Volume 2015, Issue 1 767161

Research Article

Open Access

A New CDS Structure for High Density FPA with Low Power

Xiao Wang,

Corresponding Author

Xiao Wang

[email protected]

Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

University of the Chinese Academy of Sciences, Beijing 100049, China ucas.ac.cn

Key Laboratory of Opto-Electronic Information Processing, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Search for more papers by this author

Zelin Shi,

Zelin Shi

Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Key Laboratory of Opto-Electronic Information Processing, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Search for more papers by this author

Xiao Wang,

Corresponding Author

Xiao Wang

[email protected]

Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

University of the Chinese Academy of Sciences, Beijing 100049, China ucas.ac.cn

Key Laboratory of Opto-Electronic Information Processing, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Search for more papers by this author

Zelin Shi,

Zelin Shi

Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Key Laboratory of Opto-Electronic Information Processing, Chinese Academy of Sciences, Shenyang 110016, China cas.cn

Search for more papers by this author

First published: 01 February 2015

https://doi.org/10.1155/2015/767161

Citations: 4

Academic Editor: Jose Silva-Martinez

Share a link

Email
Wechat
Bluesky

Abstract

Being an essential part of infrared readout integrated circuit, correlated double sampling (CDS) circuits play important roles in both depressing reset noise and conditioning integration signals. To adapt applications for focal planes of large format and high density, a new structure of CDS circuit occupying small layout area is proposed, whose power dissipation has been optimized by using MOSFETs in operation of subthreshold region, which leads to 720 nW. Then the noise calculation model is established, based on which the noise analysis has been carried out by the approaches of transfer function and numerical simulations using SIMULINK and Verilog-A. The results are in good agreement, demonstrating the validity of the present noise calculation model. Thermal noise plays a dominant role in the long wave situation while 1/f noise is the majority in the medium wave situation. The total noise of long wave is smaller than medium wave, both of which increase with the integration capacitor and integration time increasing.

1. Introduction

Infrared detectors have a wide range of applications in areas of military, research, and manufacture, whose core part is an infrared focal plane assembly. The assembly mainly consists of two parts: focal plane arrays (FPAs) that function to convert radiation to current signal and readout integrated circuits (ROIC) that are responsible for realization of serial read and processing of signals sampled by the FPA.

Being an essential part of infrared readout integrated circuit, correlated double sampling (CDS) circuits play important roles in both depressing reset noise and conditioning integration signals [1–3]. Applications of focal planes of large format and high density put forward more harsh demand on low power dissipation and small layout area of a ROIC unit cell. Based on the theory that MOSFETs operating in the subthreshold region consume much less dissipation than those in the depletion region, this paper proposed a low power CDS structure that contains only one sampling capacitor, two switches, and two operation amplifiers (OPs), which saves the layout area [4, 5]. Then the noise calculation model is established, based on which noise analysis has been carried out by the approaches of transfer function and numerical simulation using SIMULINK and Verilog-A, whose results are in good agreement.

2. Circuit Design

2.1. Operating Principle

The proposed CDS circuit is shown in Figure 1. It comprises two Ops A₁ and A₂ that are connected as buffers, a sampling capacitor Csh and two complementary switches S₁ and S₂. A₁ and A₂ are standard two stage OPs, which are shown in Figure 2. They can provide high gain in order to reduce the error caused by the transmission process of the signals, in the meanwhile guarantee low noise.

Details are in the caption following the image — **Figure 1 (a)**
Open in figure viewer PowerPoint

Operating principle: (a) structure and (b) operating timing.

The clock timing waveforms of the CDS circuit are also illustrated in Figure 1. After the integrator resets, S₁ and S₂ are both switched on at “t₀” and the reset voltage of the integrator is coupled on the “V₁” node, which is the first sample; then the two switches are off at “t₁,” so the charge of Csh remains unchanged until “t₂”; after the integration duration, S₁ is turned on while S₂ remains off at “t₂,” and the second sampled signal is stored on “V₁” node. Because of the law of conservation of charge, the voltage of “V₂” node jumps by the difference value between the two sampling processes, which cuts off the error that resulted by the reset process of the integrator and suppresses low frequency noise. The proposed circuit structure is easy to implement, where using OP provides the conditions that no extra bias voltage is needed. As is known, capacitors occupy the most layout area; thus the design of only one capacitor saves much area. Besides subthreshold technology applied makes the proposed CDS circuit suitable for ROIC unit cells of the large format FPAs.

2.2. Power Optimization

Subthreshold technology is operating transistors in subthreshold region by providing gate-to-source voltage lower than threshold voltage (V_gs < V_th). Ideally, when V_gs is lower than V_th, the channel between source and drain is shut down. Nevertheless, some electrons still flow across the two ports, known as subthreshold current. Research demonstrates that the subthreshold current is increasing exponentially with the V_gs increasing, like the current in BJT. The relationship can be expressed as

(1)

where I₀ is the drain current when V_gs = V_th, V_T is the thermal voltage, and V_ds represents the drain-to-source voltage [6]. It is worth noting that the behavioural model of MOSFETs in subthreshold region is not accurate enough when the process of ICs goes into deep submicron, like 0.18 um. To do the calculation precisely, all the parameters adopted should be those obtained through simulations.

MOSFETs operating in subthreshold region have larger gm-to-channel current ratio than those in saturation region, which implies that subthreshold technology can be applied to optimization power dissipation of analog ICs with the guarantee for sufficient gain [7]. Table 1 is the comparison of the performance for the OPs consisting of MOSFETs working in different regions, where the supply voltages are 0 Volts to 3 Volts. It can be seen that the dissipation of the OP with the design of subthreshold technology succeeds in reducing at least one order of magnitude at the price of tradeoff with frequency character like small SR and GBW.

Table 1. Performance comparison.

Working region	Gain (dB)	GBW (Hz)	SR (V/us)	Power (W)
Saturation	78	50.5 M	10.2	622 u
Saturation	72.5	1.7 M	0.62	5.5 u
Subthreshold	75.6	1.58 M	0.57	720 n

Figure 3 shows the transient response of the proposed CDS circuit with the two OPs A₁ and A₂ designed by the subthreshold technology, in which the reset voltage is 1 Volt, the output of the integrator is 3 Volts, and S₁ is switched on at “t₂” After slight oscillation the output of the CDS circuit reaches 2 Volts, which is the integration voltage.

3. Noise Analysis

3.1. Noise Calculation Model

The calculation model of noise for the proposed CDS circuit is illustrated in Figure 4, involving four noise sources, which are from the two OPs and the two switches, respectively. The noise sources of the switches are thermal noise and the noise of the OPs is composed of thermal noise and 1/f noise.

Referred to the noise, voltage difference appears on C_s at “t₁.” Based on the law of charge conservation, the voltage across C_s maintains the same, so the noise voltage of the “V₂” node can be derived from

(2)

For the form of integration of frequency spectrum,

(3)

The transfer function of H_CDS(f) is given by

(4)

where

and

are noise power spectrum density (PSD) of “V₁” and “V₂” nodes at “nT_S,” respectively [8, 9], which can be found by (5), due to the independence of each noise source. Here

, H_1,i(f), and H_2,i(f) are transfer functions to the “V_i” node where i can be 1 or 2 for the following three noise sources:

, and

, respectively, which are the reference noise of A₁, the resistance-on noise of S₁, and the resistance-on noise of S₂, respectively. They are described in detail in the appendix:

(5)

The noise of the end “V₂” goes through A₂ with finite GBW, added by the noise source of A₂. The output noise of the CDS circuit is given by

(6)

where H_UG(f) is the transfer function of the buffers constituted by A₁ or A₂, and GBW represents gain-bandwidth product as seen in a number of textbooks for CMOS design:

(7)

3.2. Noise Calculation

The noise of the OP and the switch is introduced by MOSFETs. With regard to S₁ and S₂, MOSFETs in linear region produce thermal noise similar to resistance [6], which is given by

(8)

where k is the Boltzmann constant, T is the absolute temperature, and the MOSFETs of the OP are in the subthreshold region that generates not only thermal noise but also 1/f noise. The whole noise of the OP can be modeled as the reference input noise at the input port of the OP, which can be described by

(9)

where R_on⁡i represents the on-resistance of the switches and K_1ei and K_2ei represent thermal noise factor and 1/f noise factor for the OPs, respectively [7]. Here i is equal to 1 or 2. The calculation of noise adopts the parameters in Table 2, where R_o is the output resistance of OP.

Table 2. Parameters of noise calculation.

R_on⁡1, R_on⁡2	2.2 kΩ
K_1e1, K_1e2	4.0 × 10⁻¹⁷ V²/Hz
K_2e1, K_2e2	4.2 × 10⁻¹¹ V²/Hz
GBW₁, GBW₂	1.58 MHz
R_o	560 kΩ

Detectors capable of different wavebands produce a variety of densities of photocurrent, which leads to different integration time needed at certain ability of charge processing. We define K_int⁡ = T_int⁡/C_int⁡ as the integration factor, determined by the value of photocurrent and output swing of the ROIC. T_int⁡ means integration time, that is, the time interval between the two sampling processes of CDS, and C_int⁡ is integration capacitor. Figure 5 gives the output noise and its component noise of the proposed CDS circuit as functions of C_int⁡ under the situations of applications of long wave and medium wave, respectively. K_int⁡ is 50 K for typical long wave and 1 MEG for medium wave. As can be seen, noise increases with C_int⁡ increasing for long wave detection, in which 1/f noise has greater growth than thermal noise; when C_int⁡ < 2pF, thermal noise dominates. For medium wave situation, owing to the radiation weaker than that for long wave, larger T_int⁡ is needed at the same C_int⁡. The fact mentioned above results in longer interval between the two sampling processes of CDS, thus causing inferiority of suppressing 1/f noise. We can see that under medium wave situation 1/f noise is larger than thermal noise and rises with C_int⁡ increasing. By comparison of the two situations, we conclude that the noise that CDS circuit brings into the signal chain is larger for medium wave application than that for long wave.

Figure 6 shows the noise varying as functions of T_int⁡ at fixed C_int⁡, in which we can see 1/f noise is increasing with T_int⁡ increasing; while thermal noise nearly remains the same, the reason can be concluded through analysis that the increasing of T_int⁡ results in the increasing of in its low frequency area; thus more noise of low frequency is transmitted to the output node of the CDS circuit.

4. Simulation Experiment

The transient analysis model of the proposed CDS circuit is constructed in SIMULINK, which is shown in Figure 7. Thermal noise can be presented directly using the module in the simulator and 1/f noise is modeled by the approach brought out in [10]. HSPICE provides the possibility to simulate circuit noise in AC response by computing the PSD but cannot give the waveform of noise in transient response directly, whereas we carried out an approach to model time domain noise source using Verilog-A in this paper [11], as is shown in Figure 8(a). The noise is filtered by an RC filter, which settles its bandwidth. The waveform can be seen in Figure 8(b).

At last we averaged the RMS of the output noise from the scope in Figure 7 and output noise obtained by Verilog-A method to compare with those that were calculated by the equations in Section 2, and the unit of noise is uV. The three sets of results, which are given in Table 3, are in good agreement, therefore proving the feasibility of the method of transfer function noise analysis.

Table 3. Comparison of the two methods of noise analysis.

T_int⁡	100 ns	1 us	10 us	100 us	150 us	200 us
Theoretical results	28.03	29.39	29.41	31.20	34.02	46.62
SIMULINK results	27.44	28.57	28.82	30.71	34.05	47.52
Verilog-A results	27.19	29.11	28.23	30.26	32.80	49.57

5. Conclusion

For the applications of FPAs of large format and high density, a new structure of CDS circuit is proposed, whose power dissipation has been optimized by subthreshold technology, which leads to 720 nW. Because of using only one sampling capacitor, the proposed CDS circuit occupies small layout area. Then the noise calculation model is established, based on which the noise analysis has been carried out by the approaches of transfer function and numerical simulation using SIMULINK and Verilog-A. The results are in good agreement, demonstrating the validity of the present noise calculation model. Thermal noise plays a dominant role in the long wave situation while 1/f noise is the majority in the medium wave situation. The total noise of long wave is smaller than medium wave, both of which increase with the integration capacitor and integration time increasing.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors thank the Key Laboratory of Opto-Electronic Information Processing for the continuous supporting of the research on noise of readout integrated circuits.

Appendix

Explanation of (5)

The model of noise source of A₁ transmitting to the nodes across C_s is shown in Figure 9, whose transfer functions that appear in (5) are given by (A.1) and (A.2), respectively:

(A.1)

(A.2)

The model of noise source of S₁ transmitting to the nodes across C_s is shown in Figure 10, whose transfer functions that appear in (5) are given by (A.3) and (A.4), respectively:

(A.3)

(A.4)

The model of noise source of S₁ transmitting to the nodes across C_s is shown in Figure 11, whose transfer functions that appear in (5) are given by (A.5) and (A.6), respectively:

(A.5)

(A.6)

References

1 Fossum E. R. and Pain B., Infrared readout electronics for space-science sensors: state of the art and future directions, 2020, Infrared Technology, November 1993, 262–285, Proceedings of SPIE, https://doi.org/10.1117/12.160549.
10.1117/12.160549
Google Scholar
2 Richwine R., Balcerak R., Rapach C., Freyvogel K., and Sood A., A comprehensive model for bolometer element and uncooled array design and imaging sensor performance prediction, 6294, Infrared and Photoelectronic Imagers and Detector Devices II, August 2006, Proceedings of SPIE, https://doi.org/10.1117/12.682836, 2-s2.0-33750452326.
10.1117/12.682836
Google Scholar
3 Hsieh C.-C., Wu C.-Y., Jih F.-W., and Sun T.-P., Focal-plane-arrays and CMOS readout techniques of infrared imaging systems, IEEE Transactions on Circuits and Systems for Video Technology. (1997) 7, no. 4, 594–605, https://doi.org/10.1109/76.611171, 2-s2.0-0031210262.
10.1109/76.611171
Web of Science® Google Scholar
4 Calhoun B. H., Wang A., and Chandrakasan A., Device sizing for minimum energy operation in subthreshold circuits, Proceedings of the IEEE Custom Integrated Circuits Conference (CICC ′04), October 2004, 95–98, 2-s2.0-17044396646.
Google Scholar
5 Tsividis Y., Operation and Modeling of the MOS Transistor, 1999, 2nd edition, McGraw-Hill, New York, NY, USA.
Web of Science® Google Scholar
6 Razavid B., Design of Analog CMOS Integrated Circuits, 2000, McGraw, New York, NY, USA.
Google Scholar
7 Gray P. R., Hurst P. J., Lewis S. H., and Meyer R. G., Analysis and Design of Analog Integrated Circuits, 2008, Wiley.
Google Scholar
8 Schreier R., Silva J., Steensgaard J., and Temes G. C., Design-oriented estimation of thermal noise in switched-capacitor circuits, IEEE Transactions on Circuits and Systems I: Regular Papers. (2005) 52, no. 11, 2358–2368, https://doi.org/10.1109/tcsi.2005.853909, 2-s2.0-28444478100.
10.1109/TCSI.2005.853909
Web of Science® Google Scholar
9 Johnson J. F. and Lomheim T. S., Hybrid infrared focal-plane signal and noise modeling, 1541, Infrared Sensors: Detectors, Electronics, and Signal Processing, July 1991, 110–126, Proceedings of SPIE, 2-s2.0-0026391357.
Google Scholar
10 Butler Z. C. and Amarasinghe N. V., Random telegraph signals in deep submicron metal-oxide-semiconductor field-effect transistors, Noise and Fluctuations Control in Electronic Devices, 2002, American Scientific, 187–199.
Google Scholar
11 Kawai N. and Kawahito S., Noise analysis of high-gain, low-noise column readout circuits for CMOS image sensors, IEEE Transactions on Electron Devices. (2004) 51, no. 2, 185–194, https://doi.org/10.1109/ted.2003.822224, 2-s2.0-0442311257.
10.1109/TED.2003.822224
Web of Science® Google Scholar

Citing Literature

All articles

A New CDS Structure for High Density FPA with Low Power

Abstract

1. Introduction