Double sampling active pixel sensor with double sampling temperature sensor

Abstract

A system which operates to determine temperature of an image sensor using the same signal chain that is used to detect the image sensor actual outputs. A correlated double sampling circuit is used to obtain the image outputs. That's same correlated double sampling circuit is used to receive two different inputs from the temperature circuit, and to subtract one from the other. The temperature output can be perceived, for example, once each frame.

Description

BACKGROUND

[0002] Image sensors strive for more accuracy in the image readout chain. Different techniques are used, including techniques to cancel out various kinds of noise. Different characteristics of image sensors are also dependent on temperature. Accordingly, temperature compensation may also be used to monitor for, and correct for, errors in the acquired signal.

[0003] The present application teaches a new technique allowing reading out a signal that is proportional to the temperature of an image sensor. This temperature may be used to compensate for the effect of the temperature on an image sensor pixel array.

BRIEF DESCRIPTION OF THE DRAWINGS

[0004] These and other aspects will now be described in detail with reference to the accompanying drawings, wherein:

[0005] FIG. 1 shows a block diagram of the system;

[0006] FIG. 2 shows a circuit diagram of a bandgap cell;

[0007] FIG. 3 shows a startup circuit for the bandgap cell; and

[0008] FIG. 4 shows an overall circuit.

DETAILED DESCRIPTION

[0009] An embodiment is shown in FIG. 1, which illustrates a block diagram of an image sensor with an included temperature sensor. Basically, this system provides an image sensing system in which outputs can represent either the output of the image sensor, and/or at temperature of the image sensor, e.g. the temperature of the substrate on which the image sensor is formed. The system includes an improved temperature sensor circuit which determines the temperature of the substrate, e.g. the silicon.

[0010] The system of FIG. 1 shows an active pixel sensor, which may be formed using CMOS circuitry for example. However, these techniques may also be applied to any other family or type of image sensor. An image sensor pixel array 100, for example an image sensor array having “m” rows and “n” columns, is driven by a control signal generator 105 that generates control signals and clock pulses for the pixel array. The output of the pixel array 110 is provided in parallel form to a double sample and hold circuit 115, that is, one which holds two values. Sample and hold circuit 115 may carry out a correlated double sampling from the image sensor, to produce an output signal that is proportional to the difference between the value of each pixel prior to light integration, and the value of the pixel after the light integration is complete. The difference circuit 120 may determine the difference between the two signals. Controlling element 105 may also produce the control signals for the difference circuit 120. The output of difference circuit 120 is amplified by a gain circuit 125, and output as an analog signal 130. The final output signal may be this analog signal 130. Alternatively, an A/D converter 135 may be used to produce a digital output 140 indicative of the analog signal 130.

[0011] A second input to the double sample and hold circuit 115 comes from a temperature sensor 150. The output 151 of the temperature sensor is also received by the sample and hold circuit 115, and passes through the signal chain in the same way as the image sensor outputs.

[0012] In this way, a signal which is directly proportional to temperature can be received from the temperature sensor 150. This may be done, for example, during a time slot while the image readout is inactive. It may be done for example at the beginning of each image, or at the beginning or end of each one frame, or every few lines, or any other interval of pixels or time. In this way, changes in temperature which fluctuate on a relatively short time frame may be used as correction, as often as desired.

[0013] In a typical implementation of an image sensor, such as the one described herein, a hotter chip provides a whiter image, or put another way, the black level of an image pixel has a higher voltage than the white level. Increasing the temperature causes a correspondingly decreased pixel signal voltage. This is the typical case when a pixel photodiode is implemented in a P type silicon or P type diffusion well. In the opposite case, where an N type substrate or N type well is used to embed the photoreceptor, an increasing voltage may correspond to a higher temperature. A relationship between the temperature and the amount of compensation of image output may be stored.

[0014] A so-called bandgap cell is shown in FIG. 2. This cell includes the temperature and voltage stabilized output labeled as V—REF. The output V—EB,Q6 is a voltage drop away from the reference voltage, and has a linear and negative temperature coefficient relative to that reference voltage. In this system, a startup transient current input is required at the node labeled “START”. After reaching steady-state, the currents in transistors Q5 and Q6 eventually equalize.

[0015] The FIG. 2 circuit is based on the Brokaw type bandgap reference circuit which is well-known.

[0016] In FIG. 2, the two NMOS transistors 200, 202 share the same gate voltage by virtue of their common gate node 206. The transistors 200, 202 are matched to have the same or similar transconductance. Therefore, the source potentials will be the same when they conduct the same current.

[0017] The CMOS transistors 210,212 form a current mirror keeps the source potential of the two NMOS transistors 200, 202 constant. The current mirror is also part of a closed looped amplifier which insures that the source potential of the NMOS transistors will be kept low due to feedback. This loop should be kept stable.

[0018] Equal currents are hence forced through the two base P-N junctions of the diode-coupled transistors 220,222. These transistors have different areas, with the area ratio between transistor 222 and 225 being 4:32 equals 1:8. Because of this area difference, there will be a difference in the P-N junction voltage drop across the junctions according to

ΔV EB =V EB,Q5 −V EB,Q6   (1)

[0019] It can be shown that

ΔVEB=−VT·ln(⅛)=−(kT/q)·ln(⅛)=−25.84·ln(⅛)mV=+53.74 mV  (2)

[0020] at T=300 K.

[0021] Therefore, Δ VEB has a positive temperature coefficient proportional to absolute temperature. The VT is called the thermal voltage, K is Boltzmann's constant, T is absolute temperature in degrees Kelvin and Q is the charge of an electron. The P-N junctions have negative temperature coefficients of about 2 mV per degrees K. By balancing these two coefficients at a chosen temperature T=Tθ, a close to 0 temperature coefficient can be obtained at that temperature.

[0022] In order for the two currents in FIG. 2 to be equal, the resistor R6 must be greater than the resistor R5. The value ΔR is defined as the difference R6-R5. The two operating currents are then given by

I Q5,Q6 =ΔV EB /ΔR

[0023] Since R6=R5+ΔR, the output reference voltage will be:

V — REF=V EB,Q6 +ΔV EB +R 5·|Q5, Q6

V — REF=V EB,Q6 +ΔV EB +(R5/ΔR)·ΔVEB  (3)

V — REF=V EB, Q6 =(1+R5/ΔR)·ΔVEB

[0024] The operating currents and current densities of Q5 and Q6 may be selected to provide a negative temperature coefficient for the VEB determined in equation 3. This can be balanced against the positive temperature term by the resistor ratio R5/ΔR and also by changing the area ratio between Q5 and Q6. In this particular embodiment this ratio 1:8.

[0025] The last part of equation 3 also shows that the last term is independent of any common production tolerance in the absolute value of the resistors. However, the operating current will still vary around the target design value. There will be a logarithmic variation in the first term VEB,Q6 over multiple process runs, and hence also in the output voltage. In most cases, this variation is acceptable. There is also an acceptable variation in the negative temperature coefficient of VEB,Q6.

[0026] According to this finding, the present application uses the double sampling part of the analog signal processing chain of an image sensor to obtain the difference between the voltage V_REF and VEB,Q6, in order to output a signal directly proportional to the absolute temperature of the sensor as

V — PTAT =(1+R5/ΔR)·ΔVEB=(1+R5/ΔR)·(kT/q)·ln(AQ6/AQ5)

[0027] Where AQ6/AQ5 are respective emitter areas of Q6 and Q5.

[0028] This enables temperature measurement to be carried out independently of process variations according to a first order. However, there may be second order variations in the term ΔVEB.

[0029] FIG. 3 also shows a startup circuit for the bandgap cell shown in FIG. 2. In the FIG. 3 cell, the start node 300 begins with a relatively low potential during startup. Prior to start up, the gate potential of transistor M14 is high so that the transistor does not conduct current. Transistor M11 is a relatively long transistor and can be used as a resistor. M11 will hence always be conducting. This causes the transistor M10 to conduct and provide the start up current. When the FIG. 2 bandgap cell has started, this sets the gate potential of M14 and therefore M14 conducts current. That current is mirrored by the transistors M12, M13 to pull down the source node of M11 so that M10 stops providing its start up current. The circuit also has two buffers and level shifters as shown in FIG. 4. The level shifters bring the two output voltages up to the normal voltage range used for the output of the pixel source followers. These level shifters also lower the output impedance of the bandgap cell. Level shifting needs to be done using carefully matched transistor pairs and matched current sources for the source followers.

[0030] Accordingly, the bandgap cell has an inherent start current provided by the start current generating circuit thereby providing a temperature sensed output.

[0031] Although only a few embodiments have been disclosed in detail above, other modifications are possible.

Claims

1. A system, comprising: an image processing chain, having parts which are adapted to automatically remove a first sampled part from a second sampled part to produce an output indicative of a difference between said first sampled part and said second sampled part, said image processing chain having a first input adapted for receiving said first and second sampled parts from an image sensor pixel array, and a second input adapted for receiving said first and second sampled parts from a temperature sensor circuit.

2. A system as in claim 1, further comprising an image sensor pixel array, producing a plurality of outputs to said first inputs, and a temperature sensor circuit, producing outputs to said second input.

3. A system as in claim 1, wherein said image processing chain includes a double sample and hold circuit, which receives the first and second values, and samples and holds said first and second values.

4. A system as in claim 3, further comprising a difference producing circuit, coupled to an output of said double sample and hold circuit, and producing a difference output indicative of a difference between said first and second values.

5. A system as in claim 3, wherein said double sample and hold circuit receives values from said image sensor indicative of image sensor values prior to image integration and after image integration.

6. A system as in claim 5, wherein said double sample and hold circuit receives first and second temperature values at said second inputs.

7. A system as in claim 6, wherein said first and second temperature values are subtracted, to provide a temperature signal that is proportional to compensated temperature of the sensor independent of process runs.

8. A system as in claim 7, wherein said first and second temperature values include values which are each dependent on variations in process runs, but whose difference is less dependent on temperatures of process runs.

9. A system as in claim 8, wherein said first and second temperature values include a value of a voltage stabilized output from a temperature sensitive bandgap cell, and a value related to a temperature coefficient-related value from said bandgap cell.

10. A system as in claim 2, further comprising a double sampling and hold circuit within said image processing chain, which determines a difference between subsequent values from said image sensor, and determines a difference between subsequent values from said temperature sensor circuit.

11. A system as in claim 10, wherein said temperature sensing circuit produces a first output which is temperature related, and a second output which is related to a voltage reference, and said double sampling and hold circuit determines a difference between said first and second outputs.

12. A system as in claim 2, wherein said temperature circuit includes a start circuit which initially produces a current to a start node, which reduces once the circuit is operating.

13. A system, comprising: an image sensing element, including an array of image sensors, producing outputs, including a first output being produced prior to image integration, and a second output being produced subsequent to image integration; a temperature sensing element, located on the same substrate as the image sensing element, and producing a first output indicative of a temperature thereof and a second output indicative of a signal that depends on a process variation of formation; and a signal processing chain, producing an output indicative of a difference between the first and second signals, connected to receive said first and second outputs of said image sensing elements at a first time, and connected to receive said first and second outputs of said temperature sensing elements at a second time.

14. A system as in claim 13, wherein said image sensing element is a CMOS image sensing element.

15. A system as in claim 13, wherein said signal processing chain is on a same substrate as said image sensing element and said temperature sensing element.

16. A system as in claim 15, further comprising a control signal generator, also on a same substrate as said image sensing element, producing control signals for said image sensor element, and for said signal processing chain.

17. A system as in claim 13, further comprising an analog to digital converter, receiving an output of said signal processing chain, and producing a digital signal indicative thereof, which signal represents a digital temperature at one time, and a digital image value at another time.

18. A system as in claim 13, wherein said temperature sensing element is a bandgap cell which produces a temperature and voltage stabilized reference voltage at one of said outputs, and produces a temperature signal with a temperature coefficient relative to said voltage reference at another of said outputs.

19. A system as in claim 18, further comprising a start signal producing circuit, which produces a start voltage for said bandgap cell during initial operation, which start voltage is gradually reduced in absolute value after said initial operation.

20. A system as in claim 18, wherein said signal processing chain is a double sampling and hold circuit that receives image sensor pixels at said first time, and receives said reference voltage and temperature signal at said second time.

21. A temperature sensing circuit, comprising: a bandgap circuit, which produces an output related to a temperature of at least one component in the circuit, and requiring a transient start up current for the circuit; and a start up current producing circuit, producing an initial start current and sensing operation of said bandgap circuit and reducing said start current relative to said sensing.

22. A temperature sensing circuit as in claim 21, wherein said startup circuit includes a current mirror which conducts current once the bandgap circuit operation has started, to reduce a value of said start current.

23. A method, comprising: using a correlated double sampling circuit to produce an output indicative of an output of an image sensor; and using the same correlated double sampling circuit to produce a temperature output of a temperature sensor.

24. A method as in claim 23, wherein said output of said temperature sensor is an output which is compensated for process variations among different circuits.

25. A method as in claim 24, wherein said output of said image sensor is an output which is compensated for a difference between charge prior to image integration and charge subsequent to image integration.

26. A method as in claim 23, wherein said output of said correlated double sampling circuit and said output of said temperature sensor are in analog form.

27. A method as in claim 26, further comprising converting said output signals to a digital form.

28. A method as in claim 23, further comprising producing said temperature outputs at specified intervals between image sensor outputs.

29. A method as in claim 28, wherein a new said temperature outputs are produced for each frame.

30. A method as in claim 28, wherein a new said temperature outputs are produced for each specified amount of image sensor outputs.

31. A method as in claim 23, further comprising producing a start signal for said temperature sensor which is initially conducting and subsequently less conducting, as the temperature sensor begins to operate.

32. A method, comprising producing outputs indicative of image pixels; and producing temperature outputs indicative of a temperature of a system that is producing said image pixels, which temperature outputs are compensated for process variations among circuits.

33. A method as in claim 32, wherein said producing outputs and said producing temperature outputs comprises using a same correlated double sampling circuit to produce both said outputs and said temperature outputs.

34. A method as in claim 33, wherein said correlated double sampling circuit receives a first temperature output that is indicative of a temperature, and receives a second temperature output that is indicative of a non temperature related signal, but where said first and second temperature outputs are both dependent on a same process variation in the circuit, and said correlated double sampling operates to subtract one of said outputs from said other of said outputs.

35. A method, comprising: receiving image signals from an image sensor indicative of blocks of pixels from said image sensor; receiving signals from said image sensor indicative of a temperature of said image sensor, said temperature signals being received every specified time during the operation of said image sensor, so that said temperature signal is received for each specified group of pixels; and using said temperature signals from said image sensor to correct said image signals.

36. A method as in claim 35, wherein said using comprises using said temperature sensors during every frame to compensate said signals.

37. A method as in claim 35, wherein said using comprises using said sensed temperature during each line to compensate said signals.