[0014]The invention provides for a clock control circuit and a method to completely remove the ISI in the DAC output waveform without any significant increase in power consumption and silicon area of the DAC. Further, the invention does not increase the requirement for slew rate and bandwidth of the amplifier as typically found in prior art.
[0015]The present invention provides for a method to eliminate inter-symbol interference error at an I-to-V output stage of a current steering DAC having a plurality of current cells, wherein the output stage comprises at least an I-to-V amplifier having summing junctions SJP and SJN, feedback resistances R1 and R2, and feedback capacitances C1 and C2. In this embodiment, the method comprises the steps of: (a) disconnecting the feedback resistors R1 and R2 and the current cells from the summing junctions SJP and SJN of the amplifier in the output stage; (b) connecting the feedback resistors R1 and R2 together to form a resistor string across outputs of the output stage, with the feedback capacitors C1 and C2 creating a hold period to hold output voltage of the I-to-V amplifier when the I-to-V amplifier is not connected to the current cells of the current steering DAC; (c) using the I-to-V amplifier as a buffer to maintain a CM level at drains of the current cells during the hold period by connecting the outputs of the current cells to midpoint of the resistor string; and changing DAC codes while outputs of the current cells are in the hold mode, and (d) disconnecting the outputs of the current cells from each other after the DAC codes are changed and re-connecting the feedback resistors and appropriate outputs of the current cells back to the summing junctions of the I-to-V converter.
[0016]The present invention also provides for an apparatus to eliminate inter-symbol interference error at an I-to-V output stage of a current steering DAC having a plurality of current cells, wherein the output stage comprises at least an I-to-V amplifier having summing junctions SJP and SJN, feedback resistances R1 and R2, and feedback capacitances C1 and C2. In this embodiment, the apparatus comprises: (a) means for disconnecting the feedback resistors R1 and R2 and the current cells from the summing junctions SJP and SJN of the amplifier in the output stage; (b) means for connecting the feedback resistors R1 and R2 together to form a resistor string across outputs of the output stage, with the feedback capacitors C1 and C2 creating a hold period to hold output voltage of the I-to-V amplifier when the where the I-to-V amplifier is not connected to the current cells of the current steering DAC; (c) means for using the I-to-V amplifier as a buffer to maintain a CM level at drains of the current cells during the hold period by connecting the outputs of the current cells to midpoint of the resistor string; and changing DAC codes while outputs of the current cells are in the hold mode, and (d) means for disconnecting the outputs of the current cells from each other after the DAC codes are changed and re-connecting the feedback resistors and appropriate outputs of the current cells back to the summing junctions of the I-to-V converter.
[0017]The present invention also provides for a circuit to eliminate inter-symbol interference error at an I-to-V output stage of a current steering DAC having a plurality of current cells, wherein the circuit comprises: (a) an I-to-V amplifier having summing junctions SJP and SJN; (b) a first feedback resistance R1; (c) a second feedback resistance R2; (d) a first feedback capacitance C1; (e) a second feedback capacitance C2; (f) a first switch S1 used to connect / disconnect R1 to the summing junction SJP and a second switch S2 used to connect / disconnect R2 to the summing junction SJN; (g) a third switch S3 used to connect / disconnect the feedback resistors R1 and R2 together and, when connected, R1 and R2 forming a resistor string across outputs of the output stage, with the feedback capacitors C1 and C2 creating a hold period to hold output voltage of the I-to-V amplifier when the I-to-V amplifier is not connected to the current cells of the current steering DAC; and wherein the I-to-V amplifier acts as a buffer to maintain a CM level at drains of the current cells during the hold period by connecting the outputs of the current cells to midpoint of the resistor string and, after DAC codes are changed, the outputs of the current cells are disconnected from each other and the feedback resistors and appropriate outputs of the current cells are reconnected back to the summing junctions of the I-to-V converter.
[0018]The present invention also provides for a circuit to eliminate inter-symbol interference error at an I-to-V output stage of a current steering DAC having a plurality of current cells, wherein the circuit comprises: (a) a first switch S1 used to connect / disconnect feedback resistance R1 to a summing junction SJP of a I-to-V amplifier; (b) a second switch S2 used to connect / disconnect feedback resistance R2 to the summing junction SJN of the I-to-V amplifier; (c) a third switch S3 used to connect / disconnect the feedback resistors R1 and R2 together and, when connected, R1 and R2 forming a resistor string across outputs of the output stage, with the feedback capacitors C1 and C2 creating a hold period to hold output voltage of the I-to-V amplifier when the I-to-V amplifier is not connected to the current cells of the current steering DAC; and wherein the I-to-V amplifier acts as a buffer to maintain a CM level at drains of the current cells during the hold period by connecting the outputs of the current cells to midpoint of the resistor string and, after DAC codes are changed, the outputs of the current cells are disconnected from each other and the feedback resistors and appropriate outputs of the current cells are reconnected back to the summing junctions of the I-to-V amplifier.