Cell array circuitry

Details Cell array circuitry Cell array circuitry

1998-08-21

2001-05-22

Tokar, Michael (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Digital to analog conversion

C341S153000

Reexamination Certificate

active

06236346

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
The subject application is related to the following U.S. and respective foreign priority applications:
DIFFERENTIAL SWITCHING CIRCUITRY filed concurrently and claiming priority from Great Britain application No. 9800387.4 filed Jan. 8, 1998 assigned to Fujitsu Microelectronics Limited;
THERMOMETER CODING CIRCUITRY filed concurrently and claiming priority from Great Britain application No. 9800384.1 filed Jan. 8, 1998 assigned to Fujitsu Microelectronics Limited;
ELECTROSTATIC DISCHARGE PROTECTION IN SEMICONDUCTOR DEVICES filed concurrently and claiming priority from Great Britain application No. 9804588.3 filed Mar. 4, 1998 assigned to Fujitsu Limited; and
MIXED-SIGNAL CIRCUITRY AND INTEGRATED CIRCUIT DEVICES filed concurrently and claiming priority from Great Britain application No. 9804587.5 filed Mar. 4, 1998 assigned to Fujitsu Limited.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cell array circuitry for use, for example, in digital-to-analog converters (DACs).
2. Description of the Related Art
FIG. 1
of the accompanying drawings shows parts of a conventional digital-to-analog converter (DAC) of the so-called “current-steering” type. The DAC
1
is designed to convert an m-bit digital input word (D
1
-Dm) into a corresponding analog output signal.
The DAC
1
includes a plurality (n) of identical current sources
2
1
to
2
n
where n=2
m
−1. Each current source
2
passes a substantially constant current I. The DAC
1
further includes a plurality of differential switching circuits
4
1
to
4
n
corresponding respectively to the n current sources
2
1
to
2
n
. Each differential switching circuit
4
is connected to its corresponding current source
2
and switches the current I produced by the current source either to a first terminal, connected to a first connection line A of the converter, or a second terminal connected to a second connection line B of the converter.
Each differential switching circuit
4
receives one of a plurality of control signals T
1
to Tn (called “thermometer-coded signals” for reasons explained hereinafter) and selects either its first terminal or its second terminal in accordance with the value of the signal concerned. A first output current I
A
of the DAC
1
is the sum of the respective currents delivered to the differential-switching-circuit first terminals, and a second output current I
B
of the DAC
1
is the sum of the respective currents delivered to the differential-switching-circuit second terminals.
The analog output signal is the voltage difference V
A
-V
B
between a voltage V
A
produced by sinking the first output current I
A
of the DAC
1
into a resistance R and a voltage V
B
produced by sinking the second output current I
B
of the converter into another resistance R.
In the
FIG. 1
DAC the thermometer-coded signals T
1
to Tn are derived from the binary input word D
1
-Dm by a binary-thermometer decoder
6
. The decoder
6
operates as follows.
When the binary input word D
1
-Dm has the lowest value the thermometer-coded signals T
1
-Tn are such that each of the differential switching circuits
4
1
to
4
n
selects its second terminal so that all of the current sources
2
1
to
2
n
are connected to the second connection line B. In this state, V
A
=0 and V
B
=nIR. The analog output signal V
A
-V
B
=−nIR.
As the binary input word D
1
-Dm increases progressively in value, the thermometer-coded signals T
1
to Tn produced by the decoder
6
are such that more of the differential switching circuits select their respective first terminals (starting from the differential switching circuit
4
1
) without any differential switching circuit that has already selected its first terminal switching back to its second terminal. When the binary input word D
1
-Dm has the value i, the first i differential switching circuits
4
1
to
4
i
select their respective first terminals, whereas the remaining n−i differential switching circuits
4
i+1
to
4
n
select their respective second terminals. The analog output signal V
A
-V
B
is equal to (2i−n)IR.
FIG. 2
shows an example of the thermometer-coded signals generated for a three-bit binary input word D
1
-D
3
(i.e. in this example m=3). In this case, seven thermometer-coded signals T
1
to T
7
are required (n=2
m
−1=7).
As
FIG. 2
shows, the thermometer-coded signals T
1
to Tn generated by the binary-thermometer decoder
6
follow a so-called thermometer code in which it is known that when an rth-order signal Tr is activated (set to “1”), all of the lower-order signals T
1
to Tr−1 will also be activated.
Thermometer coding is popular in DACs of the current-steering type because, as the binary input word increases, more current sources are switched to the first connection line A without any current source that is already switched to that line A being switched to the other line B. Accordingly, the input/output characteristic of the DAC is monotonic and the glitch impulse resulting from a change of 1 in the input word is small.
It will be appreciated that the number of current sources
2
and corresponding differential switching circuits
4
in the
FIG. 1
architecture is quite large, particularly when m is greater than or equal to 6. When m=6, for example, n=63, and 63 current sources and 63 differential switching circuits are required. In order to deal with such a large number of current sources, and to enable the thermometer signals to be delivered efficiently to the different differential switching circuits, it has been proposed to arrange the current sources and differential switching circuits as a two-dimensional array of cells, each cell including one current source and its associated differential switching circuit. This arrangement is shown in FIG.
3
.
In
FIG. 3
, 64 cells CL
ij
are arranged in an 8×8 square array having eight rows and eight columns. In
FIG. 3
, the first digit of the suffix applied to each cell denotes the row in which the cell is located and the second digit of the suffix denotes the column in which the cell is located. Thus, the cell CL
18
is the cell in row
1
, column
8
.
Each cell CL
ij
includes its own current source
2
and its own differential switching circuit
4
. The respective first terminals of the cells of the array are connected together to a first connection line A of the DAC and the respective second terminals of the cells of the array are connected together to a second connection line B of the DAC, as in the
FIG. 1
DAC.
In order to avoid having to generate and supply different respective thermometer-coded signals to all the cells of the array, a two-stage decoding process is adopted to convert the binary input word D
1
-D
6
into the respective thermometer-coded control signals T required by the differential switching circuits
4
in the different cells. The first stage of this two-stage decoding process is carried out by respective row and column decoders
12
and
14
, and the second stage is carried out by a local decoder
16
provided for each cell.
The three lower-order bits D
1
-D
3
of the binary input word are applied to the column decoder
14
which derives therefrom seven thermometer-coded column selection signals in accordance with FIG.
2
. The row decoder
12
receives the three higher-order bits D
4
-D
6
of the input word and derives therefrom seven thermometer-coded row selection signals, also in accordance with FIG.
2
. The row and column selection signals are distributed to the cells of the array.
In each cell the local decoder
16
combines the row and column selection signals to derive therefrom the required local control signal T for the differential switching circuit
4
of the cell concerned. In practice, the local decoder
16
in each cell does not need to employ all seven row and column selection signals to produce the required local control signal T. This is because, for any digital input word, the rows of the

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