Variable quantization ADC for image sensors

• US 7,148,831 B2
• Filed: 10/27/2003
• Issued: 12/12/2006
• Est. Priority Date: 10/27/2003
• Status: Expired due to Fees

• Current Assignee: Round Rock Research LLC
• Original Assignee: Micron Technology, Inc.
• Inventors: Krymski, Alexander
• Primary Examiner(s): NGUYEN, LINH V
• Application Number:
  US10/693,462
• Publication Number:
  US 20050088331A1
• Time in Patent Office: 1,142 Days
• Field of Search: 341/110, 341/118, 341/120, 341/122, 341/126, 341/155, 341/165, 341/161, 341/163
• US Class Current: 341/155
• CPC Class Codes: H03M 1/367 Non-linear conversion
• CPC Class Codes: H03M 1/56 Input signal compared with linear ramp

Abstract

An A/D converter suitable for use in a system in which the signal power of noise increases with the signal power of the signal, such as an imaging system, utilizes a variable quantization system for converting analog signals into digital signals. The variable quantization is controlled so that at low signal levels the quantization is similar or identical to conventional A/D converters, while the quantization level is increased at higher signal levels. Thus, higher resolution is provided at low signal levels while lower resolution is produced at high signal levels.

Claims

1. An analog to digital (A/D) converter, comprising:

a counter circuit for storing a digital word;

a ramp generator for generating a sequence of reference voltages which vary in accordance with at least a first transfer function of said digital word and a second transfer function of said digital word;

a comparator for comparing the magnitude of one of said reference voltages with a magnitude of an input signal; and

a control circuit for determining the digital word corresponding to the input signal by repeatedly;

comparing the magnitude of the input signal with the magnitude of a most recently generated reference voltage of said sequence,

incrementing said counter, and

causing said ramp generator to generate a new one of said sequence,

until the magnitude of the most recently generated reference voltage of said sequence exceeds the magnitude of said input signal.

2. The converter of claim 1, wherein said first transfer function maps each digital word stored in said counter below a first threshold to a corresponding reference signal in a linear manner.

3. The converter of claim 2, wherein said second transfer function maps a set of non-sequential and increasing digital words stored in said counter each having a magnitude at least equal to said first threshold to corresponding reference signals in a linear manner.

4. The converter of claim 2, wherein said second transfer function maps each increasing digital word stored in said counter having a magnitude at least equal to said first threshold to corresponding reference signals in a logarithmic manner.

5. The converter of claim 1, wherein said counter circuit comprises:

a controller for receiving a reset signal and a clock signal;

a register, coupled to said controller; and

a memory, coupled to said controller,

wherein;

when a clock signal is supplied to said controller, said controller reads a next value from said memory and stores said next value in said register, and

said memory stores a plurality of numbers in a non-sequential and increasing manner.

6. The converter of claim 5, wherein said memory is a non-volatile memory.

7. The converter of claim 1, wherein said counter circuit comprises:

a counter; and

circuitry for incrementing said digital word stored in said counter by one.

8. The converter of claim 1, wherein said ramp generator comprises:

a plurality of capacitor banks, each bank comprising;

a plurality of capacitors having equal capacitance,

a bank output line coupled to a first plate of each capacitor, and

a bank control circuit, coupled to a second plate of each capacitor, and for switchably coupling the second plate of any one of said capacitors to either a first voltage source or a second voltage source;

a master output line, coupled to each bank output line; and

a master controller, for sending control signal to each bank control circuit to a master voltage at said master output line to generate said sequence of voltages.

9. The converter of claim 8, wherein a capacitance of any capacitor in a first capacitor bank is different from a capacitance of any capacitor in a second capacitor bank.

10. The converter of claim 8, wherein the capacitance of any capacitor in a first capacitor bank is a power of 2 of a capacitance of any capacitor in a second capacitor bank.

11. A imaging system, comprising:

a pixel array;

a sample and hold circuit, coupled to said pixel array;

a driver, coupled to said sample and hold circuit;

an analog to digital (A/D) converter, coupled to said sample and hold circuit, said A/D converter comprising;

a counter circuit for storing a digital word;

a ramp generator for generating a sequence of reference voltages which vary in accordance with at least a first transfer function of said digital word and a second transfer function of said digital word;

a comparator for comparing the magnitude of one of said reference voltages with a magnitude of an input signal; and

a control circuit for determining the digital word corresponding to the input signal by repeatedly;

comparing the magnitude of the input signal with the magnitude of a most recently generated reference voltage of said sequence,

incrementing said counter, and

causing said ramp generator to generate a new one of said sequence,

until the magnitude of the most recently generated reference voltage of said sequence exceeds the magnitude of said input signal;

a digital processing circuit, coupled said A/D converter;

a storage circuit, coupled to said digital processing circuit; and

a control circuit, coupled to said pixel array, sample and hold circuit, driver, A/D converter, digital processing circuit, and storage circuit.

12. The imaging system of claim 11, wherein said first transfer function maps each digital word stored in said counter below said first threshold to a corresponding reference signal in a linear manner.

13. The imaging system of claim 12, wherein said second transfer function maps a set of non-sequential and increasing digital words stored in said counter each having a magnitude at least equal to said first threshold to corresponding reference signals in a linear manner.

14. The imaging system of claim 12, wherein said second transfer function maps each increasing digital word stored in said counter having a magnitude at least equal to said first threshold to corresponding reference signals in a logarithmic manner.

15. The imaging system of claim 11, wherein said counter circuit comprises:

a controller for receiving a reset signal and a clock signal;

a register, coupled to said controller; and

a memory, coupled to said controller,

wherein;

when a clock signal is supplied to said controller, said controller reads a next value from said memory and stores said next value in said register, and

said memory stores a plurality of numbers in a non-sequential and increasing manner.

16. The imaging system of claim 15, wherein said memory is a non-volatile memory.

17. The imaging system of claim 11, wherein said counter circuit comprises:

a counter; and

circuitry for incrementing said digital word stored in said counter by one.

18. The imaging system of claim 11, wherein said ramp generator comprises:

a plurality of capacitor banks, each bank comprising;

a plurality of capacitors having equal capacitance,

a bank output line, coupled to a first plate of each capacitor, and

a bank control circuit, coupled to a second plate of each capacitor, and for switchably coupling the second plate of any one of said capacitors to either a first voltage source or a second voltage source;

a master output line, coupled to each bank output line; and

a master controller, for sending control signal to each bank control circuit to a master voltage at said master output line to generate said sequence of voltages.

19. The imaging system of claim 18, wherein a capacitance of any capacitor in a first capacitor bank is different from a capacitance of any capacitor in a second capacitor bank.

20. The imaging system of claim 18, wherein the capacitance of any capacitor in a first capacitor bank is a power of 2 of a capacitance of any capacitor in a second capacitor bank.

21. A processor based system, comprising:

a bus;

a processor coupled to said bus;

a imaging subsystem, coupled to said bus;

wherein said imaging subsystem comprises;

a pixel array;

a sample and hold circuit, coupled to said pixel array;

a driver, coupled to said sample and hold circuit;

an analog to digital (A/D) converter, coupled to said sample and hold circuit, said A/D converter comprising;

a counter circuit for storing a digital word;

a ramp generator for generating a sequence of reference voltages which vary in accordance with at least a first transfer function of said digital word and a second transfer function of said digital word;

a comparator for comparing the magnitude of one of said reference voltages with a magnitude of an input signal; and

a first control circuit for determining the digital word corresponding to the input signal by repeatedly;

comparing the magnitude of the input signal with the magnitude of a most recently generated reference voltage of said sequence,

incrementing said counter, and

causing said ramp generator to generate a new one of said sequence,

until the magnitude of the most recently generated reference voltage of said sequence exceeds the magnitude of said input signal;

a digital processing circuit, coupled said A/D converter;

a storage circuit, coupled to said digital processing circuit; and

a second control circuit, coupled to said pixel array, sample and hold circuit, driver, A/D converter, digital processing circuit, and storage circuit.

22. The system of claim 21, wherein said first transfer function maps each digital word stored in said counter below said first threshold to a corresponding reference signal in a linear manner.

23. The system of claim 22, wherein said second transfer function maps a set of non-sequential and increasing digital words stored in said counter each having a magnitude at least equal to said first threshold to corresponding reference signals in a linear manner.

24. The system of claim 22, wherein said second transfer function maps each increasing digital word stored in said counter having a magnitude at least equal to said first threshold to corresponding reference signals in a logarithmic manner.

25. The system of claim 21, wherein:

said counter circuit comprises;

a controller for receiving a reset signal and a clock signal;

a register, coupled to said controller; and

a memory, coupled to said controller;

when a clock signal is supplied to said controller, said controller reads a next value from said memory and stores said next value in said register; and

said memory stores a plurality of number in a non-sequential and increasing manner.

26. The system of claim 25, wherein said memory is a non-volatile memory.

27. The system of claim 21, wherein said counter circuit comprises:

a counter; and

circuitry for incrementing said digital word stored in said counter by one.

28. The system of claim 21, wherein said ramp generator comprises:

a plurality of capacitor banks, each bank comprising;

a plurality of capacitors having equal capacitance,

a bank output line, coupled to a first plate of each capacitor, and

a bank control circuit, coupled to a second plate of each capacitor, and for switchably coupling the second plate of any one of said capacitors to either a first voltage source or a second voltage source;

a master output line, coupled to each bank output line; and

a master controller, for sending control signal to each bank control circuit to a master voltage at said master output line to generate said sequence of voltages.

29. The system of claim 28, wherein a capacitance of any capacitor in a first capacitor bank is different from a capacitance of any capacitor in a second capacitor bank.

30. The system of claim 28, wherein the capacitance of any capacitor in a first capacitor bank is a power of 2 of a capacitance of any capacitor in a second capacitor bank.

31. A method for converting an analog signal to a digital word, comprising:

measuring a magnitude of said analog signal;

mapping said magnitude to a digital word with a first and second transfer function only, wherein;

if said magnitude is less than a predetermined threshold, mapping said magnitude to said digital word exclusively with said first transfer function,

if said magnitude is at least equal to said predetermined threshold, mapping said magnitude to said digital word exclusively with said second transfer function,

said first transfer function is not included in said second transfer function, and

said second transfer function is not included in said first transfer function.

32. The method of claim 31, wherein said first transfer function maps each magnitude below said predetermined threshold to a corresponding reference signal in a linear manner.

34. The method of claim 32, wherein said second transfer function maps each magnitude at least equal to said predetermined threshold to corresponding reference signals in a logarithmic manner.

33. A method for converting an analog signal to a digital word, comprising:

measuring a magnitude of said analog signal;

if said magnitude is not greater than a predetermined threshold, mapping said magnitude to a digital word in accordance with a first transfer function; and

if said magnitude is at least equal to said predetermined threshold, mapping said magnitude to the digital word in accordance with a second transfer function;

wherein;

said first transfer function maps each magnitude below said predetermined threshold to a corresponding reference signal in a linear manner, and

said second transfer function maps a set of non-sequential and increasing magnitudes each at least equal to said predetermined threshold to corresponding reference signals in a linear manner.

35. A method for operating an imaging system, comprising:

receiving an analog pixel signal from a pixel;

converting said analog pixel signal into a digital word, wherein said converting comprises;

measuring a magnitude of said analog signal;

mapping said magnitude to a digital word with a first and second transfer function only, wherein;

if said magnitude is less than a predetermined threshold, mapping said magnitude to said digital word exclusively with said first transfer function,

if said magnitude is at least equal to said predetermined threshold, mapping said magnitude to said digital word exclusively with said second transfer function,

said first transfer function is not included in said second transfer function, and

said second transfer function is not included in said first transfer function.

36. The method of claim 35, wherein said first transfer function maps each magnitude below said predetermined threshold to a corresponding reference signal in a linear manner.

38. The method of claim 36, wherein said second transfer function maps each magnitude at least equal to said predetermined threshold to corresponding reference signals in a logarithmic manner.

37. A method for operating an imaging system, comprising:

receiving an analog pixel signal from a pixel;

converting said analog pixel signal into a digital word, wherein said converting comprises;

measuring a magnitude of said analog signal;

if said magnitude is not greater than a predetermined threshold, mapping said magnitude to a digital word in accordance with a first transfer function; and

if said magnitude is at least equal to said predetermined threshold, mapping said magnitude to the digital word in accordance with a second transfer function,

wherein;

said first transfer function maps each magnitude below said predetermined threshold to a corresponding reference signal in a linear manner, and

said second transfer function maps a set of non-sequential and increasing magnitudes each at least equal to said predetermined threshold to corresponding reference signals in a linear manner.