Data multiplexing in mixed-signal circuitry

Details Data multiplexing in mixed-signal circuitry Data multiplexing in mixed-signal circuitry

2000-08-08

2001-04-17

Young, Brian (Department: 2819)

Coded data generation or conversion

Analog to or from digital conversion

Digital to analog conversion

C341S141000

Reexamination Certificate

active

06218974

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to data multiplexing techniques for use in mixed-signal circuitry and integrated circuit devices, for example digital-to-analog converters (DACs). Such mixed-signal circuitry and devices include a mixture of digital circuitry and analog circuitry.
2. Description of the Related Art
FIG. 1
of the accompanying drawings shows parts of a conventional DAC integrated circuit (IC) 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
contains analog circuitry including 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 analog circuitry 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 digital 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 digital circuitry including 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
of the accompanying drawings 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.
However, when it is desired to operate such a DAC at very high speeds (for example 100 MHz or more), it is found that glitches may occur at one or both of the first and second connection lines A and B, producing a momentary error in the DAC analog output signal V
A
−V
B
. These glitches in the analog output signal may be code-dependent and result in harmonic distortion or even non-harmonic spurs in the output spectrum. Some of the causes of these glitches have been determined to be as follows.
Firstly, the digital circuitry (the binary-thermometer decoder
6
and other digital circuits) is required to switch very quickly and its gate count is quite high. Accordingly, the current consumption of the digital circuitry could be as much as 20 mA per 100 MHz at high operating speeds. This combination of fast switching and high current consumption inevitably introduces a high degree of noise into the power supply lines. Although it has previously been considered to separate the power supplies for the analog circuitry (e.g. the current sources
2
1
to
2
n
and differential switching circuits
4
1
to
4
n
in
FIG. 1
) from the power supplies for the digital circuitry, this measure alone is not found to be wholly satisfactory when the highest performance levels are required. In particular, noise arising from the operation of the binary-thermometer decoder
6
can lead to skew in the timing of the changes in the thermometer-coded signals T
1
to Tn in response to different changes in the digital input word D
1
to Dm. For example, it is estimated that the skew may be several hundreds of picoseconds. This amount of skew causes significant degradation of the performance of the DAC and, moreover, being data-dependent, the degradation is difficult to predict.
Secondly, in order to reduce the skew problem mentioned above, it may be considered to provide a set of latch circuits, corresponding respectively to the thermometer-coded signals T
1
to Tn, between the digital circuitry and the analog circuitry, which latches are activated by a common timing signal such that the outputs thereof change simultaneously. However, surprisingly it is found that this measure alone is not wholly effective in removing skew from the thermometer-coded signals. It is found, for example, that data-dependent jitter still remains at the outputs of the latch circuits and that the worst-case jitter increases in approximate proportion to the number of thermometer-coded signals. Thus, with (say) 64 thermometer-coded signals the worst-case jitter may be as much as 20 picoseconds which, when high performance is demanded, is excessively large.
These problems have been addressed in our copending U.S. applications Ser. Nos. 09/227,201 and 09/382,459 (corresponding respectively to United Kingdom patent publication nos. GB-A-2335097 and GB-A-2341287), the entire contents of which are incorporated herein by reference, which disclose DACs having the configuration as shown in
FIG. 3
of the accompanying drawings. The
FIG. 3
circuitry is divided into three sections: a digital section, a latch section and an analog section. The latch section is interposed between the digital and analog sections.
The digital section comprises decoder circuitry
10
, which is connected to other digital circuitry (not shown) to receive an m-bit digital input word D
1
~Dm. The decoder circuitry
10
has an output stage made up of n digital circuits DC
1
to DCn which produce respectively thermometer-coded signals T
1
to Tn based on

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