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ISSCC 2011 / SESSION 17 / BIOMEDICAL & DISPLAYS / 17.1

17.1 A 160W 8-Channel Active Electrode System for EEGMonitoring

Jiawei Xu1,2, Refet Firat Yazicioglu3, Pieter Harpe1, Kofi A.A. Makinwa2,Chris Van Hoof3

1imec - Holst Centre, Eindhoven, The Netherlands,2Delft University of Technology, Delft, The Netherlands, 3imec, Leuven, Belgium

An important drawback of current biopotential monitoring systems is their

dependence on gel electrodes, which can dry out, cause skin irritation, andnecessitate skilled personnel. These associated drawbacks increase the runningcosts and significantly hamper their use in consumer healthcare and lifestyleapplications. Unfortunately, the use of gel-free, or dry, electrodes increases theelectrode-tissue contact impedance, thus exacerbating the effects of interferenceand cable motion artifacts. A solution is the use of active electrodes, i.e. elec-trodes in which an amplifier with high input impedance, low noise and goodelectrode offset rejection is co-integrated (Fig. 17.1.1). Previous active elec-trodes employed voltage buffers to facilitate the inter-channel gain matchingnecessary to achieve high CMRR [1]. However, low-noise buffers consume sig-nificant power and due to their lack of gain still require a low-noise and thuspower-hungry back-end to keep the total integrated noise at acceptable levels.To reduce the total power dissipation, this paper proposes a biopotential moni-toring system based on active electrodes with gain.

The proposed system consists of eight electrodes based on eight chopper-sta-bilized amplifiers with selectable gain (Fig. 17.1.2). To improve the CMRRbetween electrode pairs, which would otherwise be limited by gain mismatch,the back-end consists of a capacitive summing amplifier that feeds the averageoutput CM voltage of all eight amplifiers back to their non-inverting inputs. Thisreduces the effective CM voltage applied to each amplifier and thus boosts theCMRR between electrode pairs by 20log(Av), where Av is the nominal voltagegain of each electrode. This is similar to the traditional Driven-Right-Leg (DRL)circuit [2], which can also be used to improve the CMRR of active electrodes.However, DRL circuits suffer from stability problems and increased power dissi-pation, since the CM signal must be fed back through an electrode with signifi-cant impedance.

State-of-the-art biopotential instrumentation amplifiers are not well suited foruse in active dry electrodes. The input impedance of a chopper-stabilized capac-

itively coupled amplifier [4] is limited by the switched-capacitor impedanceformed by the input choppers and the input capacitors. Moving the chopper tothe amplifiers virtual ground solves this problem [6], at the expense of CMRR,which is now limited by capacitor mismatch [6]. A current-feedback amplifierhas higher input impedance [5], but its DC-servo loop limits the maximum elec-trode offset (EO) that it can handle to tens of mV.

The proposed active electrode (Fig. 17.1.2) consists of a chopper-stabilizedinverting amplifier, an input-impedance boosting loop, and two calibration loopsto improve the input impedance and compensate the offset of the amplifierrespectively. The amplifiers midband gain AV=C2/C1 and different gains can berealized by switching between different values of C2. The AC coupling capacitorC1 effectively rejects any EO, while the pseudo-resistor R2 and capacitors C2implement a high-pass characteristic. However, the switched-capacitor imped-ance formed by the input chopper and the amplifiers input capacitance accentu-ates its low-frequency noise [6]. To mitigate this C

1must be large (300pF), but

this reduces the input impedance of the amplifier. Therefore, a positive feedbackloop (Fig. 17.1.3) is used to boost the amplifiers input impedance [3]. Via Cfb,the loop supplies part of the current in C1 (Iin), thus reducing the current drawnfrom the recording electrode (Iel). Cfb is implemented as the combination of acoarse and fine capacitor array. At the various gain settings, the capacitors of thecoarse array are switched in tandem with the value of C2. The fine array can thenbe adjusted to compensate for the current that flows into the bond-pad and otherparasitic capacitances.

Two design challenges associated with chopper-stabilized amplifiers are thereduction of their output ripple and residual offset. The output ripple is due tothe amplifiers up-modulated offset, and resembles a low-pass filtered squarewave (Fig. 17.1.4). Compared to the V level biopotential signals, the ripple canbe quite large and therefore limits the amplifiers headroom. The amplifier in [3]

employs a ripple-reduction loop to sense and compensate output ripple.However, it operates continuously and thus increases the amplifiers power con-sumption. A further challenge is the residual offset caused by the clockfeedthrough or charge injection of the input chopper. This generates an offsetcurrent in pseudo-resistor R2, which can lead to a large output DC voltage drift.In [6], this offset current is canceled by a Gm-C servo-loop. However, it uses alarge off-chip capacitor (>10F) to realize a sufficiently low cutoff frequency.

In the proposed active electrode, both the ripple and offset challenges are metby digitally assisted calibration loops: a ripple-reduction loop (RRL) and a DC

servo loop (DSL) (Fig. 17.1.4). These exploit the fact that both the ripple and theoffset are relatively static and so do not need to be continuously reduced. Thecalibration starts with the RRL: the peak ripple levels (Va and Vb) are synchro-nously sampled, and its polarity is determined by a comparator. A logic circuit,implementing the SAR algorithm, then sets the input to a 7b current DAC inseven clock cycles. The DAC is implemented by two current mirror arrays CA1and CA2. The DSL operates in a similar manner: Vout is sampled and comparedto Vref and the results are used to set the input of another pair of current DACs.Their outputs are chopped in order to generate a modulated compensation cur-rent. Once the calibration is finished, the inputs to the current DACs are frozen,the calibration loops are shut-down and normal signal amplification starts.Thereafter, the power dissipation of the RRL and DSL circuits is mainly deter-mined by the DAC currents, and is less than 400nW.

The front and back-end of the eight-channel active electrode system were imple-

mented in a standard 0.18m CMOS process, and consumes a total 160W froma 1.8V supply. Fig 17.1.7 shows the chip micrograph of one active electrode andback-end CMFB. Figure 17.1.5 shows the measurement results. The top-left plotshows the amplifiers noise. Chopping at 500Hz leads to a total input-referrednoise of 0.8Vrms (0.5-100Hz). The top-right plot shows that at 50Hz the back-end CMFB improves the CMRR by 35dB, to 82dB. The bottom-left plot showsthe 2G input impedance achieved by the impedance-boosting loop. The bot-tom-right figure shows that the RRL reduced the output ripple from 40 to 2mV.Although not shown, the DSL reduces the output drift from 280 to 20mV. Figure17.1.6 compares the performance of the active electrode system with the state-of-the-art. Occipital EEG measurement result using the presented activeelectrodes is shown below the table.

In conclusion, the proposed biopotential amplifier is well suited for use in gel-free active electrode applications. Its performance exceeds the state-of-the-art interms of its CMRR, input impedance, offset rejection, and power-efficient offsetcompensation. This is realized through the use of an AC-coupled chopper-stabi-lized amplifier, in conjunction with back-end CM feedback, input impedanceboosting and digital offset calibration.

Acknowledgment:

The authors would like to thank Q. Fan, C. van Liempd, M .Pertijs and J.H.Huijsing for valuable discussion and comments.

References:

[1] M. Fernandez and R. Pallas-Areny, A simple active electrode for power lineinterference reduction in high resolution biopotential measurements, IEEEEMBS, vol.1, pp.9798. 1997.[2] Bruce B. Winter, John G. Webster, Driven-Right-Leg Circuit Design, IEEETransaction on Biomedical Engineering, Vol. 30, No. 1, pp.62-66, Jan. 1983[3] Q. Fan et al., A 1.8W 1V-Offset Capacitively-Coupled Chopper

Instrumentation Amplifier in 65nm CMOS, to be published in IEEE ESSCIRC,2010.[4] T. Denison et al., A 2.2W 94nV/Hz, Chopper-Stabilized InstrumentationAmplifier for EEG Detection in Chronic Implants, ISSCC Dig. Tech. Papers, pp.162-594, Feb. 2007.[5] R. F. Yazicioglu et al., A 60 W 60 nV/Hz Readout Front-End for PortableBiopotential Acquisition Systems, IEEE J. Solid-State Circuits, pp. 1100-1110,May 2007.[6] N. Verma et al., A Micro-Power EEG Acquisition SoC With Integrated FeatureExtraction Processor for a Chronic Seizure Detection System, IEEE J. Solid-State Circuits, pp. 804-816, April 2010.[7] X. Zou et al., A 1-V 450-nW Fully Integrated Programmable BiomedicalSensor Interface Chip, IEEE J. Solid-State Circuits, pp. 1067-1077, April 2009.

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Figure 17.1.1: EEG monitoring systems based on passive electrodes, activeelectrodes (buffer) and the proposed active electrodes (with gain).

Figure 17.1.2: Block diagram of the proposed 8-channel active electrodesystem.

Figure 17.1.3: Input-impedance-boosting circuit.

Figure 17.1.5: Measurement results.Figure 17.1.6: Comparison table with published state-of-the-art and EEG sig-nal measurement result.

Figure 17.1.4: Current-DAC based offset-calibration circuit.

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Figure 17.1.7: Photograph of prototype ASIC (one active electrode and back-endCMFB).