Bio Potential Readout System

8/3/2019 Bio Potential Readout System

1/26

Sami ur Rehman


2/26

Following slides are prepared with mainhelp from a thesis titled

A 0.8-V Low Power Analog Front-End IC for

Biomedical Signal Recording

By

Shuo-Ting Kao


3/26

Electrical Signals generating within ourbodies are termed as bio signals

These low voltage signals propagatealong nerve cells or muscle fiber

They are detected by placing electrodes

on the body surface silver -silver-chloride electrodes (Ag -

AgCl) are most commonly usedelectrodes


4/26


5/26

Also called brain waves

EEG Electroencephalogram (EEG refers to the recording ofthe brain's spontaneous electrical activity)

several channels of the EEG are recorded simultaneouslyfrom various locations on the scalp for comparative analysisof activities in different regions of the brain


6/26

The ECG is the electricalmanifestation of thecontractile activity of the

heart The rhythm of the heart in

terms of beats per minute{bpm) may be easilyestimated by counting thereadily identifiable waves


7/26

Muscular potential

Skeletal muscle fibers are considered to be twitchfibers because they produce a mechanical twitch

response for a single stimulus and generate apropagated action potential.


8/26


9/26


10/26

Generally, in order to read out the bio-potentials, two skin electrodes sticking onthe body. However, bio-potential are

commonly low frequency small signals, andthere are three issues.

First, the major one is the flicker noise. Dueto low frequency signals, the chargecarriers are trapped easily by dangling

bonds, which appear at the interfacebetween gate oxide and silicon substrate,and later released by the energy states,introducing flicker noise in the drain current.


11/26

Second, another issue is power-lineinterference. Power line signal 50/60Hzcoupled to the human body is not

negligible compared to the bio-potentialsignals.

Third, the other one is the differentialelectrode offset (DEO), which comes from

the difference of two electrodes DC level.For conventional AgCl electrodes, the DEOcan be as high as 50mv and the DEOchanges with time slowly.


12/26

ELIMINATING FLICKER NOISE ANDCOMMON MODE NOISE:

BY THE USE OF INSTRUMENTATIONAMPLIFIER WITH VERY HIGH CMRR

ELIMINATING DIFFERENTIAL ELECTRODE

VOLTAGE (DEO), ALSO CALLED DC OFFSETVOLTAGE:

THROUGH THE MOTHOD OF CHOPPING,ELABORATED FURTHERAHEAHD


13/26

TYPES: DEVICE NOISE.NOISE INHERENT TO THE STRUCTURE OF THE

DEVICE. INCLUDES FLICKER NOISE AND THERMAL NOISE

THERMAL NOISE: ALSO CALLED WHITE NOISE its noise power spectrum density is constant over a given frequency thermal noise comes from the

random motion of electrons in conductors Flicker Noise The flicker noise spectral density is inversely proportional to

frequency, so it is also called 1/f noise.


14/26

INPUT SHIFTED TO HIGH FREQUENCY, LOWFREQUENCY INPUT SIGNAL IS

MODULATED, AMPLIFIED AND THENBROUGHT BACK TO LOW BASEBANDFREQUENCY.

OFFSET IS MODULATED TO HIGHFREQUENCY AND THE BASEBAND SIGNALIS THEN EXTRACTED USING LOW PASSFILTER.


15/26


16/26


17/26


18/26

It includes a fully-differential folded-cascodeopamp, a gm-c filter, three NMOS chopperand capacitors. Since the

CMOS chopper is unable to work at a lowsupply voltage environment, the bootstrappedNMOS chopper is adopted without reliabilityissue. There are two

feedback paths in the system. One is for settingthe closed-loop gain and the other is forcancelling DEO. The capacitive closed-loop

configuration dissipates less power than theresistive feedback one. Note that thedemodulation of the signal is inside the opamp


19/26


20/26

Following slides are prepared with helpfrom a paper titled

A 90nm CMOS Bio-Potential Signal ReadoutFront-End with

Improved Powerline Interference Rejection

By Chon-Teng Ma, Pui-In Mak, Mang-I Vai, Peng-Un Mak,

Sio-Hang Pun, Wan Feng and R. P. Martins


21/26

This paper describes a 90nm CMOS low-noise low-power biopotential

signal readout front-end (RFE). The front-stage instrumentation

amplifier (IA) features a chopper; an AC-coupler and a novel chopper

notch filter for minimizing the dc-offset; transistors' flicker noise and

50Hz powerline interference concurrently. A noise-aware transistor

selection (thin- and thick-oxide) in the IA enables a flexible tradeoff

between noise and input impedance performances. The 2nd stage is a

spike filter clocked by a parallel use of two non-overlapping clock

generators, effectively tracking and suppressing the chopper spikes.

The last stage is a gain-bandwidth-controllable amplifier for boostingthe gain and alleviating different bio-potential signal measurements

through simple digital controls. Simulation results showed that the

RFE is capable of tolerating a differential electrode offset up to

50mV, while achieving 140dB CMRR and 51.4nV/Hz inputreferred

noise density. The notch at 50Hz achieves 41dB rejection.

The entire RFE consumes 16.55 to 35.5A at 3V.


22/26


23/26


24/26


25/26


26/26

AC coupling is used to generate thesame spectrum of DEO and IA offset and

applied in feedback, resultantly bothoffsets are cancelled.

Notch filter need to be used to eliminateAC signal interference

Now the noise free signal can bedemodulated and fed to Delta SigmaModulator for digitizing purpose

Documents

Bio Potential Readout System