data acquisition unit for an implantable multi- channel...

eit2007 – Electro/Information Technology Conference

Chicago, IL, USA, 17-20 May, 2007.

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Abstract— Design and development of a data acquisition unit for

an implantable multi-channel optical glucose sensor is described.

The sensing technology involves sampling of the interstitial fluid

in a micro-fabricated chamber and measurement of the

absorbance of the fluid in a non-destructive and reagent free

manner. The glucose levels are estimated based on the absorbance

data. This new technology relies on the unique optical

characteristics of glucose in a near infrared spectrum. The sensor

element will be implanted in the subcutaneous tissues of the

human body. The data acquisition unit acquires optical data from

the sensor and converts it into spectral data for processing. This

new technology will be used as the sensing technology in a

feedback controlled insulin delivery system for the in situ

treatment of diabetes. The sensor-controller system together with

an insulin pump will function as artificial pancreas and help

maintain tight glycemic control in patients suffering from

diabetes.

Index Terms— Transmission spectrum, data acquisition,

optical glucose sensor.

I. INTRODUCTION

iabetes is a potentially devastating disease [1], the

medical complications of which can be diminished by

early diagnosis and tight glycemic control. The goal of tight

control is to maintain one’s blood glucose levels within a

physiologically acceptable range. Tight control requires

frequent blood glucose measurements, which provides the

information needed to administer insulin or glucose properly.

The glucose levels are measured using a blood glucose

monitor, which is typically made of an electrochemical glucose

sensor interfaced to an electronic support unit. The pain, cost

and inconvenience of state-of-the-art glucose monitoring

technology impede frequent monitoring and are primarily

responsible for the failure of patients to maintain tight control.

It has been recognized for several decades that the ideal

treatment of diabetes would involve a closed-loop insulin

delivery system that is implanted within the patient's body. The

so-called artificial pancreas consists of an insulin delivery

pump coupled with some type of glucose-sensing technology.

Manuscript received February 16, 2007. This work is supported by a grant

from the National Institutes of Health under Grant DK64659.

K. S. Kanukurthy and M. B. Cover are students with the dept. of Electrical

& Computer Engineering at the University of Iowa, Iowa City, IA, IA 52242

USA (phone: 319-335-1608; e-mail: [email protected]).

D. R. Andersen is a Professor with the departments of Electrical &

Computer Engineering, Physics & Astronomy at the University of Iowa, Iowa

City, IA, IA 52242 USA (phone: 319-335-2529; e-mail: [email protected]).

Insulin is delivered continuously in response to detected

changes in the blood glucose concentrations. For this to work,

the glucose sensing component must be able to provide

accurate and rapid blood glucose values to a micro-processing

unit, which computes the amount of insulin required and then

controls insulin delivery. The key limitation to the successful

development of an artificial pancreas is the implantable

glucose sensing technology and the electronic support needed

to control the instrumentation.

This paper describes the design and development of a data

acquisition unit [DAU] for an implantable glucose sensor that

provides continuous and reagent-free optical analysis of

interstitial fluid (ISF). The DAU is the front-end of the

Controller [2, 3], an electronics system designed to provide the

necessary electronics support to the sensor. The Controller

operates from an inductively rechargeable battery based power

supply [4] and provides support for the sensor to work

unobtrusively for extended durations.

The sensing technology relies on the unique optical

characteristics of glucose in the near infrared spectrum [5, 6].

The optical glucose sensor [7] consists of a broadband LED

light source, an optical sampling chamber, and a spatially

variable wavelength filter bonded to an array of photodetector

elements. Transmission spectrum of the interstitial fluid (ISF)

in the NIR region is collected by modulating the LED and

measuring the currents through the photodiode channels. Each

photodiode channels corresponds to a different band of

wavelengths in the NIR spectrum with the total spectrum

covering the entire region of interest. Glucose concentration

levels are estimated using the measured transmission spectrum

based on Beer-Lambert relationship.

II. DAU SYSTEM

The DAU provides the physical interface between the optical

sensing elements and the glucose related data. The DAU is

interfaced to the photodiode channels of the sensor on one end

and the user electronics such as a PDA or a PC on the other

end. The DAU performs the function of converting the

instantaneous output currents of the sensor’s photodiodes into

meaningful glucose concentration values and conveying that

information to the user. During the measurement process, LED

of the sensor is modulated by the DAU. Each detector element

reacts accordingly by producing an output current. The optical

sensor has 32 photodiode channels that output currents of the

magnitude of the order of 10 nA. The current is converted to a

voltage with preset transimpedance gain and digitized by a

Data Acquisition Unit for an Implantable Multi-

Channel Optical Glucose Sensor

Kiran Kanukurthy, Student Member, IEEE, Mathew B. Cover, David R. Andersen, Member, IEEE

D



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high resolution A/D converter (ADC). The transimpedance

gain and sampling rate can be varied as required. This process

occurs simultaneously for all the photodiode channels. The

results of the A/D conversion from all of the channels are used

to determine the glucose concentration. The following are the

requirements for the DAU interfaced to the glucose sensor.

a) Transimpedance gain: A transimpedance gain of 108 Ω is

required in order for the converted voltage to be about half the

full scale range of the ADC to achieve high SNR.

b) SNR: High SNR is required to obtain quality spectral data.

Preliminary calculations show that a SNR of 48 dB is

necessary to obtain good quality spectral data so that changes

in glucose concentration levels can be accurately measured.

Sources of noise in this case are the Johnson noise from the

electronic components, shot noise of the detector, and the

environment noise.

c) Multiple Channels Support: As each channel corresponds to

a different wavelength region, all 32 photodiode channels of

the sensor need to be sampled and converted simultaneously

by the DAU.

d) Resolution of Data: Output data of each channel should

have a resolution greater than 16-bits to result in high SNR.

III. IMPLEMENTATION

The following section discusses the hardware and software

aspects of the DAU system design along with the device

selection criteria for the front-end of an optical data

acquisition system with photodiodes as sources at its input

channels.

A. Transimpedance Amplifier Selection

Primary parameters in interfacing any sensor to an electronic

device are the voltage levels of the sensor, current output of

the sensor, and the impedance of the sensor. These factors

contribute significantly to the decision to use a particular type

of electronic device, mostly a particular kind of ADC and/or

an operational amplifier. Other factors such as the real estate

are to be considered as well when multi-channel sensors are

involved. The real estate being the cost of the circuit, the

size/area of the circuit, complexity, and the power supply

consumption. For an optical sensor whose output signal is a

current, the electronic circuit traditionally consists of a

transimpedance amplifier followed by an ADC. The detector

elements of the sensor output currents of a magnitude of the

order of 10 nA. A transimpedance gain of about 108 Ω is

required to output a voltage of half the full scale range of the

A/D converter. There are two choices for the transimpedance

amplifier vis-à-vis transresistance amplifier and switched

capacitor transimpedance amplifier. In the case of a

transresistance amplifier, the gain is provided by a resistor in

the feedback loop. A gain of 108 Ω translates to a resistance in

the range of 100 MΩ. Transresistance amplifiers with such

huge resistors in the feedback loop suffer from instabilities

apart from requiring dual power supplies. Besides, a circuit

consisting of transresistance amplifiers followed by ADCs for

32 photodiode channels requiring dual power supply is

expensive in terms of the real estate and is not a suitable

choice for an implantable sensor system.

Switched capacitor transimpedance amplifiers provide the

transimpedance gain by integrating the current on a capacitor

over a period of time. The transimpedance gain can be varied

by varying the capacitance of the integrating capacitor and the

integration window. High transimpedance gains of the order of

108 Ω can be achieved easily by using a switched capacitor

circuit. The integrating behavior of a switched capacitor

amplifier reduces noise by averaging the input noise of the

sensor, amplifier, and external sources. Integration of the input

signal for a fixed period produces a deep null (zero response)

at the frequency 1/TINT and its harmonics, where TINT is the

duration of integration. An AC input current at this frequency

(or its harmonics) has zero average value and therefore

produces no output. This property can be used to position

response nulls at critical frequencies. For example, a 16.67ms

integration period produces response nulls at 60Hz, 120Hz,

180Hz, etc., which will reject ac line frequency noise and its

harmonics. Response nulls can be positioned to reduce

interference from system clocks or other periodic noise.

A suitable choice for the front-end of a DAU for a multi-

channel photodetector array would consist of a switched

capacitor transimpedance amplifier IC operating on a single

power supply while providing multiple input channels. A

single IC offering more than one input channel with each

channel consisting of a switched capacitor transimpedance

amplifier circuit followed by an ADC would be an ideal

solution for a multi-channel photodetector array. DDC118 [8],

a current input ADC from Texas Instruments Inc., is one such

IC that can work with up to 8 photodiode channels. Since the

ADCs are current-input, additional transimpedance amplifiers

for current-to-voltage conversion are not needed. The photo

detector currents are integrated over time, converted to a

voltage, sampled and measured by the A/D converter, and

output as high precision 20-bit digital codes. Transimpedance

gain, the gain involved in current-to-voltage conversion, is

varied by varying the integration times and integrator

capacitances.

B. Hardware Design

The architecture and implementation of the DAU is shown in

the block diagram in Fig. 1.

Fig. 1. Block diagram of DAU

The design is composed of four DDC118 ICs, each of which

interfaces to eight photodiode channels. Channels are not

connected sequentially but in a way so as to achieve a clean



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layout. Odd channels are connected to the DDC118 ICs on the

left of the sensor while the even channels are connected to the

ones on the right. Each DDC118 input has two independent

integrators (sides A & B) followed by one ADC. This feature

can be utilized in such a way that continuous integration of the

input signal becomes possible eliminating the possibility of the

loss of a portion of the signal. While side A is being integrated

with the input signal, side B undergoes conversion and a flag is

generated indicating that the results are available on the SPI

interface. At the end of integration time, side B is put under

the integration mode and side A undergoes conversion. The

DDC118s have an SPI interface for communication with a

microcontroller and function as a slave device. The

microcontroller used is AT89C51ID2 [9], an 8-bit

microcontroller based on Intel’s 8051 architecture. The

microcontroller was chosen based on the ready availability of

the device and related compiler tools. The DDC118 ICs are

cascaded so that the digital data output of one IC is connected

to the digital data input of the other. The advantage of a

cascaded design is that the microcontroller in charge of the

unit has to communicate with only one DDC118 for the data

corresponding to all DDC118 ICs. Thus the hardware

connections are made simple with just one SPI slave connected

to the microcontroller. The reference voltage for the ADCs is

generated by REF3140 [10] voltage reference IC followed by

an OPA350 [11] operational amplifier in a buffer

configuration. The devices are low output noise components

and are ideal for the reference voltage circuit of the DDC118

IC. A FIFO/USB interface based on FT2232C [12], a USB

serial communication IC from FTDI Technologies Inc., is

provided to handle high data transfer rates. The

microcontroller reads the data from the DDC118s via the SPI

interface and transfers the data to a PC via the FIFO/USB

interface. The data is read from the USB interface and written

into a file for data analysis. An RS-232 interface is provided to

be used primarily for configuring the system and also for

programming and debugging purposes. A prototype of the

DAU along with the photodetector array is shown in Fig. 2.

Fig. 2. Prototype of DAU with the photodetector array connected to the DAU

via a custom 1” x 1” header. The gold square is a standard microscope glass

slide coated with gold and the blobs on either side are wire bonds from the

header to the photodetector array, the small black unit in the center.

C. Firmware Design

LED modulation, integration time, integration capacitance,

and data acquisition without loss of data are controlled by

features implemented in the firmware. The integration

capacitors on side A and side B are not guaranteed to be

identical on the DDC118 devices. The mismatch is

compensated for in the firmware. The firmware makes use of

the dual integrators on DDC118 IC (sides A & B) in the

following way. The LED is enabled and the integration process

is started on side A. At the end of the integration time,

integration process is halted on side A and started on side B.

The result of the integration on side A with the LED on,

denoted by is acquired during integration of the signal on side

B. At the end of the integration period, the LED is disabled

and integration of the signal on side B is halted. Integration of

the signal with the LED off is begun on side A. The result

from side B integration, BON is acquired and added to AON to

result in data corresponding to the duration the LED is on. The

process is repeated with the LED disabled. The net results of

(AON + BON) and (AOFF + BOFF) are stored in long integer

arrays ON and OFF respectively. Each array has one element

corresponding to the particular channel. Preset number of

samples corresponding to the configured average time are

accumulated and transferred to a PC via the USB interface.

This technique reduces the load on the 8-bit microcontroller by

transmitting data once every few seconds (configured average

time) as opposed to continuously transmitting data via the

USB-FIFO device. Timing jitters because of delays due to full

buffers on the USB-FIFO are eliminated resulting in uniform

integration windows for both LED on as well as off cases. The

signal however is continuously acquired without loss although

at a reduce sample frequency. Limited resources of the 8-bit

microcontroller are efficiently used and the averaging

mechanism improves the noise performance. The LED is

modulated at 50 % duty cycle at the sample frequency

configured by the user. The integration signal that controls

which side is being integrated is modulated at 50 % duty cycle

at twice the frequency of the LED signal. The integration

period is set by an interrupt based timer to result in uniform

integration windows. Subtracting the LED disabled data from

the LED enabled data eliminates constant background noise.

Averaging the sampled data improved noise performance. The

signals from the photodiodes are continuously integrated with

no loss of signal.

IV. RESULTS

The DAU was successfully implemented and integrated with a

prototype of glucose sensor as shown in Fig. 3. An adapter

board was designed to accommodate the change in the glucose

sensor module packaging. A heat sink and temperature

controller were mounted on the glucose sensor for efficient

heat dissipation and to maintain constant working temperature

respectively. Experiments were conducted to evaluate the

performance specifications of the DAU particularly with

respect to noise characteristics and SNR characteristics in the

presence of a signal. Since ISF is composed mainly of water,

spectral data were recorded with water as the analyte in the

initial stages. The results are as follows.



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Fig. 3. Prototype of the Sensor-DAU system. Glucose sensor module is

plugged in a ZIF socket on the modified header of the DAU board. The red

and black wires are LED terminals; the other two wires (thin) are for

monitoring the temperature of the sensor module. The clear tube is the flow

channel for the analyte. Wires on the DAU board are to monitor signals

during the debugging phase.

A. Noise Characteristics

Two experiments were conducted to evaluate the noise

characteristics of the system. The first experiment measured

the noise under open circuit conditions with all of the 32

channels open i.e. no connections to the sensor at all. The

second experiment measured the noise in closed circuit

conditions under the absence of a signal. Since photodiodes

are sensitive to ambient light and can produce small currents

even when shielded from light, a suitable way to measure noise

is to simulate the presence of a photodiode by connecting a

resistor and a capacitor in parallel in lieu of the diode. A

photodiode can be modeled as a current source in parallel with

a resistor and a capacitor. Thus, such an arrangement is a

valid. Resistors of different values were connected across a

channel chosen arbitrarily from the 32 channels and the noise

was measured. The results from the open channel experiment

are depicted in Fig. 4.

Fig. 4. Noise measured with the input channels open-circuited. Noise values

are for sensor capacitance of 100 pF.

The manufacturer’s specifications for device noise (from

DDC118 data sheet) are plotted for comparison. Results of the

closed channel noise measurement are depicted in Fig. 5a and

Fig. 5b.

Fig. 5a. Noise measured across resistors connected to the input channels

of the DDC118. The open channel noise is provided for comparison.

Fig. 5b. DAU noise plotted as a function of the impedance. Impedance of the

photodiodes of the sensor is about 1 kΩ.

B. Spectral Data

Spectral data acquired using the DAU system with water as the

analyte is depicted in Fig. 6.

Fig. 6. Transmission spectrum of water in the NIR region collected using the

DAU system. The data shown are 5s averages collected over a period of 10

minutes. The thickness of the curve is an estimate of the drift or noise of the

system. The picture also shows mismatch in hardware conditions among the

DDC118s.

Resistor Terminated Channel Noise

-50

0

50

100

150

200

250

1 13 25 37 49 61 73 85 97 109 121 133 145 157 169 181 193 205 217 229 241 253 265 277 289 301 313 325

Sample No.

No

ise

in

mV 1K

10K

47K

Open



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The input signal was continuously integrated as explained in

the firmware design. Transimpedance gain was set at 8 x 107

Ω. The data corresponds to 5 second averages collected for

about 10 minutes. The thickness of the spectrum shows the

drift or noise of the DAU system with SNR of about 20 dB.

Also, saw edges can be noticed for channels 1-16. The crests

are even channels 2-16 connected to one DDC118 while the

troughs are odd channels 1-15 connected to a different

DDC118. This could be due to variations seen in reference

voltage and analog power supply seen at the two DDC118s

and also due to difference in capacitor sizes on different

DDC11s resulting in slightly different full scale range. We

continue to investigate the issue.

C. Discussion

It can be seen from the noise characteristics that the DAU

performs well when the channels are open-circuited but the

performance deteriorates when interfaced to the glucose

sensor, irrespective of the presence or absence of a signal. This

behavior can be attributed to the bias voltage applied by the

DDC118 to each of its input channels. The following

experiment justifies the reasoning. The input channels of the

DAU were terminated with resistors of different values and

subjected to the same integration times, and integrating

capacitances with no external signal applied to them. The data

collected in such a way is primarily composed of the Johnson

noise across the resistors along with the noise internal to the

DDC118 IC.

As shown by Fig. 5a and Fig. 5b, it can be seen that the

measured noise decreases with increasing resistance. This

counter-intuitive scenario can be explained as follows. The

DDC118 IC is a current input device whose input channel

circuitry can be modeled as shown by Fig. 7.

Fig. 7. Model of a resistor connected to the input channel of DDC118

It can be seen that the actual current being integrated on the

integrator capacitor is the sum of the resistor noise current and

the bias noise current, the current induced in the resistor

because of the bias voltage. The resistor noise increases with

resistance. However, in the case of low valued resistors, the

resistor noise is very small when compared to the bias current

shot noise. Thus the measured data primarily reflects the noise

current due to the bias voltage. The effect of the bias voltage

i.e. the noise current induced in the resistor, decreases with

resistance and becomes negligible for large values of

resistance. The DDC118 is designed for use with commercial

silicon photodiodes generally under reverse bias conditions to

with impedance (Rd in Fig. 8) in the order of few tens of

Megaohms. Reverse bias increases both the dark current as

well as the response time. For commercial photodiodes, signal

current is very high compared to the dark current and increase

in the dark current is generally not an issue compared to the

faster response times achieved due to reverse bias. Due to the

high impedance in such a scenario, the noise current induced

by Vbias is very small and can be neglected compared to the

signal current. In our case, the photodiodes operate under zero

bias conditions with impedance of the order of 1 KΩ thus

leading to low signal to noise ratios. The photodetector array

[13] is not a production part but a research grade prototype

with performance improvements being achieved over each

implementation. Zero bias is necessary to limit the shot noise

component due to the dark current.

Fig. 8. Model of a photodiode connected to the input channel of DDC118

D. Bias Compensation & Improved DAU

The DDC118 IC is designed for photodiodes often working

under reverse bias conditions and thus having impedances in

the mega ohms range. Because of the high impedance, the bias

voltage applied to the photodiode at the input channel does not

cause performance degradation. The photodiodes in the

detector array of the glucose sensor operate under zero bias

conditions to limit the magnitude of the dark current and have

impedance in the range of 1 kΩ. Dark currents under reverse

bias conditions can be in the order of few µA. A high dark

current increases the shot noise, one of the components of the

noise generated by a photodiode the other two being Johnson

noise and flicker noise.

When using DDC118 with low impedance photodiodes, the

effect of the bias voltage has to be compensated. Doing so

using a dedicated circuit in hardware is not suitable due to the

following reasons. Firstly, additional components increase the

real estate, which is not suitable for an implantable system.

Secondly, it is not possible to design a simple circuit that can

compensate for the bias voltage without itself applying a

voltage to the photodiode or loading the photodiode. However,

the effect of bias voltage can be compensated in software using

the same existing hardware set-up. The collected data can be

processed through a digital filter to estimate the contribution

from the actual signal. A better way is to incorporate a digital

filter in the firmware. A first order recursive Butterworth filter

with coefficients that are powers of 2 can be incorporated in

firmware [14]. A SNR of about 38 dB can be achieved with

such a filter. Though the filter coefficients are powers of 2, the



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compiler has to be smart to differentiate between coefficients

that are powers of 2 and coefficients that are not. A second

order recursive Butterworth filter [15] with a lower cut-off

frequency than the first order filter provides even better

performance and would be ideal for the sensor-DAU system.

The calculations involving a 2nd

order filter are too complex to

be implemented on an 8-bit microcontroller. The filter

coefficients were calculated using Matlab and the raw data was

processed using the filter on a PC. SNR of about 50 dB was

achieved using such a filter. The following figure depicts the

performance of the system with the second order digital filter.

Fig. 9. Digitally filtered water transmission spectrum with improved SNR.

The picture shows reduction in the thickness of the plot indicating better

noise performance.

V. FUTURE WORK

The processing requirements necessary to incorporate digital

filtering in firmware are beyond the feature set of an 8-bit

microcontroller. An improved DAU design based on a 16-bit

microcontroller with a dual core, a microcontroller core in

addition to a DSP core, has been developed and being tested

currently. The new PCBs are laid out in a way that limits the

digital switching noise induced in the analog side. The other

sources of noise are thermal noise, shot noise, and flicker noise

of the photodiodes. Thermal (or Johnson noise) is inversely

related to the impedance of the photodiode. Improving the

impedance of the photodiodes will lower the thermal noise and

also limit the effect of the bias voltage at the input channels of

DDC118 leading to better SNR. Flicker noise possesses a 1/f

spectral density and may dominate when the bandwidth of

interest contains frequencies less than 100 Hz. Higher

sampling rates will limit the flicker. A new generation of

photodetector array with impedance an order of magnitude

more than the present array is being developed by the research

group in charge of the fabricating the photodiodes. This will

decrease the effect of the bias voltage and improve the SNR by

an order of magnitude. Also, the DSP based design will permit

high sampling rates and make it possible to run complex filter

algorithms to further improve the SNR.

Efforts are being made to collect ISF spectral data and

measure glucose concentration levels under in-vivo conditions

using a laboratory rat.

VI. CONCLUSION

A DAU prototype supporting 32 photodiode channels has

been successfully implemented and interfaced with the glucose

sensor. The spectral data collected using the system was of a

quality comparable to the data collected using a commercial

state of the art National Instruments data acquisition bench top

module. Average SNR of 20.2 dB was obtained with the

existing set-up and the current generation photodetectors prior

to digital filtering. SNR of 50.7 dB was achieved using digital

filtering techniques with a scope of at least 15 dB enhancement

in the near future with improved photodetector array, better

processing capabilities, and improved hardware design.

ACKNOWLEDGMENT

The authors would like to thank Jon Olesberg of the Optical

Science & Technology Center and Dan Cooley of the

Embedded Systems & Non-Linear Optics Lab at the Iowa

Advanced Technology Labs of the University of Iowa for their

help during the course of the project.

REFERENCES

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Coretsopoulos, "Wireless NIR Glucose Sensor Controller," Proc.

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7-10, 2005

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M.A. Arnold, and C. Coretsopoulos, "Controller for a Continuous

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Feb.8-10, 2005

[4] J. Wu, K. Kanukurthy, and D. R. Andersen, "Continuous,

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[8] DDC118 data sheet, http://www.ti.com

[9] Atmel AT89C51ID2 data sheet,

http://www.atmel.com/products/8051/

[10] REF3140 data sheet, http://www.ti.com

[11] OPA350 data sheet, http://www.ti.com

[12] FT2232C data sheet, http://www.ftdichip.com/FTProducts.htm

[13] M.H.M Reddy, J.T. Olesberg, C.Cao, J.P. Prineas, “MBE-grown

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back illumination”, Semiconductor Sci . Technol . 21 , 267-72

(2006)

[14] A.A Dorrington, R. Kunnemeyer, “A simple microcontroller

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signals”, Proc. of Electronic Design, Test and Applications, 29-31

Jan. 2002 Page(s):486 – 488

[15] A. Antoniou, Digital Filters: Analysis and Design, 2nd Edition

(McGraw Hill, New York, 1993)

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