| 摘要 |
A driver circuit (10) for selecting particular values of backmatch impedance (Z) for a functional or in-circuit test system. The test system (100) generates the backmatch impedance select signals (32A), the control signals (32B), and the drive data (32C). A decode circuit (200) receives the backmatch impedance select signal and the control signals from the test system for activating one or more of a plurality of individual drivers. Each of the plurality of drivers has a three state control input (202A), a data input (212), and an output (214). Each output of the drivers is connected to an impedance (Z). Hence, a plurality of impedances are provided which are commonly tied together and connected to the transmission line (70). The test system connects one or more of the drivers together. When one driver is selected, that particular value of backmatch impedance is connected to the transmission line. When more than one driver is connected, a resultant parallel impedance value is interconnected to the transmission line. Thus, precise value of backmatch impedance can be connected to the transmission line.
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