500nW A LOW POWER SWITCHED CAPACITOR BASED ACTIVE LOW PASS FILTER FOR BIOMEDICAL APPLICATIONS

International Journal of VLSI design & Communication Systems (VLSICS) Vol.7, No.5/6, December 2016
DOI : 10.5121/vlsic.2016.7603 25
500nW A LOW POWER SWITCHED CAPACITOR
BASED ACTIVE LOW PASS FILTER FOR
BIOMEDICAL APPLICATIONS
U. Gnaneshwara chary, L.Babitha and Vandana.Ch
Department of Electronics & Communication Engineering, BVRIT, Narsapur, Telangana
ABSTRACT
This paper presents a low power, high gain, low area low pass filter design. The pre-processing block in
this filter is the two-stage operational amplifier which is designed using the DTMOSFETS. This filter is
operated at very low supply voltages of +0.2V.The PMOS input differential pairs are used to enhance the
driving capability of the op-amp. input stage of the op-amp is the differential amplifier in which the inputs
are provided to the two PMOS transistors. The second stage is the gain stage in which common source
amplifier is used. The design parameter values are determined which optimize an objective feature
satisfying specifications or constraints. The low pass filter is designed with a cut-off frequency of 100-HZ
and a resistance of 10ohms, which occupies more layout area. In order to reduce the layout area the low
pass filter is designed using the switched capacitor. The circuit is implemented with a supply voltage of
0.4V in 0.9-um technology, with a power consumption of 300nW and the simulations are performed using
HSPICE simulator and layouts are designed using CUSTOM DESIGNER tool.
KEYWORDS
DTMOS, Switched-capacitor, Low pass filter.
1. INTRODUCTION
With rapid development of micro-electrons in the recent past years, more and more applications
require an ultra-low amplitude signal measurement module, such as implantable devices in
biomedical applications. Filters are the most important blocks in many biomedical devices. The
design of low pass filter is presented in this paper. The op-amp is the most important pre-
processing block in the filter. The op-amp designed using the CMOS transistors will operate
supply voltage of 1V, which exhibits more power dissipation. To lower the power dissipation in
the op-amp the voltages must be scaled down. Scaling down the voltages will lead to less
threshold voltages, which increases the leakage current as reported in [1]. The other alternative to
decrease the power dissipation at low supply voltages is to use the DTMOS technology[6][7].
The most interesting feature of the DTMOSFET is the dynamic variation of the threshold voltage
of the MOSFET, which reduces the leakage power when the transistor is at off state. This method
lowers the threshold voltage when the transistor is turned on and Increases the threshold voltage
when the transistor is off. Thus, the method can reduce leakage current when the transistor is off.

26
2. LOW POWER TWO-STAGE OPAMP DESIGN
The existing two-stage opamp is designed at a supply voltage of 1V, with a input swing of 0.5v to
0.8v. The gain obtained by the opamp using CMOS transistors is very low i.e. 22.5dB. To
increase the gain of the opamp the voltage need to be reduced, which increases the leakage power
and reduces the speed of the opamp. The CMOS two-stage opamp is as given below:
Figure:1 Two-stage opamp using CMOS
The W/L ratios used in the design of the opamp are as listed below:
Device Type W(um) L(um)
M1&M2 P 10 0.378
M3&M4 N 18 0.556
M5&M8 P 35 0.460
M7 P 80 0.530
M6 N 15 0.100
3. LOW VOLTAGE DTMOS BASED OPAMP
In order to reduce the power the op-amp is designed at low supply voltages[8]. The proposed
two-stage op-amp is designed with a supply voltage of +0.2v. The input stage of the op-amp is
the differential amplifier stage, in which the PMOS transistors are used. The usage of PMOS at
the input will improve the driving capability of the op-amp. The current mirror is used as the load
in the differential amplifier. The second stage of the op-amp is the common source amplifier,
which is driven by the output of the differential amplifier. The input stage transistors are replaced
by the DTMOSFETs to achieve high gain at the low supply voltages. The design of the DTMOS
based two-stage op-amp is as given below:

27
Figure: 2 Two-stage op-amp using DTMOS op-amp
This op-amp will operate at input common mode voltage range of 0.17v to 0.35v. The design
parameter values are determined from the input specifications chosen as 0.5v/µs slew rate from
which the current obtained as 500nW. The W/L ratios of the DTMOSFETs used in the design of
two-stage op-amp are as listed below:
Device Type W(um) L(um)
M1&M2 P 5 0.380
M3&M4 N 45 0.480
M5&M8 P 18 0.555
M7 P 10 0.380
M6 N 25 0.100
The gain obtained by the op-amp designed using the DTMOSFET is 42.5dB, which is very high
compared to the op-amp designed using he CMOS transistors.
Comparison of CMOS op-amp with DTMOS op-amp:
The usage of DTMOS over the CMOS in op-amp design will improve the gain of the op-amp at
low supply voltages. The following are the results obtained from the HSPICE simulator.

28
Figure: 3 Gain response of CMOS op-amp
The above is the gain response of the op-amp using the CMOS transistors in which the gain is
obtained as 22.5dB. The gain response of the DTMOS op-amp is as plotted below:
Figure: 4 Gain response of DTMOS op-amp
4. ACTIVE LOW PASS FILTER DESIGN
The low pass filter is designed using the DTMOS two-stage op-amp as the pre-processing block.
The parameters required to design a low pass filter are the resistance and capacitance. The
frequency of the low pass filter will depend on these parameters. The relation between the
resistance, capacitance and frequency is given as below:
f = 1/2*π*R*C
The low pass filter is designed for biomedical applications. As the biomedical signal frequencies
are low, here the cut-off frequency chosen to design the low pass filter is 100HZ. The R & C

29
values are calculated by the above relation based on the chosen cut-off frequency. The values of
the parameters used in the low pass filter are as given below:
F = 100HZ
R = 100ohms
C = 890pf
The low pass filter is designed by using the DTMOS op-amp and the circuit diagram of the low
pass filter using DTMOS op-amp along with resistor and a capacitor is as shown below:
Figure: 5 Low pass filter without switched-capacitor
Designs of low pass filter using switched capacitor:
The low pass filter using the switched capacitor is designed in which the resistance in the filter is
replaced by the switched capacitor. This filter is designed with the same specifications as the
previous low pass filter which is designed with the resistor and capacitor. The switched capacitor
is designed with two transistors and a capacitor, which provides the resistance of 10 ohms. This
filter is designed in order to reduce the layout area of the filter which is designed with a resistor.
Since the resistor occupies most of the design area, it is replaced by the switched capacitors
which occupy less layout area.
The circuit diagram of the switched capacitor based low pass filter in which the resistor is
replaced by the switched capacitor is as given below:

30
Figure: 6 Low pass filter using switched capacitor
The main advantage of switched capacitor based LPF over the low pass filer designed without
switched capacitor is the reduction in the layout area.
5. RESULTS & ANALYSIS
The layouts have been designed for low pass filters with a resistor as well as for a low pass filter
using switched capacitor. These layouts are designed in the CUSTOM DESIGNER tool. The
below are the layouts of the LPF with and without switched capacitor:
Figure: 7 Layout of the LPF without Switched-capacitor
The area obtained by the above LPF is the 23380um2
. The layout of the LPF with switched
capacitor is as given below:

Figure: 7 Layout of the LPF with Switched
The area obtained by the LPF with switched
the layout area of the LPF without Switched
Comparison of various parameters results are as listed below:
6. CONCLUSION
The Active switched-capacitor based low pass filter using Two
low voltage and low power is designed. This two
be used in low power, low voltage High CMRR and PSRR applications such as Biomedical
instruments and a small battery operated devices. The circuit has been designed in CUSTOM
DESIGNER using 0.90um CMOS technology. We have described an ECG amplifier with a
300nW of power consumption, and good cardiac signal fidelity. The layouts have been designed
and the areas have been compared. The Proposed two
compensation is well suited to biomedical systems such as cardiac pacem
(ECG) where low-power consumption is of primary concern.
Figure: 7 Layout of the LPF with Switched-capacitor
The area obtained by the LPF with switched-capacitor is 7975um2
which is very less compared to
the layout area of the LPF without Switched-capacitor.
of various parameters results are as listed below:
capacitor based low pass filter using Two-stage operational amplifier with
low voltage and low power is designed. This two-stage amplifier with Miller compensation can
and the areas have been compared. The Proposed two-stage operational amplifier with Miller
compensation is well suited to biomedical systems such as cardiac pacemaker, electrocardiogram
power consumption is of primary concern.
31
which is very less compared to
stage operational amplifier with
stage amplifier with Miller compensation can
stage operational amplifier with Miller
aker, electrocardiogram

32
REFERENCES
[1] Achigui, H.F,; Fayomi, C.J.B,: Sawan, M, “A 1v low-poer lo-noise DTMOS based class AB op-
amp,” IEEE NEWCAS, vol.10, pp.307-310, Aug. 2005.
[2] Shuenn-Yuh Lee; Shyh-Chyang Lee,” Design of low-power switched-capacitor filter with switched-
op-amp technique,” IEEE Circuits and systems, vol.1, pp.241-244, Dec 2004.
[3] J. Mahattanankull, “Design procedure for two-stage CMOS operational amplifiers employing current
buffer,” IEEE Trans. Circuits and Systems II, Vol. 52, No. 11, pp. 776-770, Nov. 2005.
[4] C. Nanda, J. Mukhopadhyay, D. Mandal, and S. Chakrabarti, “A CMOS instrumentation amplifier
with low voltage and low noise for portable ECG monitoring systems,” IEEE Int. Conf. on
Semiconductor Electronics, pp. 54-58, Nov. 2008.
[5] Van Mieghem, C; Sabbe, M; Knockaert, D (2004). "The Clinical value of the ECG in no cardiac
conditions” Chest 125(4):1561–76.Doi:10.1378/chest.125.4.1561.PMID 15078775.
[6] F. Assaderaghi, D. Sinitsky, S. Parke, J. Bokor, P. K. Ko, and C. Hu, “A Dynamic Threshold voltage
MOSFET (DTMOS) for ultra- low voltage operation,” in Int. Electron Devices Meeting, Techn.
Digest, 1994, pp.b809–812
[7] F. Assaderaghi, “DTMOS: Its derivatives and variations, and their potential applications,” in Proc.
12th Int. Conf. Microelectron., 2000, pp.9–10.
[8] F. Maloberti, F. Francesoni, P. Malcovati, and O. J. A. P. Nys, “Design considerations on low voltage
low-power data converters,” IEEE Trans. Circuits Syst. I, Fundam. Theory Appl., vol. 42, no. 11,
pp.853–863, Nov. 1995.

500nW A LOW POWER SWITCHED CAPACITOR BASED ACTIVE LOW PASS FILTER FOR BIOMEDICAL APPLICATIONS

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