Precision Ccs

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Proceedings of the 25 Annual International Conference of the IEEE EMBSCancun, Mexico September 17-21,2003Precision Constant Current Source for Electrical Impedance Tomography

Jeong Whan Lee, Tong In Oh, Sang Min Paek, Jae Sang Lee, and Eung Je WooCollege o f Electronics and Information, Kyuny Hee University, KoreaAbstract- I n Electrical Impedance Tomography EIT),we inject currents thro ugh electrodes placed on the su rfac e ofa subject and try to reconstruct cross-sectional resistivityimages. This requires a precision constant cur rent source. Inthis paper, we present a design and performance analysis of aconstant current sonrce using an FPGA-based waveformgenerator and improved Howland current pump circuit. Thedesigned current source was implemented and tested to verifythe total harmonic distortion of 0.0907% and amplitudestability of 0.2361 . The output impedance of the currentsource was bigger than 64 Mfl at de and smaller than 500 KRa t 50 KHz. We need to further improve the output impedanceat the frequency of 50 KHz or higher employing a bettermethod of stray capacitance cancellation.

Keywords- EIT, cur rent source, waveform generator

1. INTRODUCIIONIn Electrical Impedance Tomo graphy (EIT), we use one

or multiple constant current sources to inject currentsthrough electrodes placed on the surface of a subject,Measured boundary voltage data are used to reconstructcross-sectional resistivity or conductivity images. In mostEIT systems, the performance o f the current source is one o fthe m ajor limiting factors determining its accuracy.When multiple current sources are used it is mostimportant to accurately calibrate all of them so that the sum

of all currents is always zero. This requires a v ery accuratecontrol over amplitudes and phases of multiple currentsources [ l 21. In this paper, we propose using one currentsource with a switching circuit to m inimize errors caused bymultiple current sources. In order to reduce the dataacquisition time, we suggest fast multi-channel voltmeters.The current source usually consists of a waveformgenerator and voltage-to-current converter. In this paper, wedescribe the design and implementation of the currentsource using a digital waveform generator and improvedHowland current pump circuit with stray capacitancecancellation capability [3].

11. METHODOLOGYTable 1 shows the desired specifications of the currentsource to be used in our EIT system. We determined thespecifications to achieve the 16-bit accuracy at least within

the current source. The output impedance of 64 Mi2 isneeded assuming the maximal load impedance of 1KR. Fig.

shows the block diagram of the proposed current sourcedesign.TABLE . conrtant Current Bourcc

Feature SpecificationAmplitude ofcurrent -2.5 -+2.5 m b PFrequency of Current 1 KHz - 1 MHzOutput impedance 64 MC2Calibration procedure AutomaticDigital wavefom data 16-bit

E+Fig.1. Block diagram ofthe current source

A . Digitd Waveform G ?nerntorIn order to obtain a very low total harmonic distortion(THD) and high stability, we designed a digital waveformgenerator using an FPCiA and DAC shown in Fig. 1. Thisalso enables us to change the waveform and its frequency.

In the design, we used the hardware description language(HDL) P I

waveformAddressGenerator

WaveformDataDecoder

rocess JDistanceFig. 2. Block diagram of HDL code in the FPGA

0-7803-7789-3/03/ 17.002003 IEEE 1066


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TABLE2. Comparison of two waveform generatorsWaveform generatorFPlhlW- Analog(%) Digital ( j

Stabilhy 0.84-2.2 0.1607. .THD' 1.5-2.4 0:0275 ' .

',To ld Harmonic Distortion

I ' I

43 5 , , , , ,

10 m . . a 40 54 Ea 1 10. I/ , , , ICmby,(b)

Fig. 5. (a) Sinusaldal and (b) rectangular waveform

. ... .

Fig. 6 Amplihlde linemiry of th e variable mp liht de waveform generator

As described before, we can easily change the frequencyand amplitude of the v oltage. signal fiom the digitalwaveform generator. Throughout the frequency range of 1KHz to 1MH z, we found the similar performance as show nin Table 2.Fig. 5 shows typical voltage signals from thedigital waveform generator. Fig. 6show s the linearity of theamplitude as we controlled the amplitude.using the MDA C.B. Voltage to Current Converter

,

, ... . , . :In.or der.t o ohtain'an output resistance of 64 MQ, wemust match the resistors. in- Fig. 3 within 0.039%. Using a

10 KR digi-pot with 128 steps and a' 3 KR resistor in.parallel, we could actjust the resistance value with aresolution of 17.3 Q. Fig. 7(a) shows the change of theoutput resistance R as a function of digi-po t steps. In theory;'we may get R as high as 35 G Q . The measured value at dcwas 3.3 GO.Fig. 7(b) shows 'the change, of the capacitance in thenegative, capacitance ciicuit as a func tiodo f digi-pot steps.In ' the current:design, il,viyies in the range of10 pF. Combin ing th : effect of both Rq .an d negativecapacitance circuit, we found that the implemented. currentsource has an output impedance of 3.3 GQ at dc. However,it was less than 500KR at 5 0 ' k z .

. . , .

18.7 to -

Fig. 7. a)Output mistance ofthe voltage-tomrent converter and @)negative capacitanci: of the negative capacitance circuit

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IV. DISCUSSION AND CONCLUSIONThe performance of the constant current source is one of

the major limiting factors determining the accuracy of anEIT system. Based on our speculation that using one currentsource may outperform a system using multiple currentsources, we designed and implemented the constant currentsource including the FPGA-based waveform generator,voltage-to-current converter, and automatic calibrationcircuit.We found that the digital waveform generator must beused instead of the a nalog oscillator since it provides muchbetter stability and low total harmonic distortion. It allowsus to easily change the operating frequency and, therefore,is also a more effective solution for an EIT system usingmultiple frequencies.In order to maximize the output impedance of thecurrent source, it is essential to use digi-pots or any similarmeans for fine tunings. We found that it is little moredifficult to compensate the stray capacitance using anegative capacitance circuit rather than finding thematching condition of resistors. Without any propercompensation of the stray capacitance, we found that it isnot possible to achieve an output impedance greater than500 Kn at 50 KHz or higher. Considering the needs formultiple frequency operation up to around 1 MHz at least,an improved technique for the stray capacitancecancellation should be studies in our future works.

ACKNOWLEDGMENTThis work was supported by the grant Rll-2 002 -103from K orea Science and Engineering Foundation.

REFERENCES

[I] . D.Cook, G 1 Saulnicr, D.G Gisser, 1. C. Goble, 1. C.Newell, and D. saacsan, ACT3: a high-speed, high-precisionelectrical impedance tomography , IEEE Tmns. Biomed Eng.vol .41 ,no:8 ,pp.713-7 22, 1994.[21 A. S. Ross, G . J. Saulnier, I. C. Newell, and D. saacsonCurrent source design for electrical impedance toimography,Physiol. Meas vol. 24, pp. 509-516,2003.(31 S. Franco, Design wifh Opera fion al AmpIWiers and Ana logIntegrafedCireuirs 2 'ed., New York, McGmw-H ill, 1998.(41 D. . Smith,HDL Chip Design, Doone Publications, 1996 .

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