Design Considerations for Ground Referencingin Multi-Module Neural Implants

Dorian Haci† ?, Yan Liu† ?, Sara S. Ghoreishizadeh† ?, Timothy G. Constandinou† ?

†Department of Electrical and Electronic Engineering, Imperial College London, SW7 2BT, UK?Centre for Bio-Inspired Technology, Institute of Biomedical Eng., Imperial College London, SW7 2AZ, UK

Email: {d.haci14, yan.liu06, s.ghoreishizadeh14, t.constandinou}@imperial.ac.uk

Abstract—Implantable neural interfaces have evolved in thepast decades from stimulation-only devices to closed-loop record-ing and stimulation systems, allowing both for more targetedtherapeutic techniques and more advanced prosthetic implants.Emerging applications require multi-module active implantabledevices with intrabody power and data transmission. Thisdistributed approach poses a new set of challenges relatedto inter-module connectivity, functional reliability and patientsafety. This paper addresses the ground referencing challenge inactive multi-implant systems, with a particular focus on neuralrecording devices. Three different grounding schemes (passive,drive, and sense) are presented and evaluated in terms of bothrecording reliability and patient safety. Considerations on thepractical implementation of body potential referencing circuitryare finally discussed, with a detailed analysis of their impact onthe recording performance.

I. INTRODUCTION

In the past few decades, Implantable Medical Devices(IMDs) have shown remarkable success both as therapeuticdevices in treating debilitating conditions, and as prostheticdevices, in restoring functions by bypassing dysfunctionalorgans/pathways. The vast majority of clinical grade IMDsare electrical stimulators. A well known example is the cardiacpacemaker, which uses periodic electrical pulses as a stimu-lation means for heart contraction [1]. More recently, neuro-modulation devices such as cochlear implants for deaf peopleand deep brain stimulators (DBS) for Parkinson’s disease andessential tremor (ET), have demonstrated significant impact tothe quality of life of millions of patients [2], [3]. Besides theseexamples, implantable devices for treatment of Epilepsy haveproven to be effective therapeutic solutions, with vagus nervestimulation (VNS) implants from Cyberonics playing a keyrole [4]. Such neuroprosthetic IMDs are typically implantedin the upper chest and based on active electronics enclosedin a metallic or ceramic can, which houses also the battery.Implantable leads are used as means for reaching the area tostimulate and contact electrodes are responsible for deliveringthe electrical pulse to the target tissue.

The need for both stimulation and recording of the biosignalactivity, often a requirement in modern therapeutic techniques,have caused a recent surge in the generation of closed-loopneuroprostheses. This relatively new category of implantabledevices, in addition to stimulation, also introduces the needfor front end sensing, signal conditioning, and real-time pro-cessing. [5], [6]. Recently FDA-approved and now clinically-available examples of such devices include the closed-loopDBS Activa PC+S system from Medtronic and the responsiveneurostimulation (RNS) system for refractory partial epilepsyimplemented by NeuroPace [7], [8].

Closed-loop applications are now requiring more interfac-ing channels (both for recording and stimulation) and atmultiple locations. This is posing significant challenges for

Stim

ulat

ion

Stim

ulat

ion

Reco

rdin

g

Implantable lead

a) b) c)

Single-ModuleStimulation Implant

Single-ModuleStimulation Implant

Single-ModuleClosed-loop Implant

(Stim + Rec)

Single-ModuleClosed-loop Implant

(Stim + Rec)

Multi-Module Closed-loop

Implant System

Multi-Module Closed-loop

Implant System

Power & DataCommunication

Stim + RecStim + Rec

StimStim

Stim + RecStim + Rec

RecRec

Fig. 1. Comparison between different IMD generations: (a) single-modulepace-making stimulation implant, (b) single-module closed-loop stimulationand recording implant, and (3) multi-module closed-loop implant system.

the connectivity, for example, the number of implantableleads, and conductors in each lead, the number of electricalconnections into a hermetic package, the challenge of tryingto observe microvolt-level biopotential signals through lengthleads. Additionally, surgical and safety considerations placeadditional requirements on performing certain functions incertain locations. For example, a chest-mounted IMD canoccupy more volume (i.e. larger battery) and dissipate morepower (i.e. more capacity for on-board processing) than onemounted in the head.

To address this, a new generation of implantable medicaldevices is now emerging. Instead of the centralised approachthat uses a single active implantable device, the system isnow being partitioned and distributed across multiple activeimplantable devices each with specific functions, and locatedat different sites, e.g. [9], [10]. This approach addresses someof the limitations of single-module implants, however it posesa new set of challenges, mainly related to inter-module connec-tivity, functional reliability and patient safety. The implantableleads are now additionally used to facilitate transfer powerbetween modules and for bidirectional communication [11].

Moreover, having distinct active modules implies the needfor electrical isolation from each other to avoid direct currentpaths between them. This means each implantable modulehas its own power domain, and therefore different referencepotentials between the electronic circuits (module grounds).

This paper identifies the ground referencing challenge inmulti-module active implants and discusses three differentgrounding configurations for distributed neural implants, withparticular focus on their impact to the recording performanceand on device safety. The remainder of this paper is organisedas follows: Section II describes the multi-module implantconfiguration, discussing key challenges when dealing withmultiple power domains in an implant; Section III presentsthree different grounding schemes for dealing with this; Sec-tion IV analyses these (practical considerations, recording

Peripheral Implant

Control and Processing Unit

Central Implant

IntrabodyCommunication Interface

EnergyGenerationUnit

Data Conversion & Processing UnitFunction

Speci�c Unit(Sense/Stimulate)

ExternalTelemetryInterface

IntrabodyCommunication Interface

PowerManagementUnit

PowerManagementUnit

PeripheralImplant 2

PeripheralImplant 5

PeripheralImplant N

PeripheralImplant 1

PeripheralImplant 3

PeripheralImplant 4

CommunicationChannel

GndP2

GndP3

GndP4

GndP5

Central Module Ground (GndC)

Peripheral Module Ground (GndP)Earth Ground

(GndE)

Body Potential(BP)

GndPN

GndP1

GndC

PowerData

Dat

a &

Pow

er

a) b) c)

CentralImplant

Mains electricity

Fig. 2. Concept of the multi-module implantable system. (a) Conceptual body model with of a distributed implant system; (b) iter-module connectivity (onecentral implant, multiple peripheral implants) and; (c) block diagram of internal structure of both central and peripheral implants and their interfacing.

performance, safety); and Section V concludes the paper.

II. MULTI-MODULE ACTIVE NEURAL IMPLANTS

The system concept of the multi-implant system is illus-trated in Fig. 2.b. The typical configuration comprises ofone central implant, or central unit, and multiple smallerperipheral implants, or peripheral units, each located in closeproximity to the target interface sites. The former includesthe energy source (battery), power management circuitry, acontrol and processing unit, and communication interfaces totransmit wireless data externally and to communicate with theperipheral units through an intrabody network. Although thereare several research groups investigating different methodsof implementing wireless intrabody networks, all clinically-applicable devices currently require a wired connection be-tween the individual modules. We will thus focus only ona wireline connection. This is based on multi-conductor im-plantable leads serving as communication channel (bidirec-tional) and power delivery medium (from central to periph-eral). At the ‘receiving’ end of this network, the peripheralunits must also contain an intra-body communication interface,to interact with the network and to retrieve power. This willpower up: (1) the front-end function-specific units, which canbe both recording and stimulation; (2) the data conversioncircuitry (analog-to-digital and digital-to-analog); and (3) thedigital processing unit (Fig. 2.c).

A. Module Ground Referencing

In order to avoid a DC voltage bias across the conductorsin the implantable lead [12], and thus to maintain the chargebalance over time, power transmission schemes based on AC-coupling are typically required in wired implant systems. Thisprecludes unwanted current paths from being formed betweendifferent modules if damage of the insulation occurs. Accord-ingly, AC/DC power domain conversion is implemented at theperipheral side, generating a suitable DC supply voltage forthe front-end circuitry (Vdr - Vss), hence the module groundrail (GndP = Vss), as shown in Fig. 3.a. [13]. This however,creates DC isolation between central and peripheral implantsand therefore the electric potential of the module ground at theperipheral implant N (GndPn) is not referred to the moduleground at the central implant (GndC).

By definition, implantable neural systems are placed insidethe human body, hence surrounded by biological tissue. Unlikebedside hospital equipment, they are not directly connected tothe AC electric power coming from the mains (grid power),

they instead receive the electrical energy from a batteryenclosed within the implant or inductively from external powersources. Unless the body touches the mains, the generatedpotential rails are to be considered floating and not referencedto the earth ground. The human body, however has its ownelectric potential, body potential (BP), as shown in Fig. 2.a.In electrical equilibrium (no current flow within the body),such potential is considered the same over the whole body.The BP has to be taken into consideration when dealingwith ground referencing in the neural implant system, as thepotential displacement of GndC and GndP from BP may havea significant impact on: (1) safety of the patient (2) devicereliability, and (3) recording quality.

B. Grounding Impact on Recording Quality, Device Reliabilityand Patient Safety

Implantable devices that sense biopotential signals, for ex-ample neural recording systems, connect the sensing electrodesthat are in direct contact with the tissue, i.e. thus with BP, tohigh impedance amplifier inputs. The observed biopotentialthus consists of this large baseline (BP) in addition to arelatively weak AC signal (desired activity to be recorded).A differential front-end amplifier is typically used to recordsuch signals, for example, in intracortical recording: thissenses and amplifies the difference in potential between twoelectrode, while suppressing the common potential (common-mode rejection, CMR). Moreover, to deal with the unknownDC offset between the BP and recording circuit (power railsVdr and Vss/GndP), biopotential amplifier inputs are typicallyAC coupled through capacitors. This leaves the BP havinga relatively high impedance to the recording circuit powerrails that effectively makes the tissue an ‘antenna’ for pickingup common-mode noise. It is thus essential to maintain arelatively low impedance connection between the recordingcircuit power rails and BP to attenuate the observed commonmode noise (at the amplifier inputs). Furthermore, in termsof safety, a significant difference in electric potential betweenGndC/GndP and BP (voltage) may lead to corrosion/failureof encapsulation that would result in current flow into/from thetissue exceeding the safety limits defined by the internationaldirectives on active implantable devices.

For these reasons, an appropriate ground referencing schemeis required in order to ensure good recording quality, but alsodevice reliability and ultimately patient safety.

Communication Channel

Peripheral Neural Implant

a)

b)

Front-end recording sites

AC/DC Converter Voltage Regulator Front-end Recording

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ex

VdrVdrVdc

Vss

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Vss = GndP

Vss

Vss1 Vss2 Vss3 Vss4Vss5

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AC-c

oupl

ing

Capa

cito

rs

VrefC1

R1

R2C2

A+ _

Com

mun

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ion

Chan

nel

Fig. 3. Peripheral neural recording implant system: (a) power recovery andVdr and Vss DC rail generation; (b) distributed multi-module brain implants.

III. GROUND REFERENCING SCHEMES

Given that the central implant is enclosed within a caseserving as DC electrical screen from the body, with no directcontact between internal circuitry and body tissue, there is noneed for GndC to be referenced to BP. On the other hand, theperipheral implants have a direct path to the tissue throughthe recording electrodes (Fig. 3.b), hence the need for properreferencing of the DC rails to the body potential.

The most common and simple ground referencing con-figuration is the passive connection of a peripheral devicepower rail (either Vdr or Vss) to the body potential. In acuteexperiments, the peripheral module ground GndP is oftenshorted directly to the BP through a ground electrode, so toreference the tissue to the lowest potential of the module. Thepassive scheme is represented in (Fig. 4.a) with a resistivedivider. The ‘short’ is obtained from this when consideringeither R1 = 0, R2 = ∞ (BP = Vdr), or vice-versa(BP = Vss). Depending on the initial value of the bodypotential, compared to the shorted rail, there will be currentflow from/to the tissue in order to equilibrate the potentialbetween the two nodes. In the scenario of multiple peripheralimplants close to each other, it is usually preferred to shortthe BP to all the peripheral grounds (GndP1, ...GndPN ).

The second category of body referencing circuits (BRC) iscomposed of active components. This can be further dividedinto two topologies: (1) driving BRC; and (2) sensing BRC.The driving reference circuit is shown in Fig. 4.b consisting ofa reference voltage generation circuit (e.g. a resistor divider)and an output buffer with a low output impedance to drive theBP . In both passive and drive schemes, there is current flowbetween implant and tissue. This ensures the voltage betweenBP and GndP to be fixed, thus reducing the common-modesignal at the input of the recording amplifier, at the expense ofcurrent exchange between peripheral module and body. Unlikefor the passive scheme, the BRC with driver/buffer ensuresthat the current is always injected into or out of the tissueproviding a level of stability to the source (peripheral implant)from inherent fluctuations of the potential around BP .

Fig. 4.c presents a third scheme, based on a sensing BRC:this time, a front-end sensing circuit is used as body refer-ence circuit to measure the BP instant value and referencesthe peripheral rails level accordingly. The sensed voltage

Body Reference Circuit - Drive

Body Reference Circuit - PassiveFront-end Recording

Vdr1

Vss1-N

Vss1-N

toADC

c)

b)

a)

A

Body Reference Circuit - SenseFront-end Recording

VdrN

VdrN

VssN

VssN

toADC

A Sense

Rec electrode 2

Rec electrode 1

BP electrode

Rec electrode 2

Rec electrode 1

Rec electrode 2

Rec electrode 1

BP electrode

BP electrode

Front-end Recording

Vdr1

Vdr1

Vss1-N

Vss1-N

toADC

A Ref

Ref

R2

R1

Vdr1

Drive

Fig. 4. Different grounding schemes: (a) passive; (b) drive; (c) sense.

(BP − GndP ) can thus be sent to the recording amplifier,which uses it to adjust the input biasing point of the latter. Bydoing so, the body potential fluctuation itself, seen as common-mode from the recording input electrodes, is employed in factas part of the common-mode rejection of the amplifier.

IV. DISCUSSION

A. Insights to Practical Implementation

So far in the paper, the interfacing between the BRCs andthe body has been considered as ideal, with negligible parasiticelements and considering an ideal body potential electrode(BPE). When practically implementing the circuits in a specifictechnology and connecting them to the biological tissue, newchallenges arise due to: (1) electrode interface (metal-tissue)non-idealities and; (2) circuit implementation constraints.

The electrical model of the electrode needs to be takeninto account when evaluating the impact of the ground ref-erencing in the different schemes, both for recording qualityand safety limitations of current injection. A simplified modelis presented in Fig. 5. Also, capacitive coupling and currentleakage from the power rails, represented by Rleak and Ccoup,affect the implant-tissue interfacing. This depends mainly onencapsulation and circuit technology. The latter introducesanother interfacing aspect: typically, in CMOS technology,integrated circuits require electrostatic discharge (ESD) pro-tection at the interfacing pads, in order to prevent devicemalfunction or breakdown. This is usually implemented usingreverse-biased pn-junction diodes, which ensure the potentialat the pad doesn’t exceed the upper and lower limits of thepower rails (V ss < Vp < V dr). Such diodes (D1 and D2

Vdr

Vss

if BP < Vss

if BP > Vdrdown

up

I

I Body

Pot

etia

lEl

ectr

ode

(BPE

)

D2 RBPE

Rleak-Vdr

C Vdr-to-BPParassitic Impedance

Vss-BPParassitic Impedance

coup-Vdr

Rleak-Vss

Ccoup-Vss

CBPED1

Clamp diodes

Internal Interfacing Circuitry ESD Protection

C1

BodyReference

Circuit (BRC)

from

Pow

er S

uppl

y

Fig. 5. Practical implementation interfacing of the body reference circuit(BRC) with the body potential (ESD, parasitic impedances, and BPE model).

Vss

V

V

Z

“noise” source

BP

BRC ImpedenceBP Electrode Impedence

Simpli�ed equivalent schematic Common-mode signal path from BP to Vss

V

BP

Vss

NS

NS

VNS

Z

A

in-amp2

ref

in

Zbody-ele

ZBRC

Z c1-coup

Z c2-coupBody Potential

A

Zrec-ele1

Zrec-ele2

Zparassitic

in-amp1Z

Fig. 6. Common-mode signal path in an experimental setup. Noise sources(VNS) are observed at the recording amplifier inputs via stray noise couplingimpedance (ZNS). The purpose of the body reference circuit is to shunt thiscommon-mode signal to recording amplifier ground such that it is attenuated atthe recording amplifier input. There is essentially a potential divider betweenZNS and (ZBODY −ELE +ZBRC). It is therefore essential to maintain alow impedance BP drive relative to the recording circuit ground.

in Fig. 5) add to the parasitic impedances of the implant-tissue interface, nevertheless, they prove to be advantageousin clamping the body-potential within the implant rail-to-railrange by providing a ‘weak’ leakage path.

B. Recording Performance

The three ground referencing schemes proposed in theprevious section give a different contribution to the neuralrecording performance depending on the BRC implementation.As shown in Fig. 6, where a schematics of BP to V sspath is illustrated, the BRC and BPE impedances intervenein the modification of such BP to V ss total impedance. Bymaintaining the latter a low impedance path, thus reducingthe BRC common-mode gain ABRC−CM , the common-modefluctuations seen at the recording amplifier inputs are mini-mized. However due to the frequency response of the BRC,ABRC−CM may increase with frequency and possibly reachto its maximum theoretical value of 0 dB (open circuit), i.e.the common-mode noise add directly on the recording inputs(Vin and Vref ) through the recording path. This is representedin the example plot in Fig. 7 by the effective CMRR atfrequency f2. On the other hand, a zero impedance BRC (shortcircuit) would reduce to 0 the VNS potential division on BP,thus cancelling out the common-mode signal. This scenario istheoretically possible, however, due to parasitic and electrodeimpedances, it remains a practical implementation challenge.

C. Safety Considerations

Besides altering the neural recording performance, thechosen grounding scheme and the technology features arekey aspects for body safety requirements, since they de-fine the electrical behaviour at the implant-body interface(BP − GndP potential difference and current flow). Exceptfor its intended function, active IMDs are required to beelectrically neutral when in contact with the body and themaximum current density at the surface of any electrode isset to ≤0.75µA/mm2 according to the directive in [14]. Forthis reason, current limiters are necessary when employingdriving ground referencing topologies, alongside with implantencapsulation and implant-to-implant electrical isolation.

V. CONCLUSION

This paper has described the key design considerationsin developing next generation multi-module neural implants.Such systems need to cross-reference isolated power supplies,in addition to facilitating a stable body potential with respect

Recording signal bandwidthTF

CMRR ofRecordingAmpli�er

E�ective CMRR @ f2

f [Hz]

|H | [dB]

f1 f2

E�ective CMRR @ f1

Common-modeGain of BRC

Fig. 7. Common-mode Rejection Ratio (CMRR) of Recording Amplifierand Common-mode gain of Body Reference Circuit (BRC) with the effectiveobservable CMRR being the difference between these responses. The BRCconfiguration, i.e. impedance/drivability of BP relative to recording amplifierground, determines the magnitude and frequency dependence of the common-mode signal observed at the amplifier inputs (Vin and Vref). This is criticalfor experimental design.

to the power rails of the individual modules. Three differentgrounding schemes have been identified based on shorting,sensing, and driving the body potential. The function of theseschemes have been analysed by considering the couplingof common-mode interference to the recording inputs. It isrevealed how ensuring a relatively low impedance path tothe recording circuit ground has the effect of attenuatingthe common-mode observed at the electrode inputs, thusimproving recording quality. Finally, the practical and safetyconsiderations are assessed – in particular, due to the fact theyexchange electrical current with the body, it is essential toguarantee operation within safe limits.

ACKNOWLEDGEMENT

This work has been funded by the Wellcome Trust(102037/Z/13/Z) and EPSRC (NS/A000026/1). The authorswould like to thank Nick Donaldson, Reza Ramezani, AhmadShah Idil and Patrick Degenaar for helpful discussion.

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