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|Application Note 101
PCB Test – Simplify
faultfinding by storing
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|During PCB test and fault finding with the Polar PFL,
variations in signatures due to
different vendor input protection schemes can produce
false FAIL comparison results in good devices. You can reduce
spurious FAIL results by acquiring reference signatures
for each of your device vendors.
digital devices with ASA
Simple input diode protection
|Input protection circuits
The input protection circuitry for a digital IC can be as simple as the single diode configuration to the left, which protects the input transistor from large negative transients.
|For this configuration the negative portion of the PFL sinusoidal drive voltage will cause the protection diode to conduct so the signature will appear as in the diagram to the left (the actual shape will depend on the PFL voltage range).|
Positive and negative transient protection
|Most protection circuits will be more complex however. In the circuit, to the left, the input is protected from large positive and negative transients.|
|In this case the positive and
negative excursions of the PFL drive voltage will cause
both diodes to conduct so the signature will appear as to
Other protection circuits include resistances in series with one or both diodes so the signatures will assume the characteristic resistive slope. The slope will be especially apparent on the Junction range.
|Alternative vendor signatures
When you acquire your first set of reference signatures the PFL assigns them to the Default vendor (displayed in the list box on the Tool Bar). In many cases this is the only set of reference signatures you'll need. If signature variations between vendors are causing false FAIL results on good devices, acquire references for each manufacturer.
To acquire reference signatures for the same device from another vendor, clip and test the device. Press Save to store the signatures as reference and enter the new vendor name. The vendor names are shown in the list box on the Tool Bar. Now when the PFL executes a test it always tests against all the vendors in the list until a match is found. The results of an acquisition can be viewed against any of your stored vendors-just click the down arrow and select the vendor from the list.
If you have a trouble-shooting technique you would like to share with other Polar fault locator users please fax or email Polar Instruments on the number below.
Polar Instruments Ltd
Tel: +44 1481 253081 Fax: +44 1481 252476 © Polar Instruments 2002
|© Polar Instruments 2002. Polar Instruments pursues a policy of continuous improvement. The specifications in this document may therefore be changed without notice. All trademarks recognised.|