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EXERCISE 31 : DIGITAL VI
ACTION
Clip SOIC Test Clip on U16 74ABT574
Note the SETUP of the test especially V-I voltage range and tolerance.
Press STARTon the IC tester and observe results display. Familiarise yourself with the display
controls, (<< < > >>), to step through the screens and zoom into single trace made by
clicking on the required trace.
Press the ACTUAL button to toggle the MASTER traces on and off for visual comparison. Note
the percentage indication of the comparison. A good way to show this comparison is to clip
onto another IC eg U13, U17, or U8.
DESCRIPTION
The digital V-I test can be used to plot a current against voltage curve for any type of
component, but the test parameters, such as voltage and current, are optimised for use with
Detailed analysis of the resulting curve can be used to diagnose faults, but the best way to
use the digital V-I test is to compare a good and bad board using IC Live Comparison or, as
in this case, a TestFlow.
It is important however to understand that the digital V-I test shows the ACTUAL measured
curve for the component, and parts from different IC manufacturers may well have slightly
different curves because of internal process differences. A certain amount of experience is
required to allow for these differences when performing digital V-I tests.
COMMENTS
You may find during this exercise that some pins do not compare exactly using the built-in
comparison algorithm. This is because ICs from different manufacturers have slightly different
characteristics. This may even occur with ICs from different batches from the same
manufacturer, although in this case the differences would not be so great. V-I tests for
digital IC pins often look like the curve for diodes, that is a straight line with a diode curve at
the left-hand side, and possibly also at the right hand end.
The exact voltage at which the diode curve begins may vary from IC to IC, but the basic
shape should be the same. If on the other hand the curve shows a diagonal line instead of
the diode curve, this is an indication of a leaky diode.
Remember that the curves will be modified if other components, for example pull-up
resistors, are connected to the pin under test.
You should remember that the digital V-I test is not really a test, more a measurement of the
characteristic of the pin(s) under test. If an IC contains an internal fault that has no
connection to an external pin, the V-I test willnot be able to find it.
For example, a memory IC contains thousands of transistors, but may only have 16 pins. In
this case, only a very small number of the internal transistors can be checked with the V-I
test.
The V-I test is used to supplement the truth table, connections and voltage tests in order to
provide the best possible fault coverage.
