Page 76 - Building A Succesful Board-Test Strategy

62 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

10 kQ
-Wv

Source

R M = 6.67 kQ

Figure 2-6 In this classic resistor triangle, a measured valued of 6,67kO is correct,
despite the fact that R 1 is a 10-kO device.

By measuring voltages, currents, and resistances, MDAs can find a host
of failures other than gross analog-component problems. A backwards diode, for
example, would fail the test, because its reverse leakage would read as forward
drop. Resistance measurements detect missing analog or digital components,
and voltage measurements find many backwards ICs. In manufacturing opera-
tions where such process problems represent the bulk of board failures and where
access is available through board nodes, an MDA can identify 80 percent or more
of all faults.
Like shorts-and-opens testers, MDAs are fairly inexpensive, generally costing
less than $100,000 and often much less. Tests are fast, and self-learn programming
minimizes test-programming efforts. Because they provide better fault coverage
than shorts-and-opens testers, MDAs serve even better as prescreeners for in-
circuit, functional, or "hot-mockup" testing.
Again, these testers suffer because of the bed-of-nails. Contributions from
adjacent components severely limit analog accuracy, and there is no real digital-
test capability.

2.3.4 /n-C/rcuit Testers

In-circuit testers represent the ultimate in bed-of-nails capability. These
sophisticated machines attempt to measure each analog component to its own spec-
ifications regardless of surrounding circuitry. Its methods also permit verifying on-
board function of individual digital components.
Consider the resistor network in Figure 2-6 if the tester grounds node Z
before measuring R 1, as Figure 2-7 shows. Theoretically, no current flows through
resistor R 2 or resistor R 3. Therefore, R M = R 1 = lOkQ. This process of grounding
strategic points in the circuit during testing is called guarding, and node Z is known
as a guard point.
In practice, because the measurement-op-amp's input impedance is not infi-
nite and output impedance is not zero, a small current flows through the guard