Anti-eclipse circuitry with tracking of floating diffusion reset level
View Patent ↗An anti-eclipse circuit for an imager is formed from pixel circuitry over the same semiconductor substrate as the imaging pixels. More specifically, two adjacent pixel circuits are modified to form an amplifier. One input of the amplifier is adapted to receive a reset signal from one of the pixel circuits while another input is adapted to be set at a predetermined offset voltage from the output of the amplifier. The amplifier is preferably a unity gain amplifier, so that the output of the amplifier set to a voltage level equal to the predetermined offset from the voltage level of the reset signal. Accordingly, the anti-eclipse circuit outputs a reference voltage at predetermined level from the reset voltage of a pixel and does not need to be calibrated for fabrication related variances in reset voltages.
1. An anti-eclipsing circuit for adjusting a level of a reset signal from an imaging pixel in an imager when the reset signal is below a predetermined voltage, comprising:
a first source-follower transistor for adjusting said level of said reset signal in response to an input reference signal;
a non-imaging pixel for producing a nominal reset signal;
an offset voltage generator for producing an offset voltage; and
an amplifier for outputting said reference voltage equal to a difference between the offset voltage and a mirrored signal identical to said nominal reset signal.
2. The anti-eclipsing circuit of claim 1 , wherein said non-imaging pixel further comprises:
a floating diffusion node; and
a transfer transistor coupled to said floating diffusion node.
3. The anti-eclipsing circuit of claim 2 , wherein said floating diffusion node is coupled to a first source/drain of said transfer transistor, said non-imaging pixel further comprising:
a light sensitive element coupled to a second source/drain of said transfer transistor.
4. The anti-eclipsing circuit of claim 3 , wherein said light sensitive element is shielded from incident light.
5. The anti-eclipsing circuit of claim 1 , wherein the amplifier comprises a current mirror for mirroring said nominal reset signal.
6. The anti-eclipsing circuit of claim 1 , wherein the amplifier comprises two adjacent pixel circuits in said imager.
7. The anti-eclipsing circuit of claim 6 , wherein the amplifier comprises a load circuit having a transistor biased to flow twice the current of either of said two adjacent pixel circuits.
8. The anti-eclipsing circuit of claim 1 , further comprising a sample-and-hold circuit for sampling and holding said reference signal.
9. An imager, comprising:
an array of pixels including a plurality of imaging pixels for producing respective reset signals and image output signals;
an anti-eclipsing circuit for adjusting a level of at least one of said reset signals when said at least one of said reset signals are below a predetermined voltage, comprising:
a first source-follower transistor for adjusting said level of said reset signal in response to an input reference signal;
a non-imaging pixel for producing a nominal reset signal;
an offset voltage generator for producing an offset voltage; and
an amplifier for outputting said reference voltage equal to a difference between the offset voltage and a mirrored signal identical to said nominal reset signal.
10. The imager of claim 9 , wherein said non-imaging pixel further comprises:
a floating diffusion node; and
a transfer transistor coupled to said floating diffusion node.
11. The imager of claim 10 , wherein said floating diffusion node is coupled to a first source/drain of said transfer transistor, said non-imaging pixel further comprising:
a light sensitive element coupled to a second source/drain of said transfer transistor.
12. The imager of claim 11 , wherein said light sensitive element is shielded from incident light.
13. The imager of claim 9 , wherein the amplifier comprises a current mirror for mirroring said nominal reset signal.
14. The imager of claim 9 , wherein the amplifier comprises two adjacent pixel circuits in said imager.
15. The imager of claim 14 , wherein the amplifier comprises a load circuit having a transistor biased to flow twice the current of either of said two adjacent pixel circuits.
16. The imager of claim 9 , further comprising a sample-and-hold circuit for sampling and holding said reference signal.