameters, such as higher capacitance, which also affect
measurement accuracy.
24
High (-0.8 V)
Without
With
Probe
Probe
I
20 mA
18.4 mA
O
I
20 mA
20 mA
R
I
1.6 mA
P
Low (-1.75 V)
Without
With
Probe
Probe
4.2 mA
0.7 mA
4.2 mA
4.2 mA
3.5 mA
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