Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2001-11-08
2003-07-29
Williams, Howard L. (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
C341S154000
Reexamination Certificate
active
06600439
ABSTRACT:
BACKGROUND
The present application describes systems and techniques relating to reference voltage circuits for differential analog-to-digital converters (ADCs), for example, flash ADCs used in image sensors. An ADC is an electronic circuit that measures an analog signal, which typically represents some real-world phenomenon (e.g., temperature, pressure, incident light, acceleration, speed), and converts it to a digital signal by comparing the analog signal to a known reference voltage.
FIG. 1
is a functional block diagram illustrating a prior art design
100
for providing a differential reference voltage for an ADC. The design
100
uses a resistive ladder
102
with a differential analog input reference that may be provided by voltage controlled current sources
104
. The design includes circuits with either non-regulated or regulated high and low voltages V
ADC
—
HIGH
, V
ADC
—
LOW
set up around a common-mode voltage. The input references may be regulated by voltage regulators
106
,
108
, according to reference input voltages V
REF
—
HIGH
, V
REF
—
LOW
FIG. 2
is a schematic illustrating an example circuit
200
implementing the prior art design
100
shown in FIG.
1
. In
FIG. 2
, an amplifier AMP, regulates the voltage V
ADC
—
HIGH
according to a reference input V
REF
—
HIGH
. An amplifier AMP
2
regulates the voltage V
ADC
—
LOW
according to a reference input V
REF
—
LOW
. Resistors R
1
, R
2
, R
3
, R
4
constitute a resistive ladder
202
for the ADC. A capacitor C
3
is a differentially connected decoupling capacitor for the ladder,
202
. Capacitors C
1
and C
2
are phase compensating capacitors for regulators
206
,
208
, respectively.
SUMMARY
In one aspect, a reference voltage circuit for an analog-to-digital converter includes a resistive ladder having an even number of resistors and a midpoint between the resistors and a reference voltage source coupled to the midpoint. The resistive ladder may include four, eight, or more resistors. The reference voltage source may be a regulated voltage source that will accept both input and output currents. The reference voltage source may differ from a common-mode voltage of the analog input signal to the analog-to-digital converter by a small amount, such as by less than one half a least significant bit (½ LSB) of the analog-to-digital converter.
The reference voltage circuit may also include a reference resistor coupled with the reference voltage source, a voltage-controlled current source coupled with the reference resistor, a voltage regulator coupled with the first voltage-controlled current source, and a low voltage source coupled to the voltage regulator. The voltage regulator may be an amplifier coupled in a feedback loop with the voltage controlled current source. The reference voltage circuit may also include a current mirror coupled with a power supply, the voltage regulator, and the resistive ladder, wherein the current mirror scales current in the resistive ladder by a factor of eight. Resistance matching may be provided between the reference resistor and a total combined series resistance of the resistive ladder.
In another aspect, an analog-to-digital converter may be manufactured, such as on a semiconductor substrate, to include a reference voltage circuit having a resistive ladder and a reference voltage source connected with a midpoint of the resistive ladder. The analog-to-digital converter may also include a plurality of comparators coupled with the resistive ladder, and a digital output encoder.
In another aspect, a method includes forming on a substrate, a resistive ladder having an even number of resistors and a midpoint between the resistors, and connecting a reference voltage source to the midpoint. The method may further include forming a reference resistor on the substrate, forming a first voltage-controlled current source on the substrate, forming a voltage regulator on the substrate, connecting the reference resistor with the reference voltage source, connecting the first voltage-controlled current source with the reference resistor, and connecting the voltage regulator with the first voltage-controlled current source and with a low voltage source. The method may also include connecting an amplifier in a feedback loop with the first voltage-controlled current source, where the amplifier is the voltage regulator.
In another aspect, a method includes applying a reference voltage to a midpoint of a resistive ladder in a circuit to create a plurality of reference voltages for an analog-td-digital converter. The method may further include providing a power source for the circuit, and providing a ground for the circuit. The method may also include providing an analog input to a plurality of comparators coupled with the resistive ladder to generate a digital output.
REFERENCES:
patent: 4496935 (1985-01-01), Inoue et al.
patent: 4578715 (1986-03-01), Yamaguchi
patent: 6087970 (2000-07-01), Panicacci
patent: 6191714 (2001-02-01), Langenbacher
patent: 6211804 (2001-04-01), Kaplinsky
patent: 6215428 (2001-04-01), Fossum
patent: 6229469 (2001-05-01), Gaudet
patent: 6295013 (2001-09-01), Barna et al.
patent: 62-111525 (1987-05-01), None
Dickstein Shapiro Morin & Oshinsky
Micro)n Technology, Inc.
Williams Howard L.
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