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Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator



M. Rashtian, O. Hashemipour and K. Navi
 
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ABSTRACT

The low-voltage high-speed auto zeroed integrator characteristics is improved by applying current steering mechanism in the opamp structure of the integrators and utilizing the non-linear properties of switches. The proposed design results in considerable reduction of power dissipation. Based on this improvement a band-pass filter with centre frequency of 1 MHz and clock frequency of 6 MHz is designed. Furthermore a new circuit for implementation of an auto-zero low-pass filter is presented. Based on this configuration a fourth order low-pass switched capacitor filter with cut off frequency of 600 KHz and clock frequency of 6 MHz is presented. The proposed circuits are simulated using HSPICE and 0.25 μm CMOS technology at 1.5 V supply voltage.

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  How to cite this article:

M. Rashtian, O. Hashemipour and K. Navi, 2008. Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator. Journal of Applied Sciences, 8: 1771-1775.

DOI: 10.3923/jas.2008.1771.1775

URL: https://scialert.net/abstract/?doi=jas.2008.1771.1775
 

INTRODUCTION

The main difficulty of low-voltage switched-capacitor (SC) circuit design is the driving voltage of the input CMOS switches. Two conventional approaches for low-voltage SC design using standard CMOS technologies are currently implemented. The first one is the bootstrapping method (Dessouky and Kaiser, 2001) in order to generate higher clock voltages to drive the gate voltage of the input switches, although the existence of on-chip high voltages is a danger for deep-submicrometer CMOS processes (Guan et al., 2001). The second one is the switched-opamp method (Sauerbrey et al., 2002; Cheung et al., 2002). The opamp in the integrator is turned on and off to transfer voltage signals to the next integrator stage, so no input switch is required to sample the input voltage. Switched opamp method is a real low-voltage SC circuit, but it is not suitable for high-speed SC circuit applications due to turning on/off time of the opamp. Another example of non switched mode approach is the GM-C filters which are designed and presented in low voltage applications (Garcia-Ortega et al., 2007). Recently the auto-zeroed integrator (AZI) and its application for low-voltage SC circuits was reported by Wang and Embabi (2003), where the basic AZI circuit (Bidari et al., 1999) was modified for the design of low-voltage and high-speed SC without bootstrapped clock voltage. Based on the previous study reported by Wang and Embabi (2003), here a two mode clock controlled opamp is utilized in order to reduce the power consumption and settling time of the auto-zeroed integrator circuit. First the basic modified AZI block and the new opamp structure are explained. Then a band-pass filter with centre frequency of 1 MHz and clock frequency of 6 MHz based on the above idea is designed, explained and compared to the previous study. Finally a new structure for implementation of the low-pass filter using AZI is presented. The capacitor coupling input in the conventional AZI circuits does not allow designing of low-pass filters. This short coming is solved by designing an additional input stage. Based on this design a fourth order Chebyshev filter with pass frequency of 600 kHz and clock frequency of 6 MHz is designed and simulated.

IMPROVED AZI CIRCUIT

The high speed low-voltage SC filter can be realized by cascading of AZI blocks. Figure 1 shows the basic differential AZI block. The complete description of the circuit is explained by Wang and Embabi (2003) and Rashtian and Hashemipour (2006). Equation 1 shows the mathematical sequence in time domain and Eq. 2 shows the Z-domain transfer function of this circuit if the input of the AZI circuit comes from the output of another AZI block.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(1)

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(2)

It is important to notice that all switches in this circuit are nMOS transistors whose sources are connected to the common-mode voltage VCM and the gates are stimulated at the supply voltage VDD, which is expressed as VGS(on) = VDD-VCM. This means that VGS is not signal dependent, so charge injections and clock feed-through are cancelled in fully differential structure. If we assume that VCM = 0.5V to keep a reasonable output swing range to satisfy the requirement of VGS>VTH to turn on a switch (VTH ≈ 0.6V), the supply voltage should be VDD > VTH + VCM > 1V. This explains why AZI can work at 1.5 V in 0.25 μm CMOS technology.

In phase (φ1) in Fig. 1, the output and input of the op-amp are connected together by the action of switches (M6 and M8) and the final value of the output voltage is a dc voltage (VCM). Doing so, the linear action of the opamp is not critically essential and setting of the output voltage at VCM can be done by external switches. In this approach the output stage of the opamp is operated in two current mode levels by the current steering action of the transistors M5B-M8B as shown in Fig. 2.

The W/L ratio of the current steering transistors is ten times greater than M5-M8. The current steering mode is controlled by transistors M5C-M8C. In the integration phase (φ2), the output stage current is normal but in the other phase (φ1) it is under low current condition. This can be called as pseudo switched opamp configuration with an improved characteristic of power consumption without speed reduction characteristics of switched opamps. In the phase (φ1) the output voltage is set to VCM by the non-linear action of nMOS switches M17 and M18. Unlike the switched opamp technique, the nonlinear action of these switched transistors and the linear action of opamp without current steering transistors i.e., (M5B-M8B) will result in the same final steady state value with reduced power consumption.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 1: Basic auto zeroed integrator circuit

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 2: Modified op-amp

It means that the nMOS switches M17 and M18 are only used to reduce the settling time. The fast action of common mode feedback is critically important in high speed switched capacitor circuits (Hernandez-Garduno and Silva-Martinez, 2006) (Choksi and Carley, 2003). In this study, a unique dynamic common-mode detection circuit (C3~C6) working at 1.5 V without any signal-dependent switch is utilized. The detected common-mode voltage is compared to VCM through M9 and M10. The result is fed back to the input stage of the opamp through a current mirror. Both C3 and C4 are discharged in φ1 (the integrating phase) because the outputs are auto-zeroed by M17 and M18. In φ2, output voltages are sampled by C3 and C4 and integrated with C5 and C6. These four capacitors and the switches perform as an RC voltage divider.

BAND-PASS FILTER DESIGN

Figure 3 shows the schematic circuit of the band-pass filter (Wang and Embabi, 2003) with capacitor values of (C1 = 0.5, C2 = 2.31, Ci1 = 1.95, Ci2 = 2.15, Cc = 0.3, Cf = 1.92) pF. The clock frequency is set to 6 MHz with centre frequency of 1MHz and Q = 8.

Assuming the output of the first and second stage to named V1 and V2, respectively then:

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(3)

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(4)

From Eq. 3 and 4 the z-domain transfer function is given by 5:

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(5)

Output of the improved circuit and the previous study (Wang and Embabi, 2003) are compared in Fig. 4 at an input frequency of 1 MHz and Vp-p = 1.2 V. For assessment of fall time improvement in proposed method a small part of Fig. 4 is magnified in Fig. 5.

As shown in Fig. 5, the fall time is improved by the non-linear action of switches (M17 and M18) in the proposed opamp. Simulated results show a remarkable reduction (~32%) in the power consumption of this circuit compared to previous study.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 3: Band-pass SC filter

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 4: Simulated output of differential band-pass filter at 6 MHz clock frequency

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 5: Zoomed area of Fig. 4

Figure 6 is acquired by using of more than 100 transient simulation results.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 6: Band-pass filter frequency response

LOW-PASS FILTER DESIGN

The input coupling capacitor, which is used in the input of conventional AZI circuits, prevents applying the AZI circuit to be realized as a low-pass filter. This problem is eliminated by the addition of an input circuitry (OP0 and related components) as shown in Fig. 7 where this technique is used in a 2nd order low-pass filter. In φ1 the output of OP0 is set to Vcm and in φ2 OP0 is acting as a buffer with gain of (-1). It is obvious that the correct action of this stage depends on the relative accuracy of resistances and absolute accuracy is not crucially important.

Based on the above idea, Eq. 6 and 7 are obtained in the output of OP1 and OP2, respectively.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(6)

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(7)

Using the circuit shown in Fig. 7 and cascading them with the capacitor values of (C1 = 0.89, C2 = 0.61, Ci1 = 2, Ci2 = 2, Cc = 1.06 and Cf = 0.89) pF for the first stage and (C1 = 0.69, C2 = 1, Ci1 = 1, Ci2 = 2, Cc = 0.38 and Cf = 0.69) pF for the second stage, a low-pass fourth order Chebyshev filter is realized.

The circuit for the 4th order filter is further simplified by the fact that the designed input stage is no longer necessary for the input of the second cascaded biquad stage. This is due to the fact that the output of the first biquad stage is set to VCM in φ2 inherently. Using the above capacitor values and Eq. 6 and 7 the transfer function in z domain is obtained in 8.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
(8)

Figure 8 shows the differential output of the designed low-pass filter with input voltage of 1.2 V p-p and cut off frequency of 600 kHz at the clock frequency of 6 MHz. Frequency response of the designed filter is shown in Fig. 9.

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 7: Second order low-pass circuit

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 8: Differential output of the Low-pass filter with 1.2 v p-p input at 600 kHz

Image for - Design of a Low-Voltage High-Speed Switched-Capacitor Filters Using Improved Auto Zeroed Integrator
Fig. 9: Low-pass filter frequency response

This plot is drawn using more than 100 HSPICE real time simulation.

CONCLUSION

The auto zeroed integrator performance is improved by a current steering method and the application of non-linear characteristics of the switched transistors. Based on this method, a band-pass filter with centre frequency of 1 MHz and clock frequency of 6 MHz with quality factor of 8 is designed and simulated. A ~32% reduction in power consumption and a reduction in fall time are also observed. The short come of the AZI circuit in the realization of switched capacitor low-pass filter has been overcome by addition of an input circuitry. Based on this improvement, a low-pass 4th order filter with cut off frequency of 600 kHz and sampling frequency of 6 MHz with differential output swing of 1.4 volt with 1.5 volt supply is designed and simulated using 0.25 μm technology.

REFERENCES
1:  Bidari, E., F. Maloberti, U. Moon, J. Steensgaard and G. Temes, 1999. Low-voltage switched-capacitor circuits. Proc. IEEE Int. Symp. Circuits and Syst., 2: 49-51.

2:  Cheung, V.S.L., H.C. Luong and K. Wing-Hung, 2002. A 1-V 10.7 MHz switched-opamp bandpass modulator using double-sampling finite-gain-compensation technique. IEEE J. Solid State Circuits, 37: 1215-1225.
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3:  Choksi, O. and L.R. Carley, 2003. Analysis of switched-capacitor common-mode feedback circuit. IEEE Trans. Circuts Syst., 50: 906-917.
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4:  Dessouky, M. and A. Kaiser, 2001. Very low-voltage digital-audio ΔΣ modulator with 88-dB dynamic range using local switch bootstrapping. IEEE J. Solid-State Circuits, 36: 349-355.
CrossRef  |  Direct Link  |  

5:  Garcia-Ortega, J.M., E. Tlelo-Cuautle and C. Sanchez-Lopez, 2007. Design of current-mode Gm-C filters from the transformation of opamp-RC filters. J. Applied Sci., 7: 1321-1326.
CrossRef  |  Direct Link  |  

6:  Guan, H., B.J. Cho, M.F. Li, Z. Xu, Y.D. He and Z. Dong, 2001. Experimental evidence of interface-controlled mechanism of quasi-breakdown in ultrathin gate oxide. IEEE Trans. Electron 48: 1010-1013.
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7:  Hernandez-Garduno, D. and J. Silva-Martinez, 2005. Continuous-time common-mode feedback for high-speed switched-capacitor networks. IEEE J. Solid-State Circ., 40: 1610-1617.
CrossRef  |  Direct Link  |  

8:  Rashtian, M. and O. Hashemipour, 2006. An improved low-voltage high speed switched capacitor circuit. Proceedingof the International Conference on Engineering Education.

9:  Sauerbrey, J., T. Tille, D. Schmitt-Landsiedel and R. Thewes, 2002. A 0.7-V mosfet-only switched-opamp modulator in standard digital CMOS technology. IEEE J. Solid State Circuits, 37: 1662-1669.
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10:  Wang, L. and S.H.K. Embabi, 2003. Low-voltage high-speed switched-capacitor circuits without voltage bootstrapper. IEEE J. Solid State Circuits, 38: 1411-1415.
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