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A wide dynamic range CMOS image sensor with pulse-frequency-modulation and in-pixel amplification
ARTICLE IN PRESS Microelectronics Journal 40 (2009) 1496–1501
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
A wide dynamic range CMOS image sensor with pulse-frequency-modulation and in-pixel amplification Yong Chen 1, Fei Yuan , Gul Khan Department of Electrical and Computer Engineering, Ryerson University, Toronto, Ontario, Canada
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 June 2008 Received in revised form 25 July 2009 Accepted 10 August 2009 Available online 27 August 2009
This paper briefly examines the pros and cons of CMOS pulse-frequency-modulation (PFM) digital pixel sensors. A pulse-frequency-modulation digital pixel sensor with in-pixel amplification is proposed to improve the resolution of the pixel sensor at low illumination. The proposed PFM digital pixel sensor offers the characteristics of a reduced integration time when the level of illumination is low with the fill factor comparable to that of PFM digital pixel sensors without in-pixel amplification. The proposed digital image sensor has been designed in TSMC-0:18 mm 1.8 V CMOS technology and validated using Spectre from Cadence Design Systems with BSIM3V3 device models. Simulation results demonstrate that the dynamic range of the proposed PFM digital pixel sensor with in-pixel amplification is 20 dB larger as compared with that of PFM digital pixel sensors without in-pixel amplification. The increased dynamic range is obtained in the low illumination condition where PFM digital pixel sensors without in-pixel amplification cease the operation due to the low photo current. & 2009 Elsevier Ltd. All rights reserved.
Keywords: CMOS digital pixel sensor CMOS integrated circuits
1. Introduction CMOS digital pixel sensors with in-pixel analog-to-digital conversion (ADC) have been investigated intensively recently [1]. As compared with image sensors with column-level ADC, digital pixel sensors with in-pixel ADC offer the key advantage of reduced conversion time obtained from the parallel ADC operations performed in all pixels, making them particularly attractive for high-speed applications. The digital pixel sensor with in-pixel successive approximation ADC proposed by Yang et al. [2] is the first-generation digital pixel sensors with in-pixel ADC. Digital pixel sensors with an in-pixel first-order SD modulator for ADC was proposed in [3,4] where sampled free-running oscillators were used to remove the need for a 1-bit digital-to-analog converter (DAC). Most digital pixel sensors, however, use either pulse width modulation (PWM) [5,6] or pulse frequency modulation (PFM) [7,8] to perform ADC to take the advantage of their ease of implementation. Both architectures require an in-pixel comparator to determine the time instance at which the voltage across the photo diode drops below a user-defined reference voltage. A PWM digital pixel sensor also known as time-to-first spike pixel sensor [9,14] uses the output of its in-pixel comparator as a write enable signal to write the value of a global timing counter located outside the pixel sensor activated when the digital pixel sensor
Corresponding author.
E-mail address: [email protected] (F. Yuan). He is now with LeCroy Corp, Chestnut Ridge, New York, USA.
1
0026-2692/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2009.08.001
starts into the in-pixel internal memory of the pixel sensor [5]. The content of all in-pixel memories is then read in the read phase. This approach enjoys the advantages of reduced switching activities and low power consumption at the pixel level as only a single transition of the comparator’s output is needed to record the integration time [15,9]. Its performance, however, is affected by the following factors: (i) A high-speed clock is needed to drive the global timing circuit. (ii) The time delay from the output of the global timing circuit to each pixel will introduce an error on the final value of the in-pixel memories. (iii) The number of bits of the global timing circuit. When a large number of bits are used, the silicon area for routing these data lines from the global timing circuit to each pixel will be costly. (iv) The load of the global timing circuit is large as its output will be written into the in-pixel memory of all pixel sensors. Buffers, which might be needed to boost signal swing, will introduce timing errors. PFM digital pixel sensors, also known as spiking pixel sensors [13,10], avoid these difficulties by replacing the global timing circuit of PWM digital pixel sensors with in-pixel counters that are reset by the output of the in-pixel comparators. The in-pixel counter of each digital pixel sensor is reset each time the voltage of the photo diode drops below the reference voltage [8]. The level of illumination is represented by the number of times the output of the comparator flips for a given integration time. Although as compared with PWM digital pixel sensors, the in-pixel activity of PFM pixel sensors is high and the fill factor of the pixel sensor is low due to the inclusion of an in-pixel counter in the pixel sensor, the main deficiency of PFM digital pixel sensors is that when illumination is low, the integration time has to be long enough in
ARTICLE IN PRESS Y. Chen et al. / Microelectronics Journal 40 (2009) 1496–1501
order to have a good resolution. In [12], an adaptive high-gain column-level amplifier is employed before analog-to-digital conversion. This approach, however, does not have in-pixel ADC. The reconfigurable PFM digital pixel sensor proposed by Chen et al. uses a compact reconfigurable counter memory to lower the silicon consumption of the in-pixel counter subsequently increasing the fill factor [10]. The adaptive logarithmic digital pixel sensor proposed by Bermak and Kitchen employs a quantizer with variable quantization steps to boost the dynamic range of PFM digital pixel sensors [11]. In this paper, we propose a PFM digital pixel sensor with inpixel amplification to improve the resolution of PFM digital pixel sensors in the low illumination condition without requiring an over-long integration time. The in-pixel amplifier is a generic common-gate amplifier with a voltage shifter to minimize the silicon overhead subsequently the dimension of the pixel sensor. The proposed pixel sensor possesses the characteristics of PFM digital pixel sensors with an improved dynamic range. The reminder of the paper is organized as follows: Section 2 presents the principle of the proposed PFM digital pixel sensor. Section 3 details its circuit implementation. The simulation results of the proposed PFM digital pixel sensor are also presented in this section. The paper is concluded in Section 4.
2. PFM digital pixel sensors with in-pixel amplification
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the photo diode, respectively, and Tpd is the time duration from the start of the discharge process at which Vpd ¼ Vdd to the time instant at which Vpd ¼ Vref . The dark current is the current of the photo diode without illumination. It is the leakage current of the reverse biased pn-junction of the photo diode when illumination is absent. Dark current is a function of a number of factors including temperature and silicon defects. In one integration time Tint , the number of the pulses at the output of the comparator is contained from Npixel ¼
ðIpd þ Id ÞTint Tint ¼ : Tpd ðVdd Vref ÞCpd
ð2Þ
The average current of the photo diode can therefore be obtained from (2) Ipd ¼ ðVdd Vref Þ
Cpd N Id : Tint pixel
ð3Þ
It is seen from (3) that the average photo current of the photo diode is directly proportional to the number of the pulses at the output of the comparator (frequency modulation). The higher the reference voltage, the lower the average photo current. Also, the longer the integration time, the lower the average photo current of the photo diode. The larger the dark current, the lower the average photo current. 2.2. Digital pixel sensors with pulse-frequency-modulation and inpixel amplification
2.1. Digital pixel sensors with pulse-frequency-modulation The configuration of a conventional PFM digital pixel sensor with an N-bit in-pixel counter is shown in Fig. 1 where Cpd denotes the junction capacitance of the photo diode. In the reset phase where the reset pMOS transistor is ON, the voltage across the photo diode PD, denoted by Vpd, is charged to Vdd approximately. The output of the comparator is set to Logic-1 and the reset pMOS transistor turns off. In the following sensing phase, Vpd starts to drop due to the conduction of the photo diode. The photo diode current is set by the level of illumination. When Vpd reaches the reference voltage Vref set by users, the output of the comparator Vcomp will change from Logic-1 to Logic-0, forcing the content of the in-pixel counter to increment. It will also turn on the reset pMOS transistor and re-charge the junction capacitor of the photo diode to Vdd . Because Vpd 4Vref , the comparator output is set to Logic-1, switching off the pMOS transistor and the sensing phase starts again. This process repeats indefinitely, as illustrated in Fig. 2 for both high and low illumination. The level of illumination is represented by the content of the in-pixel counter for a given integration time Tint . The width of the output voltage pulses of the comparator can be obtained from Tpd ¼
ðVdd Vref ÞCpd ; Ipd þ Id
ð1Þ
where Cpd is the total capacitance at the cathode of the photo diode, Ipd and Id are the average photo current and dark current of
Mr Vpd Vref Cpd
PD
Vcomp
N-bit counter
The configuration of the proposed PFM digital pixel sensor with in-pixel amplification is shown in Fig. 3. Assume that the junction capacitor of the photo diode is charged to Vdd and the output of the amplifier Vin is set to Va initially. During the sensing phase in which Vpd drops, Vin ðtÞ ¼ Va Av Vpd ðtÞ ¼ Va Av
ð4Þ
where Av is the voltage gain of the amplifier. The pulse width at 0 , is obtained from (4) by the comparator output, denoted by Tpd 0 0 where Vref is the reference voltage letting Vin ¼ Vref 0 ¼ Tpd
0 ðVa Vref ÞCpd : ðIpd þ Id ÞAv
ð5Þ
If we set 0 ¼ Vdd Vref ¼ DV; Va Vref
ð6Þ
i.e. the pixel cells without and with in-pixel amplification have the 0 can be same voltage swing across the input of the comparator, Tpd expressed as 0 Tpd ¼
DVC pd Tpd ¼ : ðIpd þ Id ÞAv Av
ð7Þ
It can be seen from (7) that the pulse width of the PFM digital pixel sensor with in-pixel amplification is 1=Av that of the corresponding PFM digital pixel sensor without in-pixel amplification. If the two PFM digital pixel sensors have the same number of pulses at the output of the comparator, i.e. N0pixel ¼ Npixel ¼
0 Tint Tint ¼ 0 ; Tpd Tpd
ð8Þ
0 of the digital pixel sensor with inthen the integration time Tint pixel amplification is obtained from 0 Tint ¼
Fig. 1. Configuration of PFM digital pixel sensors with an N-bit in-pixel counter.
ðIpd þ Id Þt ; Cpd
0 Tint Tpd T ¼ int : Tpd Av
ð9Þ
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Vpd
Vpd
VDD
VDD
Vref
Vref t
Vcomp VDD
t
Vcomp VDD
Tpd
Tpd t
t
t
Tint High level of illumination
Tint
t
Low level of illumination
Fig. 2. Timing diagram of PFM digital pixel sensors with a constant reference voltage. (a) High level of illumination, photo diode voltage Vpd drops at a high rate in the sensing phase. (b) Low level of illumination, photo diode voltage Vpd drops at a low rate in the sensing phase.
3. Implementation and simulation results
Mr Vpd Cpd
PD
Av
Vin Vref
+ -
Vcomp
N-bit counter
Fig. 3. PFM digital pixel sensors with in-pixel amplification and an N-bit counter.
The schematic of the proposed PFM digital pixel sensor with in-pixel amplification is shown in Fig. 4. The photo diode is an n+/p-substrate diode with dimensions 13 mm 11 mm. The amplification stage consists of Ma , Mb , and Mc . Ma is a source follower that shifts the voltage of the photo diode Vpd down by one threshold voltage approximately. Mb is a common-gate amplifier with Mc the current source load. The voltage gain of the common-source amplifier is given by Av gm;b ðro;b Jro;c Þ;
The integration time of the digital pixel sensor with in-pixel amplification is only 1=Av that of the corresponding digital pixel sensor without in-pixel amplification. It should be noted that the only difference between the proposed PFM digital pixel sensor and conventional PFM digital pixel sensors is the inclusion of the in-pixel amplifier. The proposed PFM digital pixel sensor thus inherits all the attractive characteristics of conventional PFM digital pixel sensors, such as reduced system-level switching activities. Since ADC is performed at the pixel level, the output of the in-pixel memory has full voltage swing and can be readily read out without sensing amplifiers. This will greatly simplify the design of the readout circuitry. Fixed pattern noise, which is caused by device mismatches of pixel sensors, the mismatches in comparator offset voltage, and the mismatches in the capacitance of the photo-sensing node, exists. Although increasing the dimensions of the devices of the comparator will reduce mismatches, subsequently fixed pattern noise, the silicon consumption and fill factor constraints set the lower bound of the device sizes. Mismatches can be minimized by layouting the differential in an inter-digitized fashion. As pointed in [5], the mismatch-induced fixed pattern noise of digital pixel sensors can be eliminated using digital correlated double sampling performed in a post-processing step. Specifically, the pixel sensor is read twice, one with actual illumination and one without. The difference of the two readings not only yields the actual reading of the illumination level but also eliminates the effect of the mismatches of the devices.
ð10Þ
where gm;b is the transconductance of Mb , ro;b and ro;c are the output impedance of Mb and Mc , respectively. In our design, the voltage gain of the amplification stage is set to 20 dB approximately. For the same DV, the pulse width of the output of the comparator of the digital pixel sensor with in-pixel 1 that without in-pixel amplification. Following amplification is 10 the amplifier is a conventional PMOS two-stage comparator. The inverter restores the voltage swing, drives the in-pixel counter, and resets transistor Mr . A 10-bit linear-feedback shift-register (LFSR) counter with dynamic D flip-flops used in [8] is integrated in the pixel. The schematic is shown in Fig. 5. The input signal SEL is the mode switching signal. If SEL is high, the counter is in the counting mode. If SEL is low, the pixel is in the data readout mode. EXTCLK is the clock signal in the data readout mode, and SERIALIN is the data input port to reset the counter. A drawback of the dynamic D flip-flop is its need for a refresh mechanism to prevent the data loss when the illumination is low and no clock activities for a long time. A modified dynamic D flip-flop shown in Fig. 6 is used. With a weak transistor M7 (small channel width), the charge loss is reduced. The layout of the PFM digital pixel sensor with in-pixel amplification is shown in Fig. 7. The size of the pixel is 25 mm 23 mm with a fill factor of 25%, which is comparable to the dimensions and fill factor of the pixel sensor reported in [8]. For the purpose of comparison, the PFM digital pixel sensor proposed in [8], which does not have in-pixel amplification, is also implemented with an nMOS two-stage comparator. The
ARTICLE IN PRESS Y. Chen et al. / Microelectronics Journal 40 (2009) 1496–1501
Vpd
Mr
Vbias
Ma
Vbp
Mb1
Mb2
M1
Mb
M2
1499
M7 Vcomp
Vref
Vin Vbn
PD
M4
M3
Mc
M5
M6
Fig. 4. Simplified schematic of digital pixel sensors with in-pixel amplification (counter is not shown). Circuit parameters: Wr ¼ 0:5 mm, Wa ¼ 2 mm, Wb ¼ 4 mm, Wc ¼ 0:8 mm, W1 ¼ W2 ¼ 2 mm, W3 ¼ W4 ¼ 1 mm, W5 ¼ 2 mm, W6 ¼ 1 mm, W7 ¼ 2 mm, Wb1 ¼ 2 mm, Wb2 ¼ 4 mm. L ¼ 0:18 mm for all transistors except Lr ¼ 0:8 mm.
SEL SERIALIN
DQ SEL
Q0
C
DQ
Q1
C
Q5
DQ C
Q6
DQ
Q7
C
DQ
Q9
C
PIXELCLK EXTCLK Fig. 5. Schematic of 10-bit linear-feedback shift-register counter.
M7
C
M2
C
Comparator
M5 Q
D M4
M1 M3
M6
LFSR Counter
Fig. 6. Schematic of modified D flip-flops. Circuit parameters: W1 ¼ W2 ¼ W5 ¼ 1 mm, W3 ¼ W4 ¼ W6 ¼ 0:5 mm, W7 ¼ 0:25 mm. L ¼ 0:18 mm for all transistors.
Photo diode dimension of the photo diode and that of the reset transistor of the two pixel sensors are set to be the same in order to have a fair comparison. Both digital pixel sensors are implemented in TSMC0:18 mm 1.8 V CMOS technology and analyzed using Spectre from Cadence Design Systems with BSIM3V3 device models. An ideal current source connected in parallel with the photo diode is used to model the current of the photo diode. Fig. 8 plots the voltages of the two PFM pixel sensors with a 10 pA photo current and 700 ms integration time. The bias voltages are Vbias ¼ 1:3 V, Vbp ¼ 0:34 V, and Vbn ¼ 0:55 V. The reference voltage of the pixel sensor without in-pixel amplification is set to 1.6 V. Vref of the pixel sensor with in-pixel amplification is 0.2 V and Va is 1.0 V. As can be seen from the figure, the output voltage of the comparator Vcomp of the pixel sensor with in-pixel amplification has 10 pulses with pulse width 69:5 ms. The pixel sensor without in-pixel amplification has only three pulses with
23 m Fig. 7. Layout of digital pixel sensors with in-pixel amplification.
pulse width 222:5 ms. The voltage gain of the amplification stage is 22 dB approximately. The dynamic range of CMOS image sensors is defined as Imax ; ð11Þ DR ¼ 20log Imin
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106
Vpd[V]
2 1.8
105 0
100
200
300
400
500
600
700
Vin[V]
2 1 0 Vcomp[V]
0
100
200
300
400
500
600
700
1
103
101
0 100
200
300 400 Time [us]
500
600
700
100
101
102 103 Photo current [pA]
104
105
Fig. 10. Dynamic range of proposed PFM digital pixel sensor for 250 ms integration time at process corners. Bias conditions are as follows: TT: Vbp ¼ 0:34 V, Vbn ¼ 0:55 V; FF: Vbp ¼ 0:52, Vbn ¼ 0:45 V; FS: Vbp ¼ 0:37 V, Vbn ¼ 0:45 V; SF: Vbp ¼ 0:32 V, Vbn ¼ 0:64 V; and SS: Vbp ¼ 0:23 V, Vbn ¼ 0:55 V.
1.9 Vpd [V]
104
102
2
0
1.8 1.7 1.6 1.5 0
100
200
300
400
500
600
700
0
100
200
300 400 Time [us]
500
600
700
2 Vcomp [V]
Pulse width [ns]
1.6
1.5 1 0.5 0
Fig. 8. Simulated waveform of voltages of digital pixel sensors with and without in-pixel amplification. Top—with in-pixel amplification. Bottom—without in-pixel amplification.
whereas that of the conventional PFM digital pixel sensor is only 80 dB. The upper bound of the dynamic range of the two digital pixel sensors is approximately the same. This is because at high illumination, the voltage gain of the amplification gain decreases. Also observed is that when the level of illumination is low, the sensitivity is reduced. This is because at low illumination, the effect of the dark current of the photo diode cannot be neglected and the pulse width becomes less sensitive to the photo current. Fig. 10 shows the dynamic range of the proposed digital pixel sensor at process corners. The bias voltages Vbn and Vbp are adjusted to ensure that the dynamic range remains nearly unchanged. It is seen that by the proper adjustment of the biasing conditions, the effect of process variation can be minimized.
106 4. Conclusions
Pulse width [ns]
105 Conventional PFM DPS
104 103
PFM DPS with pre−amplification
102 101 100
101
102 103 Photo current [pA]
104
105
Fig. 9. Dynamic ranges of proposed PFM digital pixel sensor and that of conventional PFM digital pixel sensor for 250 ms integration time.
where Imax is the maximum photo diode current and Imin is the minimum detectable photo diode current. Imin is mainly set by the dark current and leakage currents. The dynamic range of two PFM digital pixel sensors are compared in Fig. 9 with the integration time set to 250 ms. The pixel sensor without in-pixel amplification stops working when the photo current drops below 10 pA while the pixel sensor with in-pixel amplification continues to operate until the photo current drops to 1 pA. The dynamic range of the proposed PFM digital pixel sensor is approximately 100 dB,
A pulse-frequency-modulation digital pixel sensor with inpixel amplification has been proposed to improve the resolution of the pixel sensor at low illumination. It has been shown that the proposed PFM digital pixel sensor offers the attractive characteristics of a reduced integration time when the level of illumination is low with the fill factor comparable to that of PFM digital pixel sensors without in-pixel amplification. In addition, it possesses the characteristics of conventional PFM digital pixel sensors. The simulation results of the proposed PFM digital pixel sensor implemented in TSMC-0:18 mm 1.8 V CMOS technology have demonstrated that the dynamic range of the proposed PFM digital pixel sensor with in-pixel amplification is 20 dB larger as compared with that of PFM digital pixel sensors without in-pixel amplification. The increased dynamic range is obtained in the low illumination condition where PFM digital pixel sensors without in-pixel amplification cease the operation due to the low photo current.
References [1] A. El Gamal, H. Eltoukhy, CMOS image sensors, IEEE Circuits Devices Mag. 21 (3) (2005) 6–20. [2] D. Yang, B. Fowler, A. Gamal, A Nyquist-rate pixel-level ADC for CMOS image senor, IEEE J. Solid-State Circuits 34 (3) (1997) 348–356.
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[3] B. Fowler, A. Gamal, D. Yang, A CMOS area image sensor with pixel-level A/D conversion, in: Proceedings of the IEEE International Solid-State Circuits Conference, February 1994, pp. 226–227. [4] L. McIlrath, A low-power low-noise ultra wide-dynamic-range CMOS imager with pixel-level A/D conversion, IEEE J. Solid-State Circuits 36 (5) (2001) 846–853. [5] S. Kleinfelder, S. Lim, A. El Gamal, A 10 000 frames/s CMOS digital pixel sensor, IEEE J. Solid-State Circuits 36 (12) (2001) 2049–2059. [6] A. Kitchen, A. Bermak, A. Bouzerdoum, A digital pixel sensor array with programmable dynamic range, IEEE Trans. Electron Devices 52 (12) (2005) 2891–22600. [7] E. Culurciello, R. Etienne-Cummings, K. Boahen, A Biomorphic digital image sensor, IEEE J. Solid-State Circuits 38 (2) (2003) 281–294. [8] X. Wang, W. Wang, R. Hornsey, A high dynamic range CMOS image sensor with in-pixel light-to-frequency conversion, IEEE Trans. Electron Devices 53 (12) (2006) 2988–2992. [9] C. Shoushun, A. Bermak, Arbitrated time-to-first spike CMOS image sensor with on-chip histogram equalization, IEEE Trans. VLSI Syst. 15 (3) (2007) 346–357.
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[10] S. Chen, A. Bermak, F. Boussaid, A compact reconfigurable counter memory for spiking pixels, IEEE Electron Device Lett. 27 (4) (2006) 255–257. [11] A. Bermak, A. Kitchen, A novel adaptive logarithmic digital pixel sensor, IEEE Photonics Technol. Lett. 18 (20) (2006) 2147–2149. [12] M. Sakakibara, S. Kawahito, D. Handoko, N. Nakamura, H. Satoh, M. Higashi, K. Mabuchi, H. Sumi, A high-sensitivity CMOS image sensor with gainadaptive column amplifiers, IEEE J. Solid-State Circuits 40 (5) (2005) 1147–1156. [13] J. Doge, G. Schonfelder, G.T. Streil, A. Konig, An HDR CMOS image sensor with spiking pixels, pixel-level ADC, and linear characteristics, IEEE Trans. Circuits Syst. II 49 (2) (2002) 155–158. [14] X. Qi, X. Guo, J.G. Harris, A time-to-first spike CMOS imager, in: Proceedings of the IEEE International Symposium on Circuits and Systems, vol. 4, May 2004, pp. 23–26. [15] A. Bermak, Y. Yung, A DPS array with programmable resolution and reconfigurable conversion time, IEEE Trans. VLSI Syst. 14 (1) (2006) 15–22.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
A wide dynamic range CMOS image sensor with pulse-frequency-modulation and in-pixel amplification Yong Chen 1, Fei Yuan , Gul Khan Department of Electrical and Computer Engineering, Ryerson University, Toronto, Ontario, Canada
a r t i c l e in f o
a b s t r a c t
Article history: Received 16 June 2008 Received in revised form 25 July 2009 Accepted 10 August 2009 Available online 27 August 2009
This paper briefly examines the pros and cons of CMOS pulse-frequency-modulation (PFM) digital pixel sensors. A pulse-frequency-modulation digital pixel sensor with in-pixel amplification is proposed to improve the resolution of the pixel sensor at low illumination. The proposed PFM digital pixel sensor offers the characteristics of a reduced integration time when the level of illumination is low with the fill factor comparable to that of PFM digital pixel sensors without in-pixel amplification. The proposed digital image sensor has been designed in TSMC-0:18 mm 1.8 V CMOS technology and validated using Spectre from Cadence Design Systems with BSIM3V3 device models. Simulation results demonstrate that the dynamic range of the proposed PFM digital pixel sensor with in-pixel amplification is 20 dB larger as compared with that of PFM digital pixel sensors without in-pixel amplification. The increased dynamic range is obtained in the low illumination condition where PFM digital pixel sensors without in-pixel amplification cease the operation due to the low photo current. & 2009 Elsevier Ltd. All rights reserved.
Keywords: CMOS digital pixel sensor CMOS integrated circuits
1. Introduction CMOS digital pixel sensors with in-pixel analog-to-digital conversion (ADC) have been investigated intensively recently [1]. As compared with image sensors with column-level ADC, digital pixel sensors with in-pixel ADC offer the key advantage of reduced conversion time obtained from the parallel ADC operations performed in all pixels, making them particularly attractive for high-speed applications. The digital pixel sensor with in-pixel successive approximation ADC proposed by Yang et al. [2] is the first-generation digital pixel sensors with in-pixel ADC. Digital pixel sensors with an in-pixel first-order SD modulator for ADC was proposed in [3,4] where sampled free-running oscillators were used to remove the need for a 1-bit digital-to-analog converter (DAC). Most digital pixel sensors, however, use either pulse width modulation (PWM) [5,6] or pulse frequency modulation (PFM) [7,8] to perform ADC to take the advantage of their ease of implementation. Both architectures require an in-pixel comparator to determine the time instance at which the voltage across the photo diode drops below a user-defined reference voltage. A PWM digital pixel sensor also known as time-to-first spike pixel sensor [9,14] uses the output of its in-pixel comparator as a write enable signal to write the value of a global timing counter located outside the pixel sensor activated when the digital pixel sensor
Corresponding author.
E-mail address: [email protected] (F. Yuan). He is now with LeCroy Corp, Chestnut Ridge, New York, USA.
1
0026-2692/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2009.08.001
starts into the in-pixel internal memory of the pixel sensor [5]. The content of all in-pixel memories is then read in the read phase. This approach enjoys the advantages of reduced switching activities and low power consumption at the pixel level as only a single transition of the comparator’s output is needed to record the integration time [15,9]. Its performance, however, is affected by the following factors: (i) A high-speed clock is needed to drive the global timing circuit. (ii) The time delay from the output of the global timing circuit to each pixel will introduce an error on the final value of the in-pixel memories. (iii) The number of bits of the global timing circuit. When a large number of bits are used, the silicon area for routing these data lines from the global timing circuit to each pixel will be costly. (iv) The load of the global timing circuit is large as its output will be written into the in-pixel memory of all pixel sensors. Buffers, which might be needed to boost signal swing, will introduce timing errors. PFM digital pixel sensors, also known as spiking pixel sensors [13,10], avoid these difficulties by replacing the global timing circuit of PWM digital pixel sensors with in-pixel counters that are reset by the output of the in-pixel comparators. The in-pixel counter of each digital pixel sensor is reset each time the voltage of the photo diode drops below the reference voltage [8]. The level of illumination is represented by the number of times the output of the comparator flips for a given integration time. Although as compared with PWM digital pixel sensors, the in-pixel activity of PFM pixel sensors is high and the fill factor of the pixel sensor is low due to the inclusion of an in-pixel counter in the pixel sensor, the main deficiency of PFM digital pixel sensors is that when illumination is low, the integration time has to be long enough in
ARTICLE IN PRESS Y. Chen et al. / Microelectronics Journal 40 (2009) 1496–1501
order to have a good resolution. In [12], an adaptive high-gain column-level amplifier is employed before analog-to-digital conversion. This approach, however, does not have in-pixel ADC. The reconfigurable PFM digital pixel sensor proposed by Chen et al. uses a compact reconfigurable counter memory to lower the silicon consumption of the in-pixel counter subsequently increasing the fill factor [10]. The adaptive logarithmic digital pixel sensor proposed by Bermak and Kitchen employs a quantizer with variable quantization steps to boost the dynamic range of PFM digital pixel sensors [11]. In this paper, we propose a PFM digital pixel sensor with inpixel amplification to improve the resolution of PFM digital pixel sensors in the low illumination condition without requiring an over-long integration time. The in-pixel amplifier is a generic common-gate amplifier with a voltage shifter to minimize the silicon overhead subsequently the dimension of the pixel sensor. The proposed pixel sensor possesses the characteristics of PFM digital pixel sensors with an improved dynamic range. The reminder of the paper is organized as follows: Section 2 presents the principle of the proposed PFM digital pixel sensor. Section 3 details its circuit implementation. The simulation results of the proposed PFM digital pixel sensor are also presented in this section. The paper is concluded in Section 4.
2. PFM digital pixel sensors with in-pixel amplification
1497
the photo diode, respectively, and Tpd is the time duration from the start of the discharge process at which Vpd ¼ Vdd to the time instant at which Vpd ¼ Vref . The dark current is the current of the photo diode without illumination. It is the leakage current of the reverse biased pn-junction of the photo diode when illumination is absent. Dark current is a function of a number of factors including temperature and silicon defects. In one integration time Tint , the number of the pulses at the output of the comparator is contained from Npixel ¼
ðIpd þ Id ÞTint Tint ¼ : Tpd ðVdd Vref ÞCpd
ð2Þ
The average current of the photo diode can therefore be obtained from (2) Ipd ¼ ðVdd Vref Þ
Cpd N Id : Tint pixel
ð3Þ
It is seen from (3) that the average photo current of the photo diode is directly proportional to the number of the pulses at the output of the comparator (frequency modulation). The higher the reference voltage, the lower the average photo current. Also, the longer the integration time, the lower the average photo current of the photo diode. The larger the dark current, the lower the average photo current. 2.2. Digital pixel sensors with pulse-frequency-modulation and inpixel amplification
2.1. Digital pixel sensors with pulse-frequency-modulation The configuration of a conventional PFM digital pixel sensor with an N-bit in-pixel counter is shown in Fig. 1 where Cpd denotes the junction capacitance of the photo diode. In the reset phase where the reset pMOS transistor is ON, the voltage across the photo diode PD, denoted by Vpd, is charged to Vdd approximately. The output of the comparator is set to Logic-1 and the reset pMOS transistor turns off. In the following sensing phase, Vpd starts to drop due to the conduction of the photo diode. The photo diode current is set by the level of illumination. When Vpd reaches the reference voltage Vref set by users, the output of the comparator Vcomp will change from Logic-1 to Logic-0, forcing the content of the in-pixel counter to increment. It will also turn on the reset pMOS transistor and re-charge the junction capacitor of the photo diode to Vdd . Because Vpd 4Vref , the comparator output is set to Logic-1, switching off the pMOS transistor and the sensing phase starts again. This process repeats indefinitely, as illustrated in Fig. 2 for both high and low illumination. The level of illumination is represented by the content of the in-pixel counter for a given integration time Tint . The width of the output voltage pulses of the comparator can be obtained from Tpd ¼
ðVdd Vref ÞCpd ; Ipd þ Id
ð1Þ
where Cpd is the total capacitance at the cathode of the photo diode, Ipd and Id are the average photo current and dark current of
Mr Vpd Vref Cpd
PD
Vcomp
N-bit counter
The configuration of the proposed PFM digital pixel sensor with in-pixel amplification is shown in Fig. 3. Assume that the junction capacitor of the photo diode is charged to Vdd and the output of the amplifier Vin is set to Va initially. During the sensing phase in which Vpd drops, Vin ðtÞ ¼ Va Av Vpd ðtÞ ¼ Va Av
ð4Þ
where Av is the voltage gain of the amplifier. The pulse width at 0 , is obtained from (4) by the comparator output, denoted by Tpd 0 0 where Vref is the reference voltage letting Vin ¼ Vref 0 ¼ Tpd
0 ðVa Vref ÞCpd : ðIpd þ Id ÞAv
ð5Þ
If we set 0 ¼ Vdd Vref ¼ DV; Va Vref
ð6Þ
i.e. the pixel cells without and with in-pixel amplification have the 0 can be same voltage swing across the input of the comparator, Tpd expressed as 0 Tpd ¼
DVC pd Tpd ¼ : ðIpd þ Id ÞAv Av
ð7Þ
It can be seen from (7) that the pulse width of the PFM digital pixel sensor with in-pixel amplification is 1=Av that of the corresponding PFM digital pixel sensor without in-pixel amplification. If the two PFM digital pixel sensors have the same number of pulses at the output of the comparator, i.e. N0pixel ¼ Npixel ¼
0 Tint Tint ¼ 0 ; Tpd Tpd
ð8Þ
0 of the digital pixel sensor with inthen the integration time Tint pixel amplification is obtained from 0 Tint ¼
Fig. 1. Configuration of PFM digital pixel sensors with an N-bit in-pixel counter.
ðIpd þ Id Þt ; Cpd
0 Tint Tpd T ¼ int : Tpd Av
ð9Þ
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Vpd
Vpd
VDD
VDD
Vref
Vref t
Vcomp VDD
t
Vcomp VDD
Tpd
Tpd t
t
t
Tint High level of illumination
Tint
t
Low level of illumination
Fig. 2. Timing diagram of PFM digital pixel sensors with a constant reference voltage. (a) High level of illumination, photo diode voltage Vpd drops at a high rate in the sensing phase. (b) Low level of illumination, photo diode voltage Vpd drops at a low rate in the sensing phase.
3. Implementation and simulation results
Mr Vpd Cpd
PD
Av
Vin Vref
+ -
Vcomp
N-bit counter
Fig. 3. PFM digital pixel sensors with in-pixel amplification and an N-bit counter.
The schematic of the proposed PFM digital pixel sensor with in-pixel amplification is shown in Fig. 4. The photo diode is an n+/p-substrate diode with dimensions 13 mm 11 mm. The amplification stage consists of Ma , Mb , and Mc . Ma is a source follower that shifts the voltage of the photo diode Vpd down by one threshold voltage approximately. Mb is a common-gate amplifier with Mc the current source load. The voltage gain of the common-source amplifier is given by Av gm;b ðro;b Jro;c Þ;
The integration time of the digital pixel sensor with in-pixel amplification is only 1=Av that of the corresponding digital pixel sensor without in-pixel amplification. It should be noted that the only difference between the proposed PFM digital pixel sensor and conventional PFM digital pixel sensors is the inclusion of the in-pixel amplifier. The proposed PFM digital pixel sensor thus inherits all the attractive characteristics of conventional PFM digital pixel sensors, such as reduced system-level switching activities. Since ADC is performed at the pixel level, the output of the in-pixel memory has full voltage swing and can be readily read out without sensing amplifiers. This will greatly simplify the design of the readout circuitry. Fixed pattern noise, which is caused by device mismatches of pixel sensors, the mismatches in comparator offset voltage, and the mismatches in the capacitance of the photo-sensing node, exists. Although increasing the dimensions of the devices of the comparator will reduce mismatches, subsequently fixed pattern noise, the silicon consumption and fill factor constraints set the lower bound of the device sizes. Mismatches can be minimized by layouting the differential in an inter-digitized fashion. As pointed in [5], the mismatch-induced fixed pattern noise of digital pixel sensors can be eliminated using digital correlated double sampling performed in a post-processing step. Specifically, the pixel sensor is read twice, one with actual illumination and one without. The difference of the two readings not only yields the actual reading of the illumination level but also eliminates the effect of the mismatches of the devices.
ð10Þ
where gm;b is the transconductance of Mb , ro;b and ro;c are the output impedance of Mb and Mc , respectively. In our design, the voltage gain of the amplification stage is set to 20 dB approximately. For the same DV, the pulse width of the output of the comparator of the digital pixel sensor with in-pixel 1 that without in-pixel amplification. Following amplification is 10 the amplifier is a conventional PMOS two-stage comparator. The inverter restores the voltage swing, drives the in-pixel counter, and resets transistor Mr . A 10-bit linear-feedback shift-register (LFSR) counter with dynamic D flip-flops used in [8] is integrated in the pixel. The schematic is shown in Fig. 5. The input signal SEL is the mode switching signal. If SEL is high, the counter is in the counting mode. If SEL is low, the pixel is in the data readout mode. EXTCLK is the clock signal in the data readout mode, and SERIALIN is the data input port to reset the counter. A drawback of the dynamic D flip-flop is its need for a refresh mechanism to prevent the data loss when the illumination is low and no clock activities for a long time. A modified dynamic D flip-flop shown in Fig. 6 is used. With a weak transistor M7 (small channel width), the charge loss is reduced. The layout of the PFM digital pixel sensor with in-pixel amplification is shown in Fig. 7. The size of the pixel is 25 mm 23 mm with a fill factor of 25%, which is comparable to the dimensions and fill factor of the pixel sensor reported in [8]. For the purpose of comparison, the PFM digital pixel sensor proposed in [8], which does not have in-pixel amplification, is also implemented with an nMOS two-stage comparator. The
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Vpd
Mr
Vbias
Ma
Vbp
Mb1
Mb2
M1
Mb
M2
1499
M7 Vcomp
Vref
Vin Vbn
PD
M4
M3
Mc
M5
M6
Fig. 4. Simplified schematic of digital pixel sensors with in-pixel amplification (counter is not shown). Circuit parameters: Wr ¼ 0:5 mm, Wa ¼ 2 mm, Wb ¼ 4 mm, Wc ¼ 0:8 mm, W1 ¼ W2 ¼ 2 mm, W3 ¼ W4 ¼ 1 mm, W5 ¼ 2 mm, W6 ¼ 1 mm, W7 ¼ 2 mm, Wb1 ¼ 2 mm, Wb2 ¼ 4 mm. L ¼ 0:18 mm for all transistors except Lr ¼ 0:8 mm.
SEL SERIALIN
DQ SEL
Q0
C
DQ
Q1
C
Q5
DQ C
Q6
DQ
Q7
C
DQ
Q9
C
PIXELCLK EXTCLK Fig. 5. Schematic of 10-bit linear-feedback shift-register counter.
M7
C
M2
C
Comparator
M5 Q
D M4
M1 M3
M6
LFSR Counter
Fig. 6. Schematic of modified D flip-flops. Circuit parameters: W1 ¼ W2 ¼ W5 ¼ 1 mm, W3 ¼ W4 ¼ W6 ¼ 0:5 mm, W7 ¼ 0:25 mm. L ¼ 0:18 mm for all transistors.
Photo diode dimension of the photo diode and that of the reset transistor of the two pixel sensors are set to be the same in order to have a fair comparison. Both digital pixel sensors are implemented in TSMC0:18 mm 1.8 V CMOS technology and analyzed using Spectre from Cadence Design Systems with BSIM3V3 device models. An ideal current source connected in parallel with the photo diode is used to model the current of the photo diode. Fig. 8 plots the voltages of the two PFM pixel sensors with a 10 pA photo current and 700 ms integration time. The bias voltages are Vbias ¼ 1:3 V, Vbp ¼ 0:34 V, and Vbn ¼ 0:55 V. The reference voltage of the pixel sensor without in-pixel amplification is set to 1.6 V. Vref of the pixel sensor with in-pixel amplification is 0.2 V and Va is 1.0 V. As can be seen from the figure, the output voltage of the comparator Vcomp of the pixel sensor with in-pixel amplification has 10 pulses with pulse width 69:5 ms. The pixel sensor without in-pixel amplification has only three pulses with
23 m Fig. 7. Layout of digital pixel sensors with in-pixel amplification.
pulse width 222:5 ms. The voltage gain of the amplification stage is 22 dB approximately. The dynamic range of CMOS image sensors is defined as Imax ; ð11Þ DR ¼ 20log Imin
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Vpd[V]
2 1.8
105 0
100
200
300
400
500
600
700
Vin[V]
2 1 0 Vcomp[V]
0
100
200
300
400
500
600
700
1
103
101
0 100
200
300 400 Time [us]
500
600
700
100
101
102 103 Photo current [pA]
104
105
Fig. 10. Dynamic range of proposed PFM digital pixel sensor for 250 ms integration time at process corners. Bias conditions are as follows: TT: Vbp ¼ 0:34 V, Vbn ¼ 0:55 V; FF: Vbp ¼ 0:52, Vbn ¼ 0:45 V; FS: Vbp ¼ 0:37 V, Vbn ¼ 0:45 V; SF: Vbp ¼ 0:32 V, Vbn ¼ 0:64 V; and SS: Vbp ¼ 0:23 V, Vbn ¼ 0:55 V.
1.9 Vpd [V]
104
102
2
0
1.8 1.7 1.6 1.5 0
100
200
300
400
500
600
700
0
100
200
300 400 Time [us]
500
600
700
2 Vcomp [V]
Pulse width [ns]
1.6
1.5 1 0.5 0
Fig. 8. Simulated waveform of voltages of digital pixel sensors with and without in-pixel amplification. Top—with in-pixel amplification. Bottom—without in-pixel amplification.
whereas that of the conventional PFM digital pixel sensor is only 80 dB. The upper bound of the dynamic range of the two digital pixel sensors is approximately the same. This is because at high illumination, the voltage gain of the amplification gain decreases. Also observed is that when the level of illumination is low, the sensitivity is reduced. This is because at low illumination, the effect of the dark current of the photo diode cannot be neglected and the pulse width becomes less sensitive to the photo current. Fig. 10 shows the dynamic range of the proposed digital pixel sensor at process corners. The bias voltages Vbn and Vbp are adjusted to ensure that the dynamic range remains nearly unchanged. It is seen that by the proper adjustment of the biasing conditions, the effect of process variation can be minimized.
106 4. Conclusions
Pulse width [ns]
105 Conventional PFM DPS
104 103
PFM DPS with pre−amplification
102 101 100
101
102 103 Photo current [pA]
104
105
Fig. 9. Dynamic ranges of proposed PFM digital pixel sensor and that of conventional PFM digital pixel sensor for 250 ms integration time.
where Imax is the maximum photo diode current and Imin is the minimum detectable photo diode current. Imin is mainly set by the dark current and leakage currents. The dynamic range of two PFM digital pixel sensors are compared in Fig. 9 with the integration time set to 250 ms. The pixel sensor without in-pixel amplification stops working when the photo current drops below 10 pA while the pixel sensor with in-pixel amplification continues to operate until the photo current drops to 1 pA. The dynamic range of the proposed PFM digital pixel sensor is approximately 100 dB,
A pulse-frequency-modulation digital pixel sensor with inpixel amplification has been proposed to improve the resolution of the pixel sensor at low illumination. It has been shown that the proposed PFM digital pixel sensor offers the attractive characteristics of a reduced integration time when the level of illumination is low with the fill factor comparable to that of PFM digital pixel sensors without in-pixel amplification. In addition, it possesses the characteristics of conventional PFM digital pixel sensors. The simulation results of the proposed PFM digital pixel sensor implemented in TSMC-0:18 mm 1.8 V CMOS technology have demonstrated that the dynamic range of the proposed PFM digital pixel sensor with in-pixel amplification is 20 dB larger as compared with that of PFM digital pixel sensors without in-pixel amplification. The increased dynamic range is obtained in the low illumination condition where PFM digital pixel sensors without in-pixel amplification cease the operation due to the low photo current.
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