12
1159A 1GHz Active Differential Probe
To Connect the Probe to the Circuit under Test
To Connect the Probe to the Circuit under Test
The method you use to connect the probe to the circuit under test is critical for
ensuring accurate measurements. The following examples examine the effect of
using different lengths of wire at 100MHz to connect the signal source to the
probe tip.
The Impedance of the Source
This is another instance where the symmetry of the differential circuit is
important. The impedance of the source forms a network with the input
impedance of the connection and the probe. This network determines the
frequency response for the measurement. If each side of the differential source
has a different impedance, the frequency response of each side will be different.
This lack of balance is reflected in reduced CMRR. The higher the impedance
of the source, the more critical these parasitic effects.
The Ground Connection
A poorly located ground connection allows ground loops to add to the common
mode signal. The differential probe measures the potential difference between
two locations on a PC board. Usually, it is not necessary to ground the probe.
Whether to ground the probe depends on the magnitude and frequency of the
voltage difference between the oscilloscope ground and the board ground. It is
good practice to maintain a board ground. Without this ground reference, you
could easily exceed the common mode range of the probe.
Probe Offset
The amplifiers in the 1159A probe limit the Differential Mode Range to 400mV.
If the input to the probe is approaching 400mV, there is little offset range available
for positioning the trace on screen. There are two solutions to this problem:
• Attenuate the signal into the probe with the 1/10 or 1/20 attenuator. This
mode of operation will induce some small loss in CMRR.
•Use Position to position the trace on screen.
The added feature of position control independent of Offset allows trace
positioning without calculating how much probe offset range is available. The
trace can be positioned by dragging the trace or positioning the trace under the
Probe menu.
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