MOS switches have for the most part been excluded from a large number of analog and mixed-signal test applications because of their high leakage currents and larger on-resistances. Relays, particularly low-picofarad relays and low-thermal-EMF relays, are usually the only viable choice for precision measurements. Test engineers would prefer the reliability provided by the non-mechanical switching of the MOS devices over the inevitable wear associated with using mechanical relays.
The circuit presented here helped at least part of one mixed-signal solution (in an analog function) take advantage of solid-state benefits. The AFE1224, an analog front-end for HDSL applications, has three voltage references as shown in the figure. We need to measure the reference to within a tenth of a volt just to make sure the reference is functional. However, when these measurements aren't taking place, the long traces from the references to the DUT board edge serve as antennas. These antennas pick up output and/or digital switching transients. Such interference may degrade test performance in other areas, therefore a high impedance at each voltage reference as close to the DUT as possible would achieve the desired function.
During the voltage reference test, the enable input pins on the DG411 in the figure are set low, and the test hardware has a transparent connection to the voltage references.
During the other tests, the enable pin is set high, causing an impedance in the gigohm range between the voltage references and the test hardware. This high series impedance effectively disconnects each voltage reference from those problematic board traces to the test hardware. The capacitors located on each voltage reference pin provide additional decoupling from circuit noise.
