Settling Clamp Circuit for Digital-To-Analog Converter
Original Publication Date: 1986-Jan-01
Included in the Prior Art Database: 2005-Mar-08
A circuit technique that reduces the time required for a digital-to- analog converter (DAC) to settle to the proper voltage level during a digital-to-analog conversion. The figure shows a partial schematic of a DAC. Devices Qa and Qb form a conventional PNP current mirror. Devices Q1-Qn are PNP current sources that drive the R-2R resistor ladder network depending on the state of the digital inputs BO-Bn. For example, if BO is a 'ONE', transistor Q1 is ON, sourcing current to the R-2R network. If BO is a 'ZERO', transistor Q1 is cut off and does not source current to the ladder network. The worst-case DAC settling time occurs when BO-Bn switch from all 'ONES' to all 'ZEROS'. During this case current mirror Qa and Qb and its associated nodes A and B experience the greatest change in bias conditions.