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current-measuring device (conceptually, an ammeter with zero impedance to ground) to the other component terminal. The current flow observed due to the known stimulus voltage is related to R by Ohm s law (R = V * I). See Fig. 55.5(b).
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FIGURE 55.5 An analog component on a loaded board can be accessed by two in-circuit probes that connect its terminals with tester resources. There are two approaches: force a known current and measure a voltage (a) and force a known voltage and measure a current (b).
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However, some discrete analog components may be connected to one another in ways that prevent simple measurement of the current flowing through the component due to parallel pathways that sidetrack some of the current. In Fig. 55.6, when applying a voltage to one terminal of R, a current also flows in a parallel path through Ra and Rb. The ammeter does not measure the true current through R because current from the parallel path also gets to the ammeter.
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FIGURE 55.6 Three analog devices interconnected with an integrated circuit (U1).
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The parallel path problem can be solved with a process called guarding. Guarding is accomplished by using a third nail to connect ground to the node marked C as shown in Fig. 55.7. When C is grounded, all the sidetracked current goes to ground because there is no voltage drop across Rb to attract current to or from the ammeter. All the current seen by the ammeter comes through R, so again Ohm s law gives the value of R. This is a classic three-wire measurement. It turns out that, for general component topologies, multiple guards (ground connections) may be required, but these are still considered three-wire measurements. In some cases where enhanced accuracies are needed or where there are extreme ratios of component
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FIGURE 55.7
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Equivalent circuit for Fig. 55.6 when making a guarded measurement.
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values in a network, additional sense wires are used to eliminate errors that are due to small voltage drops in fixture nail, wire, relay, and trace contributions. (See Fig. 55.8.) See Ref. 3 for a discussion of enhanced measurement accuracies.
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FIGURE 55.8 Routing of tester resources to the circuit to be tested.
Digital In-Circuit Test Digital in-circuit test focuses on the digital components residing on a board, and requires that power be applied to activate the digital logic contained within the ICs. Just as analog components
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can be tested without removing them from a board, digital components can be tested the same way. The key technology is backdriving. When applying digital stimuli to a digital device s inputs (via the bed of nails), a tester s driver must overcome the voltage levels that connected upstream devices are producing. This is done by equipping the digital in-circuit tester with powerful, low-impedance drivers that can backdrive upstream drivers with enough current to create the desired signal voltage in spite of their state. With tester receivers connected by nails to the device s outputs, the tester can monitor these outputs for expected responses to the stimuli. (See Fig. 55.9.)
FIGURE 55.9 In-circuit digital test setup with full nodal access. The device under test is most often connected to other devices.
A problem with backdriving is that the tester drivers are abusing the upstream drivers in other devices* while testing the device of interest. Studies4 have shown that this is a legitimate concern and that damage to upstream devices may occur. For example, overdriven silicon junctions or device bond wires may heat up enough to be damaged. Sometimes this damage may occur surprisingly fast (within milliseconds). This problem can be solved by careful application of tests with an eye toward their duration. If backdrive testing can be done quickly, and/or with appropriate cooling intervals, then damage can be successfully avoided.5 This has allowed digital in-circuit testing to become the dominant testing technology. A great advantage of digital in-circuit testing is that it is performed directly on the inputs and outputs of a targeted device. If the device should fail testing, this is seen directly, rather
* This abuse can be mitigated by conditioning upstream drivers. One method is to disable the upstream drivers by devoting additional tester drivers to driving states on disabling inputs (when they exist) such as output enable pins. Of course, the problem may simply be moved upstream, because the disabling inputs may also be connected to upstream drivers that are now being overdriven in place of the original drivers.This conditioning process may be carried out to several levels, if desired.
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