Using Ferrite-Based Isolation For Improved Performance

摘要

Historically, combining both high-speed AC and precision DC measurement capability within an automated test system required mechanical relays to keep the two subsystems from interfering with each other. Even with improvements in mechanical relays, it still is desirable to eliminate as many relays as possible from a test system to maximize circuit density and minimize test costs. To eliminate the relay used to disconnect the AC pin electronics circuitry, the pin-driver IC must have low input leakage current over the full voltage range of the DC measurement system. This has been realized for pin drivers operating as fast as 500 MHz. To remove the relay that disconnects the DC measurement system, the per-pin DC measurement unit (PPMU) IC and associated circuitry must be designed to preserve the AC waveform passing between the pin driver/comparator and the device under test (DUT). The primary cause of distortion in the rising and falling edges of the AC measurement system has been the parasitic capacitance presented by the DC circuitry. This distortion has been difficult to correct, especially at fast edge rates.