Page 74 - Building A Succesful Board-Test Strategy

60 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

I "M = ^«

Figure 2-5 The "apply current, measure voltage" in-circuit measurement
configuration.

Therefore, the current through the two resistors must be identical, and the value of
the unknown resistor is proportional to the value of the output voltage V 0.
In Figure 2-4, the measurement injects a known voltage V s. Measuring the
current through R Ref in the feedback loop determines the unknown resistance R M.
This version is often called "apply voltage, measure current." In Figure 2-5, the
position of the two resistors is reversed, so that the applied current is known. Again,
the value of the unknown resistor depends on the ratio of the output voltage to
Vs-, but the factor is inverted. An AC voltage source allows this technique to
measure capacitances and inductances.

2.3.2 Shorts-and-Opens Testers

The simplest application of the analog measurement technique is to identify
unwanted shorts and opens on either bare or loaded boards. Shorts-and-opens
testers gain access to board nodes through a bed-of-nails. A coarse measurement
determines the resistance between two nodes that should not be connected, calling
anything less than some small value a short. Similarly, for two points that should
connect, any resistance higher than some small value constitutes an open.
Some shorts-and-opens testers handle only one threshold at a time, so that
crossover from a short to an open occurs at a single point. Other testers permit
two crossovers, so that a short is anything less than, say, 10Q, but flagging two
points as open might require a resistance greater than 50 Q. This "dual-threshold"
capability prevents a tester from identifying two points connected by a low-
resistance component as shorted.
In addition, crossover thresholds are generally adjustable. By setting open
thresholds high enough, a test can detect the presence of a resistor or diode,
although not its precise value.
Purchase prices for these testers are quite low, generally less than $50,000.
Testing is fast and accurate within the limits of their mission. Also, test-program