Reference voltage stabilization in CMOS sensors
View Patent ↗A reference voltage generator for use in an image sensor provides a reference voltage to an S/H block during a pixel read-out operation and another reference voltage to an analog-to-digital converter (ADC) during a digitization operation. The reference voltage generator includes a variable voltage generator, a sample-and-hold circuit to sample a reference voltage prior to the pixel read-out operation or the digitization operation, and a buffer amplifier to drive the appropriate reference voltage to the relatively high impedance load presented by the S/H block and the variable impedance load provided by the ADC.
1. An imager comprising:
an image sensor including an array of pixels, said image sensor further comprising:
a first operative device for receiving signals and coupled to receive a first reference voltage;
a reference voltage generator operative to generate said first reference voltage, said reference voltage generator including a sample-and-hold circuit coupled to a voltage source and operative to sample-and-hold said voltage source to produce said first reference voltage; and
a second operative device for receiving signals from said first operative device, said second operative device coupled to a second reference voltage, said sample-and-hold circuit of said reference voltage generator operative to sample-and-hold a second voltage of said voltage source to produce said second reference voltage.
2. The imager according to claim 1 , wherein said first operative device is a pixel signal sample-and-hold block.
3. The imager according to claim 1 , wherein said second operative device is an analog-to-digital converter (ADC).
4. The imager according to claim 1 , wherein said reference voltage generator further comprises a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit and produce said second reference voltage.
5. The imager according to claim 1 , further comprising a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit to produce said first reference voltage prior to use of said first reference voltage by said first operative device and said second reference voltage prior to use of said second reference voltage by said second operative device.
6. The imager according to claim 5 , wherein said buffer amplifier comprises a voltage amplifier.
7. The imager according to claim 6 , wherein said voltage amplifier has unity gain.
8. An apparatus comprising:
a voltage generator including a voltage tree operative to generate a plurality of reference voltages from a voltage source, said voltage generator including a sample-and-hold circuit coupled to said voltage generator and operative to sample-and-hold said plurality of reference voltages;
a pixel signal sample-and-hold block for receiving a first one of said plurality of reference voltages; and
an analog-to-digital conversion circuit for converting a sampled pixel signal into a corresponding digital value, said analog-to-digital conversion circuit receiving a second one of said plurality of reference voltages.
9. The apparatus according to claim 8 , further comprising a buffer amplifier coupled to said sample-and-hold circuit and operative to amplify an output of said sample-and-hold circuit to produce said plurality of reference voltages.
10. The apparatus according to claim 9 , wherein said buffer amplifier comprises a voltage amplifier.
11. The apparatus according to claim 10 , wherein said voltage amplifier has unity gain.
12. The apparatus according to claim 8 , wherein said voltage generator comprises a resistance ladder coupled to a current source, said resistance ladder including a plurality of resistors coupled to a plurality of selection switches.