Digitally Tunable Lowpass-Notch Filter Design for Analog Front-Ends in Brain Signal Measurement Applications Undergraduate Student: Kaidi Du (BSEE) Advisor: Prof. Marvin Onabajo

Analog & Mixed-Signal Integrated Circuit (AMSIC) Research Laboratory, Northeastern University, Boston, MA

OverviewA digitally tunable Transconductance-

Capacitor Low-pass Notch Filter (LPNF) for

Electroencephalography (EEG) application is

presented. EEG signals fall into four basic

frequency bands, δ (1-4Hz), θ (4-8Hz), α (8-

13Hz), and β (13-40Hz), but the power line

interference at 60Hz negatively affects the

EEG signal measurement. For this reason, a

combination of a notch filter and a high-order

low-pass filter are employed in this work.

Digitally-assisted tuning methods are

becoming increasingly popular in integrated

circuit (IC) implementations [1]. In this

project, a LPNF was assembled on a

prototype board to perform measurements to

investigate the feasibility of digital tuning.

Tunable Filter DesignIn order to suppress power line interference,

it is necessary to have a lowpass filter with

the following characteristics:

1. A steep roll-off outside of the passband

2. High attenuation at the notch frequency

3. The size of the filter has to be small for

implementation on chips

Schematic of the fifth-order single-ended

low-pass notch filter [2]

Equations for the Null Frequencies

LPNF Diagram

Simulation Results

Transfer Function of LPNF

Measurement Results

Summary of Simulation Results

Screenshot from the Oscilloscope for Point 1

Input Signal Waveform and FFT at Point 1

Output Signal Waveform and FFT at Point 1

Screenshot from the Oscilloscope for Point 2

Input Signal Waveform and FFT at Point 2

Output Signal Waveform and FFT at Point 2

Conclusion A digitally tunable fifth-order Transconductance-Capacitor (Gm-C) Low-pass Notch Filter for EEG applications has been

presented. This design allows to digitally tune the notch frequency to be at 60Hz so that the power line interference signal can be

filtered out. Since the values of the grounded programmable capacitors are distributed in a binary manner, changing the gate

voltage of each NMOS transistor switch (i.e., changing the binary switch code) leads to tuning of the notch frequency.

References[1] S. A. Zahrai, L. Xu, C.-H. Chang, K. Wang, I. Farah, and M. Onabajo, “On-chip digital calibration for automatic input

impedance boosting during biopotential measurements”, in Proc. IEEE Intl. Midwest Symp. on Circuits and Systems (MWSCAS),

Aug. 2015.

[2] X. Qian, Y. P. Xu, and X. Li, “A CMOS continuous-time low-pass notch filter for EEG system,” Analog Integrated Circuits

Signal Processing, vol. 44, no. 3, pp. 231-238, Sep. 2005.

Department of Electrical & Computer Engineering

In Support of College Honors in Electrical Engineering