Electronic circuits typically operate on two types of signals—digital signals and analog signals. Digital signals are binary values that are typically represented in an electronic circuit by two discrete voltage or current levels (e.g., 0 and 3.3 volts). Digital signals often represent real world phenomena using binary values that are manipulated using digital logic circuits, digital signal processing circuits, or microprocessors, for example. Analog signals are continuous in time and have a continuous range of values corresponding to real world phenomena.
Digital-to-analog converters (DACs) are used to translate digital signals into analog signals. For example, a video DAC may receive a digital representation of a video signal and translate the digital video signal into an analog video signal. FIG. 1 illustrates one stage of a DAC 100. DAC 100 may include a current source 104 generating a current I1x and switches 106 and 108. Power supply voltage Vdx 102 connects to DAC 100 to supply current and voltage, which is returned through ground 120. Digital signals are used to control multiple switching stages of the type shown in FIG. 1 to drive an output pin 110. Pin 110 serves as a connection to a load, which may include resistor R1x 112 in parallel a transmission path 114 and resistor R2x 116.
DAC 100 as shown in FIG. 1 is commonly referred to as a current steering DAC. Current I1x from current source 104 is steered by opening and closing switches 106 and 108 in response to digital signals. When switch 106 is open, switch 108 is closed, and current I1x flows to pin 110. When switch 106 is closed, switch 108 is open, and current I1x flows to ground. Because I1x is always on, current steering DACs may convert digital signals to an analog signal at pin 110 at very high speeds and are useful in video applications, for example.
DAC 100 operates as long as there is enough headroom in the supply voltage Vdx 102 so that current source 104 is approximately ideal. However, as the supply voltage Vdx 102 becomes lower, voltage drops across internal devices of DAC 100 may cause the circuit to become inoperable. For example, current source 104 and switch 108 may be implemented using three series connected transistors—one for the current source and two more for a cascode current steering stage. As the supply voltage drops, the series connected transistor may not have sufficient voltage to operate properly.
Thus, it would be advantageous to provide improved DACs that are less sensitive to variations in power supply voltages.