Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2000-12-28
2003-03-18
Wamsley, Patrick (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S120000, C341S129000
Reexamination Certificate
active
06535156
ABSTRACT:
FIELD OF THE INVENTION
The field of invention relates generally to electronic circuit design; and, more specifically, to a method and apparatus for a folding analog-to-digital converter (ADC) having a coarse decoder with reduced complexity.
BACKGROUND
FIG. 1
shows a folding analog-to-digital converter (ADC) circuit
100
. A folding ADC circuit typically includes a fine ADC
101
, a fine decoder
102
, a coarse ADC
103
, a coarse decoder
104
and a mapping unit
105
. The coarse ADC
103
typically comprises a “tree” of comparators
150
that each have a corresponding threshold voltage.
The threshold voltages increase with each step up the tree (i.e., TH
1
<TH
2
<TH
3
<TH
4
). As the amplitude of the analog input signal
106
increases, the number of comparators in the tree
150
that indicate the analog signal
106
has surpassed their corresponding threshold voltage will increase. For example, if the amplitude of the analog signal
106
is between the first threshold TH
1
and the second threshold TH
2
, only the first comparator
107
will provide a logical “1”, all others will provide a logical zero.
However, if the amplitude of the analog input signal
106
is between the third threshold TH
3
and the fourth threshold TH
4
, the first through third comparators
107
through
109
will provide a logical “1”. Only the highest comparator
110
in the tree
150
will provide a logical zero. The output signal format of the coarse ADC
103
is referred to as “thermometer code” because the number of logical “1s” in the coarse ADC output signal increases in an “upward” direction, similar to the mercury in a thermometer, as the amplitude of the analog signal
106
grows.
The coarse ADC
103
is deemed coarse because the threshold voltages TH
1
through TH
4
are spaced farther apart than those associated with the fine ADC
101
. That is, the coarse ADC
103
is typically designed to help identify which of N measurement regions the input signal
106
amplitude is within. The measurement regions are spaced across a range &Dgr;V
ts
of the input signal
106
being measured. The comparator tree
150
has a comparator (and a corresponding threshold) for N−1 of the N measurement regions.
For example, as seen in the exemplary coarse ADC transfer curve
112
of
FIG. 1
, note that seven regions exist but only five measurement regions exist. Regions
0
and
6
correspond to underflow and overflow regions, respectively, where changes in input signal amplitude are not measured. That is, the thermometer output signal for both ADCs
103
and
101
corresponds to all “0s” in the underflow region
0
and all “1s” in the overflow region
6
.
In the exemplary coarse ADC transfer curve
112
of
FIG. 1
, the measurement range of the input signal &Dgr;Vts is divided into five measurement regions (regions
1
through
5
). As such, in this example, N=5 . Thus, the coarse ADC
103
has N−1=4 comparators
107
through
110
and threshold voltages TH
1
through TH
4
. If the input signal
106
amplitude reaches a level just above TH
1
, the output of the first comparator
107
is a logical “1” while all other comparator outputs are a logical “0”.
If the input signal
106
amplitude reaches a level just above TH
3
, comparators
107
through
109
will provide a logical “1” while comparator
110
will provide a logical “0”. Thus the coarse ADC
103
helps identify which measurement region the input signal
106
amplitude corresponds to. In the former case above, the coarse ADC
103
indicates the input signal
106
amplitude is within region
2
; while, in the latter case above, the coarse ADC
103
indicates the input signal
106
amplitude is within region
4
.
An exemplary transfer function
113
for the fine ADC
101
is also shown in FIG.
1
. Note that the fine ADC transfer function
113
may be viewed as being “folded” over the N measurement regions discussed above. That is, the transition from a lower region into a higher region causes the transfer function
113
to “fold” by changing its slope. The fine ADC
101
provides for finer measurement resolution because each measurement region is resolved to n different levels. This corresponds to an overall measurement resolution of &Dgr;V
ts
/(nN).
For example, the exemplary folded ADC transfer function
113
of
FIG. 1
has: 1) seven regions (regions
0
through
6
); 2) five measurement regions (i.e., N=5 where the measurement regions correspond to regions
1
through
5
as discussed above); 3) an input signal range &Dgr;Vts; and 4) resolution of the input signal to thirty two different levels within each measurement region (i.e., n=32). This corresponds to an overall input measurement resolution of &Dgr;V
ts
/160 (where nN=32×5=160).
Each level within a measurement region has a corresponding output bit in the fine ADC
101
output signal. Thus, the input signal
106
amplitude within a particular measurement region is indicated by a rising or falling thermometer signal depending on which measurement region the input signal strength corresponds to.
For example, if the input voltage is within region
0
, the fine ADC
101
output signal corresponds to thirty two zeros (i.e., 000 . . . 000 where all 26 bits replaced by the ellipsis are zero) which, as mentioned above, corresponds to the underflow region. Once the input voltage rises above Vo and surpasses Vo+&Dgr;V
ts
/160 (i.e., enters region
1
), the least significant bit of the fine ADC
101
output signal flips to a “1” (i.e., the fine ADC
101
output signal is 000 . . . 001 where all 26 bits replaced by the ellipsis are a logical 0).
If the input signal voltage continues to increase, the fine ADC
101
thermometer signal rises because the slope of the transfer curve is positive within region
1
. When the input voltage surpasses Vo+16&Dgr;V
ts
/160 the thermometer output signal will rise past “halfway” up the slope of the transfer function
113
within the first region. That is, an input signal
106
amplitude just above Vo+16&Dgr;V
ts
/160 converts sixteen of the fine ADC's thirty two output bits into logical “1s” (i.e., 000 . . . 111 where of the 26 bits replaced by the ellipsis, the 13 most significant bits are a logical 0 and the
13
least significant bits are a logical 1). This corresponds to point
114
in FIG.
1
.
If the input signal amplitude
106
continues to increase, eventually, the input signal
106
will surpass Vo+32&Dgr;V
ts
/160. That is, an input signal
106
. amplitude just above Vo+32&Dgr;V
ts
/160 converts all thirty two of the fine ADC's output bits into logical “1s” (i.e., 111 . . . 111 where all 26 bits replaced by the ellipsis are a logical 1). This corresponds to point
115
in FIG.
1
.
If the input signal continues to increase beyond the transfer curve fold observed at point
115
, the thermometer output signal begins to “drop” because the transfer curve has a negative slope within region
2
. Thus, if the input signal further increases to surpass TH
1
+16&Dgr;V
ts
/160 (where TH
1
=Vo+32&Dgr;V
ts
/160), the fine ADC
101
output signal will fall “halfway” down the slope of the transfer function
113
within the second region.
In an embodiment, the input signal
106
voltage will convert the least significant sixteen of the thirty two fine ADC output bits from logical “1s” to logical “0s” (i.e., 111 . . . 000 where of the 26 bits replaced by the ellipsis, the 13 most significant bits are a logical 1 and the 13 least significant bits are a logical 0). This corresponds to point
116
in FIG.
1
. Eventually, if the input signal
106
rises to just above TH
1
+32&Dgr;V
ts
/160, the thermometer signal will be all “0s”. This corresponds to point
117
in FIG.
1
.
Note that, whereas in region
1
the fine ADC
101
thermometer output signal may be viewed as continually adding a 1 against a backdrop of 0s for each rise in input signal amplitude, in region
2
the fine ADC
101
thermometer output signal may be viewed as continually adding a
Gyurics Tunde
Wang Dong
Blakely , Sokoloff, Taylor & Zafman LLP
Intel Corporation
Wamsley Patrick
LandOfFree
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