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Digital-To-Analog Ladder Terminator Allowing Variable Output Voltage

IP.com Disclosure Number: IPCOM000039249D
Original Publication Date: 1987-May-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Frushour, JE: AUTHOR [+2]

Abstract

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.

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Digital-To-Analog Ladder Terminator Allowing Variable Output Voltage

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. This sets the voltage on the emitter of Q1 at approximately - 0.5 volt. The emitter voltage of Q1 is a reference voltage (VX) and is used as a terminating voltage for the DAC R-2R ladder and also as the reference voltage for the voltage follower. The ladder output node voltage is set equal to reference VX by the voltage follower circuit. The load current on the voltage follower output is the selected DACI which flows from the summing node of the transimpedance stage OP AMP M2 through transistor Q2 and into the ladder output node. To minimize power, the majority of the DAC c...