Digital-To-Analog Ladder Terminator Allowing Variable Output Voltage
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
An integrated control module design has a vernier voltage on an externally controlled reference voltage. The vernier voltage is controlled by an internal digital-to-analog converter (DAC). In order to minimize module power, the following design is used. As seen in the schematic, terminating circuitry is placed in the DAC between the R-2R ladder and transimpedance stage. The terminating circuitry provides the proper voltage bias levels for the ladder and allows the selected DAC current (DACI) to flow from the transimpedance stage, through the terminator and into the ladder. The terminating circuitry is composed of a resistor divider voltage reference R1/R2, a buffer transistor Q1 and a voltage follower made up of an operational amplifier M1 and transistor Q2. The resistor divider places + 0.3 volt on the base of transistor Q1.