432 IEEE SENSORS JOURNAL, VOL. 11, NO. 2, FEBRUARY 2011

Virtual Ground Technique for Crosstalk Suppressionin Networked Resistive Sensors

Raghvendra Sahai Saxena, R. K. Bhan, Navneet Kaur Saini, and R. Muralidharan

Abstract—Two-dimensional resistive sensor arrays that utilizeshared row and column connections to simplify the interconnectcomplexity suffer from the crosstalk problem among its elementsintroduced due to the interconnection overloading. In this letter,we present a method of overcoming the problem of crosstalk byputting all of the row nodes at virtually equal potential using vir-tual ground of high-gain operational amplifiers (opamps) in neg-ative feedback. The circuit, though it requires a large number ofopamps, solves the crosstalk problem to a large extent and providesfaster scanning. We verified the circuit functionality with PSPICEsimulations. We have also derived the expressions of crosstalk re-jection and sensitivity to show that, by using high-gain, low-noiseopamps, we may get excellent performance.

Index Terms—Crosstalk, resistive sensor, sensor array, virtualground.

I. INTRODUCTION

R ESISTIVE sensor arrays, due to their implementationease, are used in variety of applications, like biometric

sensing, photoconductive image sensing arrays, and elec-tronic nose. A large number of sensors is usually required forbetter sensing resolution, but this increases the interconnectcomplexity. Sharing of rows and columns in these arrays isan attractive technique for reducing interconnect complexity,but at the cost of increased crosstalk [1]–[5]. Also, in sucharrays, the total number of nodes becomes whichcan give only nodal equations, with which wecannot directly extract the outputs for all individual sensorsand require external circuitry for that. Such circuits [3]–[5]require switching combinations of rows and columnsalong with voltage feedback that reduces readout rate. Theirperformance is also degraded when they are applied to sensorsof low resistance.

In this letter, we present a method of suppressing crosstalk inresistive sensor arrays with higher reading rate even for low re-sistances. It uses high-gain operational amplifiers (opamps),keeping all rows virtually at the same potential, to sufficientlyreduce the crosstalk. We present the practical limitations of thescheme along with the circuit simulation

II. PROPOSED SCHEME

Fig. 1 shows the proposed scheme, in which identicalopamps in negative feedback are connected to rows. The

Manuscript received March 12, 2010; revised July 06, 2010; accepted July12, 2010. Date of publication September 23, 2010; date of current versionNovember 19, 2010. The associate editor coordinating the review of thismanuscript and approving it for publication was Prof. Evgeny Katz.

The authors are with the State Physics Laboratory, Timarpur, Delhi 110054,India (e-mail: rs_saxena@yahoo.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2060186

Fig. 1. Sensor array and schematic of the circuit that suppresses the crosstalk.

virtual same potential appearing at the two inputs of eachopamp keeps the rows at zero potential. Now, applyingvoltage to any column (say, column-j) will result in a current

(for to ), in sensors of column-j only.These currents will flow through the feedback resistancesof all of the opamps, producing outputs equal to

(1)

where is the output corresponding to the th row. Thus, weget parallel outputs inversely proportional to the sensorresistances of the selected column without any interference byother sensors, thereby eliminating the crosstalk.

Now, we may scan the whole array by only state switchingof bias, using the demultiplexer as shown in Fig. 1. Reducingswitching to states improves the readout rate. Here, the min-imum detectable resistance change will be governed by the noiseat opamp’s output. From (1), we obtain

(2)

Here, we have skipped the subscripts for clarity. If the totalnoise referred at the opamp’s output is , we will obtain

(3)

where is the minimum detectable resistance change. Itrepresents the sensitivity that may be improved by using low-noise opamps.

III. SIMULATION RESULTS

To verify the scheme, we have performed the simulations of a4 4 resistor array of 1-k resistance in a PSPICE circuit sim-

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SAXENA et al.: VIRTUAL GROUND TECHNIQUE FOR CROSSTALK SUPPRESSION IN NETWORKED RESISTIVE SENSORS 433

Fig. 2. Example circuit simulated in PSPICE 16.2.

ulator by selecting practical opamp-IC AD-741. In the uniformresistance (1-k ) case, we found that all of the opamp outputswere at 1.0 V and all rows had 24.83- V potential.

Now, to analyze crosstalk suppression capability of thiscircuit, we changed three resistances R23, R32, and R41 lo-cated at different rows and different columns and applied biasto column-1. Fig. 2 shows various simulated node-voltages.Only the opamp output has changed to 909.0 mV, whichis connected to the row of the changed resistance (R41) ofthe selected column, verifying the circuit functionality. Here,we have increased the resistance assuming positive TCR, likeresistive microbolometers [1], [2], [5], however, the analysis isequally applicable for reduced resistance case also.

IV. PRACTICAL NONIDEALITIES AND THEIR SENSITIVITY

An important concern in such circuits is the dependenceof output on the elements that are not selected (known ascrosstalk). The reason for this nonideality in this circuit is thefinite open-loop gain of the opamp, due to which, when a sensorexperiences a change in its resistance, the row potentials cannotbe pinned to the same value (virtual ground). If a selectedresistance changes, the corresponding output will be given by(1), which in turn will cause the change in its row (called the“selected row” hereafter) potential as follows:

(4)

where and are the potentials of the th row and thopamp’s output node, respectively, and is the opamp’s gain.Now, let us assume that initially all of the sensors have resis-tance and the selected element’s resistance changes to .The potential difference between the selected row and any otherrow (called the “unselected row” hereafter) will be

(5)

where and are the potentials of selected and unselectedrows, respectively. This potential difference will cause a current

to flow between selected and unselected rows, given by

(6)

Now, by applying KCL at any unselected opamp’s input node,we find that current flowing through will now be lesser byan amount , changing the output by . Now, using (5)and (6), we may find that this change will be

(7)

Thus, the opamps that are not connected with the sensing el-ement will also show undesired voltage changes at the output.Here, we ignored the effect of on for simplicity andbecause it will be negligibly small. Using (2) and (7), we mayfind the ratio of desired and undesired outputs (crosstalk rejec-tion ratio) as

(8)

We see that a very high rejection of unwanted signal maybe achieved by increasing the opamp gain or by reducing feed-back resistance . Opamp gain is the most important param-eter for improvement since the ratio also changes theoverall signal gain. Furthermore, (2) and (3) suggest that, forsensors with smaller resistances, the scheme will have higheroutput and better resolution. Finally, we see that the best benefitof the scheme may be achieved with the opamps of very highgain and very low noise.

V. CONCLUSION

We have presented the virtual ground technique for solvingthe crosstalk problem in the networked resistive sensor arraysby discussing its nonidealities and limitations. The scheme pro-vides a faster scanning rate, and we have shown that it is bettersuited for sensors with low resistance, which has been a problemwith other techniques. We verified the scheme using PSPICEcircuit simulations. Additionally, we derived the expressions forsensitivity and crosstalk rejection. The sensitivity may be im-proved by reducing the overall noise and crosstalk may be re-duced by selecting high-open-loop-gain opamps.

REFERENCES

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[2] R. S. Saxena, R. K. Bhan, C. R. Jalwania, P. S. Rana, and S. K. Lomash,“Characterization of area arrays of microbolometer based un-cooled IRdetectors without using ROIC,” Sens. Actuators A, Phys., vol. 141, no.2, pp. 359–366, Feb. 2008.

[3] R. S. Saxena, R. K. Bhan, and A. Aggarwal, “A new discrete circuit forreadout of resistive sensor arrays,” Sens. Actuators A, Phys., vol. 149,no. 1, pp. 93–99, Jan. 2009.

[4] R. S. Saxena, R. K. Bhan, and A. Aggarwal, “Reducing readout com-plexity of large resistive sensor arrays,” IEEE Sensors J., vol. 8, no. 11,pp. 1862–1863, Nov. 2008.

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