Abstract
Buffer-based CMOS filters are maximally simplified circuits containing as few transistors as possible. Their applications, among others, include nano to micro watt biomedical sensors that process physiological signals of frequencies from 0.01 Hz to about 3 kHz. The order of a buffer-based filter is not greater than two. Hence, to obtain higher-order filters, a cascade of second-order filters is constructed. In this paper, a more general method for buffer-based filter synthesis is developed and presented. The method uses RLC ladder prototypes to obtain filters of arbitrary orders. In addition, a set of novel circuit solutions with ultra-low voltage and power are proposed. The introduced circuits were synthesized and simulated using 180-nm CMOS technology of X-FAB. One of the designed circuits is a fourth-order, low-pass filter that features: 100-Hz passband, 0.4-V supply voltage, power consumption of less than 5 nW, and dynamic range above 60 dB. Moreover, the total capacitance of the proposed filter (31 pF) is 25% lower compared to the structure synthesized using a conventional cascade method (40 pF).
Highlights
Buffer-based analogue filters are unity-gain amplifiers characterized by a bandwidth that is limited to the required frequency
Synthesis of a second-order response is possible, only for certain classes of buffers—for instance a few variants of source followers including undamped [1,2], super [3], flipped [4,5,6], and ones that belong to other classes [7,8,9,10]
The advantage of the buffer-based filter realizations is a substantially lower number of transistors compared to traditional Operational Transconductance Amplifier Capacitor (OTA-C) structures [11,12,13,14] since a single transistor or a differential pair replaces the entire OTA
Summary
Buffer-based analogue filters are unity-gain amplifiers (i.e., buffers, followers) characterized by a bandwidth that is limited to the required frequency. If the transmittanc2eof 13 is found to be desirable, the buffer circuit parameters (transistor transconductances, node capacitances, etc.) are selected to meet the required filter parameters. OTA-C filters of higher orders feature the analogy to unity-gain bu3foffe1r3s when synthesized from RLC ladders terminated by single resistance. OTA-C filters of higher orders feature the analogy to unity-gain buffers when. Due to iac = 0.5·gm1,2·(V+ − V−), an integration time constant is doubled, that is advantageous for low-frequency filter realizations. Cb1iAq=ua7.d10spoFf, Figure Cp2FA,=C121A.1=4 1p1F,.1C41Bp=F1,7C.110Bp=F,1C72.B10= p4.F60, CpF2,Ba=nd.I6B0=p25F0, paAndasIsBum= i2n5g0BputAterawsosruthmaipnpgroBxiumtatetirown orth ap aannddf −f−33ddBBoof f10100H0zH. z
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