Using Polar GDPS groundless differential probes
Application Note AP8507
Polar GDPS groundless differential probes for on board impedance testing
Polar GDPS probes are designed to allow the CITS880s (v15.01 or later) to measure the impedance of balanced differential traces without the need to make a ground connection to the board or coupon under test. This is an ideal compromise for situations where access to the ground plane of the pcb is impracticable or for situations where you need to test a balanced differential pair such as a broadside coupled surface trace on a flex circuit.
As the characteristics of this probe are a little different from the standard Polar IP probes it is important the following instructions are followed.
Setting up a test
Connect the probe to the CITS880s using one cable to any one of the input channels. When setting up the test in the Test Editor be sure to select “Groundless Differential Test” and pick the appropriate channel.
Enter the differential impedance and any other settings required then click OK.
Setting up the end of probe and the test region needs to be done with care. For the groundless differential probe an end of probe setting of 8 inches is recommended.
To set the test region, pick the flattest portion of the test trace before the trace goes to the top of the screen where the open at the end of the trace is detected.
Determining the end of probe
If you want to determine the end of probe with greater precision, then do so at a short and not at the open circuit of a probe held in the air. Set the probe length to zero, place the probe on a conductor so that both pins are shorted and then scroll the trace up so that the point at which the trace levels off at near zero can be seen.
Take the probe length from this point.
In the above example the test limits should be rescaled so the testing commences at about 10 inches; set the probe length to 10 inches, view the coupon and choose the test limits as described above; the length of the test region will depend on the length of the coupon.
You cannot measure the impedance of a pair of 50 Ohm reference lines (airline or semi-rigids with probe adaptor board) with a groundless differential probe as there is no coupling between them. Neither can you measure a single 100 Ohm reference line as this is not a balanced structure.
However you can measure a resistor. A single 100 Ohm 0805 SMD resistor can be probed and will give a valid result within the specification of the CITS. If the probe pin spacing is too wide the resistor may have to be soldered to a pcb but there should be no more pad or trace than is necessary to connect to the probe.
This probe is not suitable for use with the Polar Atlas Insertion Loss Test System as it has too low a bandwidth. It is not suitable for use with older CITS models due to their less sophisticated pulse processing.
Atlas impedance testing can perform groundless differential as it uses a true differential pair of pulses. For this an appropriate high bandwidth differential probe is required.
PLEASE NOTE: Even though they look like ordinary single ended IP probes and have only one cable, Polar goundless differential probes are not capable of single ended measurement.