Methods and apparatus to control current steering digital to analog converters
Methods and apparatus to control current steering digital to analog converters are described herein. In one example, a digital to analog converter includes a first unit cell including a positive output and a negative output, wherein the positive output of the first unit cell and the negative output of the first unit cell comprise substantially equal magnitudes and wherein the positive and negative outputs of the first unit cell are substantially one hundred eighty degrees out of phase; and a second unit cell including a positive output and a negative output, wherein the positive output of the second unit cell is substantially zero when the negative output of the second unit cell is non-zero.
1. A digital-to-analog converter (DAC) comprising:
a source follower voltage supply providing a bias voltage on a bias node;
a first unit cell including:
a first current source coupled to the bias node;
first and second switches coupled to the first current source; and
first and second cascode transistors respectively coupled to the first and second switches and respectively providing a first positive output and a first negative output;
a second unit cell including:
a second current source coupled to the bias node;
third and fourth switches coupled to the second current source; and
third and fourth cascode transistors respectively coupled to the third and fourth switches and respectively providing a second positive output and a second negative output;
a decoder coupled to the first, second, third, and fourth switches, wherein the decoder controls the first and second switches such that the first positive output and the first negative output comprise substantially equal magnitudes and wherein the first positive output and the first negative output are substantially one hundred eighty degrees out of phase, and wherein the decoder controls the third and fourth switches such that the second positive output is substantially zero when the second negative output non-zero; and
a charge injection cancellation unit that is coupled to the bias node, wherein the charge injection cancellation unit includes:
a logic gate that is coupled to the decoder;
an inverter that is coupled to the logic gate;
a capacitor that is coupled to the bias node;
a fifth switch that is coupled to the capacitor and that receives the supply voltage, wherein the fifth switch is controlled by the logic gate; and
a sixth switch that is coupled between the capacitor and ground, wherein the sixth switch is controlled by the inverter.
2. The DAC as defined by claim 1 , wherein the DAC further comprises a first group of unit cells that operate in class A mode while a second group of unit cells operates in class B mode.
3. The DAC as defined by claim 1 , wherein the logic gate further comprises a NOR gate.
4. The DAC as defined by claim 1 , wherein the capacitor further comprises a MOS capacitor.