Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
1999-12-14
2002-04-16
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S160000
Reexamination Certificate
active
06373423
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to flash analog-to-digital converters that use a plurality of comparators in performing analog to digital conversion.
2. Description of the Related Art
Analog-to-digital conversion is used to interface a system using analog signals capable of continuous variation to an electronic system using discrete digital signals. The reverse operation of digital-to-analog conversion may also be used.
FIG. 1
illustrates an embodiment of a typical prior art parallel analog-to-digital converter (ADC)
100
, also known as a flash converter. An analog input voltage signal (INPUT)
105
is input to a plurality of comparators
115
. Generally, a large number of comparators are used, such as
256
comparators. A reference voltage signal (REF)
110
is also input to the plurality of comparators
115
through a voltage divider tree that includes a plurality of resistors
111
coupled between the reference signal
110
and ground
112
. As shown, the positive input of each comparator
115
is the input signal
105
, and the negative input of each comparator
115
is a comparator reference signal with a voltage between REF
110
and ground
112
. The reference signal
110
applied to the negative input of each comparator
115
supplies a different comparator reference voltage according to the voltage divider tree.
The output of each comparator
115
is typically input to a latch
120
. The output of each comparator
115
is stored in the respective latch
120
upon a rising edge of the clock signal (CLK)
125
. The collective outputs of the comparators
115
, stored in the latches
120
, make up a thermometer code output
130
. The thermometer code
130
is input to a decoder
135
, sometimes referred to as an encoder. The decoder
135
decodes the thermometer code
130
into a multiple bit output
140
. The output
140
is a digital value corresponding to the input analog signal
105
.
A thermometer code
130
is typically a binary string of numbers, one binary value per comparator
115
. Assigning the convention of most significant bit (MSB) on the left and least significant bit (LSB) on the right, i.e. MSB to LSB, the thermometer code
130
represents the binary output string of the plurality of comparators
115
. The MSB is taken from the comparison between the input signal
105
and the reference signal
110
. The LSB is taken from the comparison between the input signal
105
and the ground
112
. In the thermometer code
130
, each successive digit of the code changes from a “0” to a “1” as the value of the thermometer code increases. Assuming a simplified example with seven comparators, the thermometer code and the decoded output
140
could be any one of the following:
Index
Thermometer Code
Decode
A
0000000
000
B
0000001
001
C
0000011
010
D
0000111
011
E
0001111
100
F
0011111
101
G
0111111
110
H
1111111
111
all other
unknown
Note that thermometer code A signifies an input signal
105
that is below one-seventh of the reference signal
110
. Thermometer code B signifies an input signal
105
that is above one-seventh of the reference signal
110
but below two-sevenths of the reference signal
110
. Thermometer code H signifies an input signal
105
that is above the reference signal
110
.
By convention, most ADCs
100
output the decode of thermometer code H as an error code since the output of the comparators
115
is other than an accepted value for a thermometer code
130
. Any value above the reference signal
110
is unmeasurable, i.e. there is no way to know how far that the input signal is above the reference signal
110
. An output that is not a proper thermometer code
130
is called a sparkle code, glitch or glitch error, or a misconversion error. For example, if the output of the comparators
115
were 0001011, then the decoder
135
would output an unknown code.
The reason for the error code output is that decoders
135
typically only look for the single transition from a “0” to a “1” in the thermometer code
130
. If two or more transitions occur, the decoder
135
cannot properly decode the thermometer code
130
, and hence the decoder
135
may output an incorrect or erroneous code. Incorrect thermometer codes may result from signal propagation delays in the circuit. For example, the output of the comparators
115
may be latched by the latches
120
at slightly different times, allowing for one latch
120
F to latch a “one” while latch
120
G latches a “zero”.
As described above, a flash A to D converter has a large number of comparators, and each comparator is typically required to compare the input voltage with a reference voltage that is very close to its neighboring comparator. When a flash A/D converter is implemented in silicon, additional problems can arise. In a silicon implementation, the flash A/D converter must have good resolution and low offset, meaning that the error in neighboring comparators should not compound to create sparkle codes. Also, the large number of comparators result in a large amount of chip space, power consumption and noise. Further, since it is necessary to drive the input of all of the comparators, low impedance results, as well as high frequency noise due to the capacitance of the comparators.
Therefore, an improved flash analog to digital converter is desired which provides good resolution while also using a reduced number of comparators.
SUMMARY OF THE INVENTION
The present invention comprises a flash analog-to-digital conversion system and method with a reduced number of comparators. The present invention may provide good resolution with reduced chip space and power requirements, as well as reduced noise and capacitance problems.
The system comprises a reduced plurality of comparators each coupled to receive an analog input signal, and a decoder coupled to receive the outputs of the comparators. Each comparator also receives a respective comparator reference signal for comparison with the analog input signal. Each comparator is configured to output a digital value indicative of the comparison between the analog input signal and the respective comparator reference signal. The decoder is configured to output a digital representation (a digital output signal) of the analog input signal based on the result.
In one embodiment, the analog-to-digital converter includes a dynamic reference controller coupled to the first plurality of comparators and the decoder. The dynamic reference controller is configured to dynamically output one or more dynamic reference voltages to the first plurality of comparators, wherein the first plurality of comparators are operable to dynamically receive different comparator reference voltages for comparing with the analog input signal. The dynamic reference controller is operable to dynamically change the dynamic reference voltage signal(s) to provide a sliding range voltage window for use in the analog-to-digital conversion process. The dynamic reference controller may adjust only the upper voltage or lower voltage of the sliding range voltage window to grow or shrink the window, or may adjust both the upper and lower voltages to move the sliding range voltage window within the larger range of the A/D converter. The dynamic reference controller may use the input signal, the digital output signal, or a combination of both to dynamically change the dynamic reference voltage signal(s). The dynamic reference controller preferably maintains the sliding range voltage window such that the input signal is within the voltage window. The dynamic reference controller provides information regarding the sliding range voltage window to the decoder, and the decoder uses this information to adjust the digital output accordingly.
A method for performing analog-to-digital conversion is also contemplated. In one embodiment, the method includes receiving an analog signal and comparing the analog signal with a plurality of comparator reference signals to form a thermometer code representative of the analog signal. The method uses a reduced number of compa
Conley Rose & Tayon PC
Hood Jeffrey C.
Lauture Joseph
National Instruments Corporation
Tokar Michael
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