The present invention relates generally to a reference voltage source and more particularly to a reference voltage source that provides a plurality of reference voltages with equal step size between voltages despite loads connected to the reference voltage source.
A plurality of reference voltages with equal step size between voltages are often used in analog-to-digital converters (ADC) where an input voltage is compared to the reference voltages through comparators. Typically generated by passing a current I.sub.r through many serially connected resistors R.sub.r, as shown in FIG. 1, these reference voltages are produced at nodes or junctions between the resistors. Unfortunately, with comparators or other loads connected to the nodes, load currents I.sub.L are drawn from the nodes and these load currents in turn change the values of the reference voltages at the nodes.
One method of preventing variations in the reference voltages due to load currents is to connect an extra voltage source V.sub.o to one of the nodes, as shown in FIG. 2. However, in a typical monolithic analog-to-digital converter, a reference source that provides 64 or more different reference voltages would require many extra voltage sources to hold all the reference voltages constant. This would undesirably increase the power consumption and the cost and complexity of the circuit.
FIG. 3 shows a method of providing constant reference voltages by means of unequal resistor values. The value of each resistor is determined according to the amount of load current which the circuit designer expects to be drawn through that resistor. As a first order approximation, the value of a given resistor is equal to EQU R.sub.r *(1-n*(I.sub.L /I.sub.r))
where n represents the location of the resistor with respect to the current source. This method fails if the load current I.sub.L is different from the expected value, for example due to fluctuations in the manufacturing process, or if the load current changes with time, for example due to temperature variations. Another drawback of this method is that resistors, with values different from each other, having the required precision are much more difficult to implement, especially in integrated circuit technologies, than resistors with values equal to each other.
To reduce the reference voltage error due to the load currents, FIG. 4 shows another method which feeds in compensation currents, as described in U.S. Pat. No. 4,804,941, issued to Yoji Yoshii on Feb. 14, 1989. This method simulates and offsets the load currents I.sub.L by a complex active compensation circuit CC that is made of three current mirrors, each current mirror replicating the input currents of 64 comparators. This approach is complex and difficult to implement economically.
Various devices using the above methods have been known for a number of years, and by way of examples, forms of such devices can be found in IEEE Journal of Solid-State Circuits, Vol SC-17, No. 6, Dec. 1982, page 1133 to 1138.
It is apparent from the foregoing that there is still a need for an efficient reference voltage source that can provide a plurality of reference voltages with equal step size between voltages despite loads connected to the reference voltage source and that can be effectively and economically implemented with various fabrication technologies.