Dynamic current mismatch accumulation schemes for digital-to-analog converters
Techniques and apparatus for determining dynamic current mismatches in a current-steering digital-to-analog converter (DAC) are provided. One example technique generally includes accumulating current mismatches between a DAC cell of a plurality of DAC cells and a reference cell using a capacitive element and changing a polarity of the capacitive element during the accumulating. The timing of the accumulating may be controlled such that a static current mismatch between the DAC cell and the reference cell is at least reduced and a dynamic current mismatch between the DAC cell and the reference cell is enhanced.
1 . A digital-to-analog converter (DAC) circuit comprising:
a plurality of DAC cells, each DAC cell comprising a current source, a first switch coupled between the current source and a DAC output node, and a second switch coupled between the current source and a complementary DAC output node;
a reference cell comprising a reference current source, a third switch coupled between the reference current source and the complementary DAC output node, and a fourth switch coupled between the reference current source and the DAC output node;
a capacitive element;
an integrator comprising a first input coupled to a first terminal of the capacitive element and a second input coupled to a second terminal of the capacitive element;
a first set of switches configured to selectively couple the first terminal of the capacitive element to the DAC output node and the second terminal of the capacitive element to the complementary DAC output node; and
a second set of switches configured to selectively couple the second terminal of the capacitive element to the DAC output node and the first terminal of the capacitive element to the complementary DAC output node.
2 . The DAC circuit of claim 1 , wherein the reference cell is one of the plurality of DAC cells.
3 . The DAC circuit of claim 1 , wherein during a first phase:
the first switch, the third switch, and the second set of switches are configured to be closed;
the second switch, the fourth switch, and the first set of switches are configured to be open; and
the capacitive element is configured to accumulate a first static current mismatch between the current source of the DAC cell and the reference current source.
4 . The DAC circuit of claim 3 , wherein during a second phase:
the second switch, the third switch, and the second set of switches are configured to be closed;
the first switch, the fourth switch, and the first set of switches are configured to be open; and
the capacitive element is configured to accumulate a first dynamic current mismatch between the current source of the DAC cell and the reference current source.
5 . The DAC circuit of claim 4 , wherein during a third phase:
the second switch, the fourth switch, and the second set of switches are configured to be closed;
the first switch, the third switch, and the first set of switches are configured to be open; and
the capacitive element is configured to accumulate a second static current mismatch between the current source of the DAC cell and the reference current source, the second static current mismatch having opposite polarity to the first static current mismatch that at least reduces an effect of the first static current mismatch.
6 . The DAC circuit of claim 5 , wherein during a fourth phase:
the second switch, the fourth switch, and the first set of switches are configured to be closed;
the first switch, the third switch, and the second set of switches are configured to be open; and
the capacitive element is configured to accumulate a third static current mismatch between the current source of the DAC cell and the reference current source.
7 . The DAC circuit of claim 6 , wherein during a fifth phase:
the first switch, the fourth switch, and the first set of switches are configured to be closed;
the second switch, the third switch, and the second set of switches are configured to be open; and
the capacitive element is configured to accumulate a second dynamic current mismatch between the current source of the DAC cell and the reference current source, the first dynamic current mismatch and the second dynamic current mismatch forming a total dynamic current mismatch for the DAC cell.
8 . The DAC circuit of claim 7 , wherein during a sixth phase:
the first switch, the third switch, and the first set of switches are configured to be closed;
the second switch, the fourth switch, and the second set of switches are configured to be open; and
the capacitive element is configured to accumulate a fourth static current mismatch between the current source of the DAC cell and the reference current source, the fourth static current mismatch having opposite polarity to the third static current mismatch that at least reduces an effect of the third static current mismatch.
9 . The DAC circuit of claim 1 , wherein the first set of switches and the second set of switches are configured to operate such that during a period for determining a total dynamic current mismatch for the DAC cell, the capacitive element has a first polarity for one half of the time in the period and a second polarity for another half of the time in the period.
10 . The DAC circuit of claim 1 , wherein the first switch is configured to be controlled by a first clock signal and wherein the second set of switches is configured to be controlled by a second clock signal having a quadrature relationship with the first clock signal.
11 . A method of digital-to-analog converter (DAC) calibration, comprising:
accumulating current mismatches between a DAC cell of a plurality of DAC cells and a reference cell using a capacitive element; and
changing a polarity of the capacitive element during the accumulating.
12 . The method of claim 11 , wherein timing of the accumulating is controlled such that a static current mismatch between the DAC cell and the reference cell is at least reduced and a dynamic current mismatch between the DAC cell and the reference cell is enhanced.
13 . The method of claim 12 , wherein the accumulating comprises using an integrator including differential inputs coupled across the capacitive element and wherein the timing of the accumulating is controlled such that the dynamic current mismatch between the DAC cell and the reference cell is integrated by the integrator.
14 . The method of claim 11 , wherein changing the polarity of the capacitive element comprises:
opening a first set of switches selectively coupling a first terminal of the capacitive element to a DAC output node and a second terminal of the capacitive element to a complementary DAC output node; and
closing a second set of switches selectively coupling the second terminal of the capacitive element to the DAC output node and the first terminal of the capacitive element to the complementary DAC output node.
15 . The method of claim 14 , wherein during the accumulating and for a period for determining a total dynamic current mismatch for the DAC cell, the first set of switches and the second set of switches are controlled to operate such that the capacitive element has a first polarity for one half of the time in the period and a second polarity for another half of the time in the period.
16 . The method of claim 11 , wherein during a first phase of the accumulating, the accumulating comprises controlling a plurality of switches to accumulate, on the capacitive element, a first static current mismatch between a current source included in the DAC cell and a reference current source included in the reference cell.
17 . The method of claim 16 , wherein during a second phase of the accumulating, the accumulating comprises controlling the plurality of switches to accumulate, on the capacitive element, a first dynamic current mismatch between the current source included in the DAC cell and the reference current source included in the reference cell.
18 . The method of claim 17 , wherein during a third phase of the accumulating, the accumulating comprises controlling the plurality of switches to accumulate, using the capacitive element, a second static current mismatch between the current source included in the DAC cell and the reference current source included in the reference cell, the second static current mismatch having opposite polarity to the first static current mismatch that at least reduces an effect of the first static current mismatch.
19 . The method of claim 18 , wherein during a fourth phase of the accumulating, the accumulating comprises controlling the plurality of switches to accumulate, using the capacitive element, a third static current mismatch between the current source and the reference current source.
20 . The method of claim 19 , wherein:
during a fifth phase of the accumulating, the accumulating comprises controlling the plurality of switches to accumulate, using the capacitive element, a second dynamic current mismatch between the current source and the reference current source, the first dynamic current mismatch and the second dynamic current mismatch forming a total dynamic current mismatch for the DAC cell; and
during a sixth phase of the accumulating, the accumulating comprises controlling the plurality of switches to accumulate, using the capacitive element, a fourth static current mismatch between the current source and the reference current source, the fourth static current mismatch having opposite polarity to the third static current mismatch that at least reduces an effect of the third static current mismatch.