Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2001-06-21
2002-08-13
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
Reexamination Certificate
active
06433721
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an arrangement of a plurality of current source cells forming an integrated circuit on a semiconductor printed circuit board and a method of selecting current source cells, and more specifically to a current source cell arrangement suited for minimizing signal distortions of a current addition type Digital-to-Analog (D/A) converter, a current source cell selection method and a current addition type D/A converter.
Many electronic devices such as cellular phones use a current addition type D/A converter that converts a digital signal into an analog signal. The current addition type D/A converter, if its differential non-linear error is large, distorts a converted analog signal.
FIG. 10
is a layout diagram showing a conventional current source cell arrangement of the current addition type D/A converter. The current addition type D/A converter for mounting on LSIs is manufactured by arranging a large number of current source cells in matrix.
FIG. 10
shows a configuration of the converter having an array of 240 current source cells
1
(16 rows×15 columns) arranged in 15 column units each consisting of 16 current source cells to represent higher 4-bit MSBs (most significant bits) of an 8-bit current addition type D/A converter.
In the following explanation of the current source cell matrix, a number “a” in matrix (a, b) represents a “row number” and a number “b” represents a “column number” unless otherwise specifically stated.
When a conventional current source cell matrix of such a configuration is to be operated as a D/A converter, the following arrangements are made. For example, if one unit of the MSB cell is to be represented by a current value of 16 current source cells, “2” units of MSBs can be represented by selecting 32 current source cells. Similarly, “N” MSBs can be represented by selecting 16×N current source cells.
At this time, according to the conventional layout of the current source cells, a vertical column is selected to electrically connect the 16 current source cells. For example, (
1
,
1
) to (
16
,
1
) are selected to represent one unit of MSB and the total current value of that one unit is used as a signal output for MCELL
1
. Next, (
1
,
2
) to (
16
,
2
) are selected to represent one unit of MSB and the total current value of that one unit is used as a signal output for MCELL
2
. The similar process is carried out until the current source cells (
1
,
15
) to (
16
,
15
) are connected vertically, thus producing signals MCELL
1
to MCELL
15
. In this way, to simplify the connection of one unit of MSB, the current source cells to be summed up are arranged in one direction (column direction).
The layout of the current source cells
1
shown in
FIG. 10
are assumed to have the same capacities. If there are variations in capability among the current source cells, the capability variations are reflected on the output signals from MCELL
1
-
15
, distorting the output signals from the D/A converter.
The capability variations of the current source cells are those with a certain tendency, rather than random variations. This is considered to result from the LSI manufacturing process. When for example the weight of the current capability of the current source cell (
1
,
1
) at the upper left corner is “1”, the current capability increases at a rate of 2% in the vertical direction and at a rate of 3% in the lateral direction as indicated by figures in the cells of
FIG. 11
, exhibiting a certain tendency of variations.
In such a current source cell matrix, when the outputs of the 16 vertically arranged current source cells are electrically connected, their total current value is “18.4” in the smallest MCELL
1
and “25.12” in the largest MCELL
15
for each MSB cell
4
.
Due to the process variations that occur when the D/A converter is manufactured in the form of LSI, the current values of constitutional current source cells differ greatly between the ends of the current source cell matrix, making it impossible to secure linearity. Particularly in the current addition type D/A converter that controls the output value by the current value, a differential non-linearity (DNL) error and an integral non-linearity (INL) error, measures of its linearity characteristics, deteriorate.
Although in the above conventional example a D/A converter has been described, the same problem occurs also when a plurality of constant current sources are manufactured on semiconductor IC circuits. When constant current sources are provided on semiconductor IC circuits, because a required output current cannot be produced by one current source cell alone, outputs of a plurality of current source cells are parallelly connected to function as a constant current source with a predetermined output. However, if a large number of current source cells provided as shown in
FIG. 10
are divided into groups each consisting of a predetermined number of cells and, in each group, the output currents of the cells are added up to manufacture a plurality of constant current sources with the same outputs, it is difficult to make the outputs of these constant current sources equal, resulting in variations.
SUMMARY OF THE INVENTION
The present invention has been accomplished to solve the problems described above and provides a current source cell arrangement, a current source cell selection method and a current addition type D/A converter, in which current value errors in the current sources due to process variations are reduced by improving the layout of the current source cells to improve a linearity and therefore characteristic of the current sources.
To solve the above problem, this invention according to the first aspect of the invention provides a current source cell arrangement which comprises: a plurality of current source cells each having a predetermined current value, the current source cells being arranged in matrix; wherein two or more of the current source cells in the current source cell matrix are combined to form constant current sources each having a predetermined current value; wherein the current source cell matrix is divided into a plurality of blocks arranged symmetrically with respect to a center of the matrix; wherein the constant current sources are formed by combining equal numbers of the current source cells selected in a row or column direction from each block.
According to the second aspect of the invention, the invention provides a current source cell arrangement in which the current source cell matrix is divided point-symmetrically with respect to a center of the matrix.
According to the third aspect of the invention, the invention provides a current source cell arrangement in which the current source cell matrix is divided line-symmetrically with respect to a center of the matrix.
According to the fourth aspect of the invention, the invention provides a current source cell arrangement in which the current source cell matrix is divided radially with respect to a center of the matrix.
According to the fifth aspect of the invention, the invention provides a current source cell selection method for forming constant current sources each having a predetermined current value by combining two or more of a plurality of current source cells, wherein the plurality of the current source cells each having a predetermined current value are arranged in matrix, the current source cell selection method comprising the steps of: dividing the current source cell matrix into a plurality of blocks arranged symmetrically with respect to a center of the matrix; and selecting an equal number of the current source cells in a row or column direction from each of the divided blocks and combining them to form the constant current sources.
According to the sixth aspect of the invention, the invention provides a current source cell selection method in which the current source cell matrix is divided point-symmetrically with respect to a center of the matrix.
According to the seventh aspect of the invention, the invention provides a
Matsushita Electric - Industrial Co., Ltd.
Pearne & Gordon LLP
Young Brian
LandOfFree
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