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Acute Technology Inc.
Copyright
2016
Connecting the probe to the test circuit
Two inputs are available at the probe tip to connect the probe to a circuit under test.
For accurate measurements, the + and – inputs both must always be connected to the test
circuit.
Positive voltages applied to the + input (red) relative to the – input (black) will deflect the
oscilloscope trace toward the top of the screen.
To maintain the high performance capability of the probe in measurement applications,
care must be exercised in connecting the probe to the test circuit.
Increasing the parasitic capacitance or inductance in the input paths may introduce a “ring”
or may slow the rise time of fast signals.
Input leads that form a large loop area will pick up any radiated electromagnetic field that
passes through the loop and may induce noise into the probe inputs.
Because this signal will appear as a differential mode signal, the probe’s common mode
rejection will not remove it.
This effect can be greatly reduced by twisting the input leads together to minimize the loop
area.
High common mode rejection requires precise matching of the relative gain or attenuation
in the + and – input signal paths.
Mismatches in additional parasitic capacitance, inductance, delay, and a source impedance
difference between the + and – signals will lower the common mode rejection ratio.
Therefore, it is desirable to use the same length and type of wire and connectors for both
input connections.
When possible, try to connect the inputs to points in the circuit with approximately the
same source impedance.
