Video Design Idea: Measure nanoamps to ensure accurate computer clocks
By Staff -- 5/10/2007
Bonnie Baker, senior applications engineer at Texas Instruments and regular EDN columnist, demonstrates a simple way to add DAC functionality to a microcontroller-based system using only an op amp and two passive components.; Bonnie Baker; DAC; EDN.com; analog design; op amp; video design idea; Mark Thoren, mixed-signal application engineering manager with Linear Technology, demonstrates an amplifier-based circuit design for a relatively inexpensive precision voltage source.; Mark Thoren; amplifier-based circuit design; linear technology; mixed-signal application; precision voltage source; Jim Williams, staff scientist with Linear Technology, explains why PC clocks are invariably wrong, and how engineers can surmount the extreme measurement challenge involved in solving the problem.; cell phones; computer clocks; jim williams; linear technology; nanoamps; quartz crystals; video design idea; EDN Tech Clips deliver technical depth and tutorial design information for engineers involved in analog circuit design, power management, embedded-system design, board-level design, signal integrity, and more. http://link.brightcove.com/services/link/bcpid1125855242http://www.brightcove.com/channel.jsp?channel=959007201
The extremely low-power quartz crystals that are prevalent in products from cell phones to PCs today work with currents that max out at 1 µA. With such a small full-scale current range, a seemingly tiny error in the current a circuit sends through the crystal can result in terribly inaccurate clocks. Designers can address that problem by accurately measuring the current through a crystal and tuning their circuit design for precision. But as Jim Williams explains, measuring current in the sub-µA range is difficult, and the measuring circuit can induce no more than 1 pF of capacitance.
Read "Measuring nanoamperes" a recent in-depth feature by Technical Editor Paul Rako, which covers this and other challenges in measuring very small currents.
