Digital-to-analogue converter using an array of current sources

• US 6,501,402 B2
• Filed: 07/20/2001
• Issued: 12/31/2002
• Est. Priority Date: 05/09/2001
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Boxho, Jean
• Primary Examiner(s): Wamsley, Patrick
• Application Number:
  US09/908,569
• Publication Number:
  US 20020167430A1
• Time in Patent Office: 529 Days
• Field of Search: 341/110, 341/118, 341/120, 341/144, 341/145, 341/119, 341/133, 324/130, 375/229, 326/37, 381/104
• US Class Current: 341/120
• CPC Class Codes: H03M 1/066 by continuously permuting the elements used, i.e. dynamic element matching
• CPC Class Codes: H03M 1/1004 without interrupting normal operation, e.g. by providing an additional component for temporarily replacing components to be tested or calibrated H03M1/1009, H03M1/1071 take precedence
• CPC Class Codes: H03M 1/747 with equal currents which are switched by unary decoded digital signals

Abstract

A current source DAC has calibration of the current sources used for providing the analog output. There are two outputs, one of which provides the output current or else a differential output is provided. The calibration is cyclic and the current source outputs switched to the output terminals are selected as a function of the point within the calibration cycle. The current stage of the cyclic calibration process is thus taken into account in the D/A conversion. For example, the average time since calibration for all current sources having outputs switched to the first output may be approximately equal to the average time since calibration for all current sources having outputs switched to the second output. In this way, the average current of the cells switched to one terminal is identical to the average current of the cells switched to the other terminal, and the average current of the cells switched to each terminal remains constant in time irrespective of the digital signal value being converted. This ensures DAC linearity.

Claims

1. A digital-to-analogue converter, comprising:

an array of current sources, each of said current sources having an output that is switchable onto one of first and second output terminals as a function of a digital input signal, wherein a first group of current source outputs are switched to the first output terminal regardless of being a most significant bits (MSB) or a least significant bits (LSB) current source and a second group of current source outputs are switched to the second output terminal regardless of being a MSB or LSB current source; and

a calibration unit, wherein any of said current source outputs are also switchable to said calibration unit, wherein the current sources are calibrated cyclically during a calibration cycle, and wherein the current source outputs switched to the first and second output terminals are selected as a function of a point within the calibration cycle.

2. The converter as claimed in claim 1, wherein an average time since calibration for said first group of said current sources is approximately equal to an average time since calibration for said second group of said current sources.

3. The converter as claimed in claim 2, wherein said first group of current source outputs is the current sources most recently calibrated and least recently calibrated and said second group of said current source outputs is a remaining set of the current sources, whereby an average time since calibration for all said current sources switched to said first output terminal is equal to an average time since said calibration for all said current sources switched to said second output terminal.

4. The converter as claimed in claim 1, wherein the array of current sources comprises said MSB current sources, the outputs of which are switched in dependence on the most significant bits of the digital input, and said LSB current sources, the outputs of which are switched in dependence on the least significant bits of the digital input.

5. The converter as claimed in claim 4, wherein the LSB current sources comprises first and second current sources, and wherein the output of said first and second current sources is divided to produce currents on a plurality of lines, the currents on the individual lines being switchable to the first or second output terminals under control of the least significant bits of the digital input signal.

6. The converter as claimed in claim 1, wherein each current source comprises:

a first current source transistor comprising a gate voltage controlled to provide adjustment of a total current output of the current source;

a second current source transistor comprising a fixed gate voltage, wherein the first current source transistor provides a fraction of the total current output current of the current source and the second current source transistor provides a majority of the total current output current of the current source;

a reference current source; and

a third current source transistor switching the total current output of the current source to the reference current source during calibration of the current source.

7. The converter as claimed in claim 6, further comprising:

a feedback control providing a gate voltage to the first current source transistor so that the current source provides a desired output current.

8. The converter as claimed in claim 1, wherein the calibration unit compares the output of each said current source with a reference source.

9. The converter as claimed in claim 1, wherein 14 bits are converted and wherein the array comprises 128 of said MSB current sources corresponding to 7 most significant bits and two of said LSB current sources corresponding to 7 least significant bits.

10. The converter as claimed in claim 1, wherein selecting the point within the calibration cycle enables an average time since calibration for all current sources switched to the first and second output terminals to be equal.

11. A digital-to-analogue converter, comprising:

an array of current sources, each said current source having an output that is switchable onto one of first and second output terminals as a function of a digital input signal, wherein a first group of current source outputs are switched to the first output terminal regardless of being a MSB or LSB current source and a second group of current source outputs are switched to the second output terminal regardless of being a MSB or LSB current source; and

a calibration unit that calibrates the current sources,

wherein the current source outputs are also switchable to the calibration unit, wherein the current sources are calibrated cyclically, and wherein the current source outputs switched to the first and second output terminals are selected such that an average time since calibration for said first group is approximately equal to an average time since said calibration for said second group.

12. A method of controlling a digital-to-analogue converter, the converter comprising an array of current sources, each of the current sources having an output that is switchable onto one of first and second output terminals as a function of a digital input signal, such that a first number of current source outputs are switched to the first output terminal and a second number of current source outputs are switched to the second output terminal, the method comprising the steps of:

cyclically calibrating the current sources;

switching a first subset of the current source outputs to the first terminal regardless of being a MSB or LSB current source;

switching a second subset of the current source outputs to the second output terminal regardless of being a MSB or LSB current source;

basing the switching on the digital input signal; and

selecting the first and second subsets as a function of a point within a calibration cycle.

13. The method as claimed in claim 12, further comprising the step of setting an average time since calibration for all said current sources in the first and second subsets as being approximately equal.

14. The method as claimed in claim 13, further comprising the steps of:

forming said first subset from the current sources most recently and least recently calibrated; and

forming said second subset from remaining ones of said current sources, excluding at least one of the current sources being calibrated.