Method and apparatus for temperature compensating a digital to analog converter
AssigneeAnalog Devices, Inc.
InventorsJeffrey G. Barrow, Adrian P. Brokaw
Life of this patent
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Abstract
A current-source ladder digital-to-analog converter is compensated for temperature changes by making the total current running through the converter proportional to absolute temperature and by terminating the parallel transistor chain forming the current source ladder with a transistor whose emitter voltage is greater than the emitter voltage of the least significant bit current source transistor by 2(KT/q)ln 2 volts. The aformentioned voltage difference is achieved by making the emitter area of the termination transistor at least eight times the emitter area of the least significant bit transistor.
Claims27
A method of operating a digital-to-analog converter having a plurality of parallel-connected transistor with emitters connected to an R-2R resistor network to cause the transistor to operate as binary-weighted current sources, said transistors being arranged in order from a most significant bit transistor to a least significant bit transistor, to compensate for errors introduced by varying emitter currents, said method comprising the steps of:
- A. drawing a current through the digital-to-analog converter which is proportional to absolute temperature, and
- B. applying to said R-2R resistor network at the least significant bit end a compensating current generated by placing across a resistor of value 2R a compensating voltage generated by means including a transistor having an emitter area which is larger than the emitter area of the least significant bit transistor so that said compensating voltage has a magnitude of 2(kT/q) ln 2 volts greater than the base-emitter voltage of the least significant bit transistor.
A method of operating a digital-to-analog converter having a plurality of parallel-connected transistors with emitters connected to a R-2R resistor network so that said parallel-connected transistors operate as binary-weighted current sources, said transistors being arranged in order from a most significant bit transistor to a least significant bit transistor having an emitter voltage and an emitter area, said method compensating for conversion errors caused by temperature-dependent variances in said parallel-connected transistor base-emitter voltages and comprising the steps of:
- A. Generating a current which is proportional to absolute temperature by applying across a series-connected diode chain including a number of diode junctions, a voltage equal to a diode voltage drop times said number of diode junctions in said diode chain;
- B. Passing said current through said digital-to-analog converter so that the total current passing through said digital-to-analog converter is equal to said current;
- and
- C. Connecting in parallel with said parallel connected current source transistors a compensating transistor having an emitter and an emitter area which is larger than said emitter area of said least significant bit transistor, the emitter of said compensating transistor being connected to said least significant bit end of said R-2R network by a compensating resistor of value 2R to inject a compensating current and wherein a compensating voltage at said compensating transistor emitter has a magnitude of 2(kT/q)ln 2 volts greater than said least significant bit transistor emitter voltage.
In an improved digital-to-analog converter of the type having a plurality of parallel-connected transistors with emitters connected to an R-2R resistor network having both series resistors and shunt resistors to cause the transistors to operate as binary-weighted current sources, said transistors being arranged in order from a most significant bit to a least significant bit, the improvement for compensating for errors introduced by varying emitter currents, said improvement comprising:
- means for producing a current which is proportional to absolute temperature, said digital-to-analog converter being connected to said current producing means so that the total current running through said digital-to-analog converter is proportional to absolute temperature;
- means including a transistor having an emitter area which is larger than the emitter area of the least significant bit transistor for applying to said R-2R resistor network at the least significant bit end a compensating current, said compensating current being generated by connecting a resistor of value 2R to said R-2R network and applying to said resistor a compensating voltage with a magnitude of 2(kT/q) ln 2 volts greater than the base-emitter voltage of the least significant bit transistor.
A digital-to-analog converter comprising:
- a R-2R resistor network comprising shunt and series resistors;
- a plurality of parallel-connected transistors with emitters connected to said resistor network, said transistors operating as current sources and being arranged in order from a most significant bit transistor to a least significant bit transistor;
- means for drawing a current which is proportional to absolute temperature through said plurality of transistors and said network so that the total current flowing through said digital-to-analog converter is proportional to absolute temperature;
- and
- compensating means connected to said network to develop a flow of compensating current through said series resistors said compensating current having a magnitude to produce across each of said series resistors a compensating voltage at least substantially equal to the difference between the base-emitter voltages of successive transistor current sources.
Dependencies are inferred from the claim text.