A new BICS for CMOS operational amplifiers by using oscillation test techniques

A new BICS for CMOS operational amplifiers by usingoscillation test techniquesJ.Font *,R.Picos,M.Roca,E.Isern,E.Garcı´a-Moreno Grup de Tecnologia Electro`nica,Universitat de les Illes Balears,Ctra.Valldemossa,km.7.5,07122Palma de Mallorca,Spain Received 15November 2002;accepted 8April 2003AbstractIn this paper,we present a built-in current sensor to test operational amplifiers that takes advantage of previous results where the negativesupply current has been taken as the test observable using the Oscillation-test technique.The sensor is applied to a variable-length chain of OTAs considering an exhaustive analysis of catastrophic defects (opens,shorts),Gate Oxide Short and Floating Gate defects.We analyse the sensitivity of both frequency and amplitude of the current consumption.Results show that the proposed sensor provides 97%fault coverage,as the previous results suggested.q 2003Elsevier Ltd.All rights reserved.Keywords:Current sensor;Oscillation-test-technique;Operational transconductance amplifier;Fault models;Built-in current sensor1.IntroductionThe growing use of analog blocks within IC,combined with the ever-reducing technology sizes,has been motivating the development of several approaches for testing these analog sections embedded in digital systems [1–3].One of the more promising techniques is the oscillation test strategy,OTS (also known as oscillation-based test,OBT)[4–7].The central idea of this technique is to convert the circuit under test (CUT)into an oscillator during the test mode.This is accomplished by adding a feedback block to the CUT and activating it in the test mode to induce self-sustained oscillations,using a convenient starting set-up.It has been reported that the presence of defects affects the oscillation frequency [4],and so it can be used to distinguish between faulty and defect-free circuits.Most of the results reported for this technique use the output voltage as the test observable [4–7].It is a well known fact that current has been shown to be a very effective test observable not only for digital circuits [8–11],but also for analog ones [12,13].In Ref.[14],OTS was used to detect the defects in a two-stageoperational transconductance amplifier (OTA).In that work,a comparison of merits between supply current and output voltage as test observables was performed.The results showed that the maximum fault coverage was achieved when the peak value of the negative supply current was used as the test observable.Thus,the next logical step seems to be the implementation of a current sensor to test the CUT when using OTS.An on-chip sensor would be preferable to an off-chip sensor due to the best signal-to-noise ratio that can be achieved,as well as for its greater speed.However,it has the drawback of an extra area consumption.This problem can be alleviated by using a sensor that can be configured to monitor several analog blocks.The test structure proposed here is based on sequentially testing structures formed by one,two and three blocks of three OTAs each.Hence,as many OpAmps as desired can be tested in a simultaneous way just by adapting the sensor,as explained in this work.The rest of the paper is organized as follows.Section 2presents an overview of the OTS.Section 3provides us with the needed DFT modifications.Section 4explains the proposed Built-In Current Sensor (BICS).Section 5presents the results obtained with the sensor and,finally,main conclusions are highlighted in the last section.0026-2692/03/$-see front matter q 2003Elsevier Ltd.All rights reserved.doi:10.1016/S0026-2692(03)00158-7Microelectronics Journal 34(2003)919–926/locate/mejo*Corresponding author.Tel.:þ34-971-173-227,fax:þ34-971-173-426.E-mail address:vdfsjfr0@uib.es (J.Font).2.Oscillation-test strategy overviewFig.1shows the block diagram of a generic oscillation-test strategy.OTS consists of adding an external circuitry to the CUT to convert it an oscillator,whose oscillating frequency depends upon the external components of the feedback block,as well as on internal components of the CUT.Any fault will potentially manifest itself by deviating the oscillation frequency from its nominal value,even in most cases causing loss of oscillation.Therefore,that deviation may be employed to detect the fault [4].The oscillation frequency of the output voltage has not been the only parameter considered to detect a hard fault of the CUT.OBT was applied in Ref.[6]to two parameters of the test signal:frequency and amplitude.The integration of the output voltage provided test information of the former parameters.In Ref.[14],it was proposed to test an OTA by monitoring,in addition to the output voltage,another test signal:the supply current.Moreover,additional parameters of both output voltage V o ðt Þand supply current I ss ðt Þwere measured,such as the oscillating frequency,amplitude (or peak-to-peak value),peak value and average value over a period of time (DC value).For each parameter it was determined a tolerance band due to design and technol-ogy tolerances and power supply fluctuations.This tolerance band,which is the threshold to discriminate between good and faulty circuits,was calculated from a Monte Carlo analysis.It has been demonstrated in Ref.[14]that the optimal choice of parameters to test the OTA is to measure the maximum peak of the supply current.Since we have to determine whether the peak current falls inside or outside the tolerance band,the current sensor has to contain a window comparator as the pass/fail stage.3.Design for testability (DFT)In order to make the circuit testable,the designer must take into account the necessary modifications in the original design.There are two kinds of modifications.The first ones concern the changes at each OTA,while the second kind of modifications are related to the external feedback network.3.1.Reconfigurable OpAmpThe modifications the designer needs to introduce into each OTA are summarized in Fig.2.The control block (Ctrl block)allows the following possibilities.In normal mode ðon =off ¼0Þ;V fext_1and V fext_2are connected to the inverting and non-inverting inputs of the OTA,respectively.In test mode ðon =off ¼1Þ;the OTA has to be isolated from the rest of the circuit.Now,two different cases for the inputs in test mode are considered.The first case ðc =f ¼1Þcorresponds to the first OTA of the chain,which is set as a level comparator with a voltage reference,with no feedback.The second case ðc =f ¼0Þconcerns the remaining OTAs,which are set in a gain-one follower configuration.3.2.Feedback networkAs it has been mentioned before,the goal is to use the OTS to test a chain of OTAs (Fig.3).We have considered a chain composed of three blocks,where each block is made of three OTAs.Depending on the control signals S 1;S 2we force oscillations in one,two or three blocks.This is carried out by setting the first OpAmp as a comparator and the rest of the chain as cascaded followers connected to a RC network at the end.4.Proposed built-in current sensorThis section is devoted to describe the structure of the current sensor (Fig.4).First,the basichigh-performanceFig.1.Oscillation test strategy of a testablecircuit.Fig.2.DFT reconfigurable OpAmp.J.Font et al./Microelectronics Journal 34(2003)919–926920sensor is presented,and then it is shown how this sensor can be used with a variable current level.4.1.High-performance sensorThe proposed sensor (Fig.4)is based on a push–pull OTA in a feedback configuration and a controllable sensing resistance.Usually,the current sensing is done by measuring the voltage drop across a passive element [8,15],thus making it difficult to scale if a high-range measurement is desired.Also,process variations (as high as a 20%in some cases)make it difficult to perform an accurate measurement.By contrast,the sensor proposed here uses MOS transistors as the sensing element.These transistors are part of a feedback system,along with an OTA.This feedback improves parameter stability against process variations,thus allowing us a greater accuracy than stand-alone passive elements.The use of transistors also allows us to easily scale the circuit by changing its sensing resistance.The biasing stage of the OTA is represented by MR1and MR2,which set the reference voltage V ref :In the normal mode,the voltage reference V ref is set to V DD ;thus effectively switching off the sensor.Transistors M1–M9form a basic OTA in a push–pull configuration.The current of the CUT (our test observable)flows through MS1-MS2-(MSi),causing a small voltage drop V in ;which is sensed at the gate of M2.The MS i s also mirror the current through M5,providing an amplified copy of our test observable.The Ctrl block turns on/off one or more of the transistors (MS1…MSi)depending on its inputs S 1…S n :At the output of the OTA a scaled copy of the current consumption of the CUT is obtained.Both this current or the output voltage (Vout)can be used as the sensor output.In the case presented here,only two sensing transistors are used (MS1and MS2),with four possible test configurations including sensor switch-off.4.2.Controllable sensing resistanceControllable sensing resistance is a valuable add-on for current testing,because of the high rangeofFig.3.Reconfigurable Phase Shift Oscillator composed by either three,six or nineOpAmps.Fig.4.Schematics of the proposed BICS,including controllable sensing resistance.J.Font et al./Microelectronics Journal 34(2003)919–926921variability of the probing current signals for test.This controllable sensing resistance allows us a greater flexibility to measure different ranges of current and we can also fine-trigger the threshold to account for process variation.Moreover,it makes possible to test more than one simple device with the same sensor,thus saving silicon area.In this proposed circuit,the sensing resistance is implemented by using the transistors named MS1-MS2in Fig.4.The size of these transistors has to be chosen for the different incoming test current levels,keeping in mind two facts:(1)the transistors are to be in the ohmic region ðV in ,V g -V th Þ;and (2)voltage drop ðV in Þhas to be minimized.Control of the sensing resistance is performed by connecting the gate of MSi to V g (in the case that S x ¼1)or to V DD (when S x ¼0).As a special case,if all the S x are zero the sensor is switched off.In this case all the gates of MS x are connected to V DD to allow the CUT to function with the minimum possible performance degradation in normal mode.All possible control cases for our CUT are shown in Table 1.5.Results 5.1.Fault modelsIn this work,we have injected an exhaustive list of faults into the netlist extracted from the OTA layout to evaluate the merits of the test method proposed.As the defects introduced are catastrophic,loss of oscillation is expected in most cases.The schematics of the operational amplifier used in the simulations is shown in Fig.5.Its netlist has been obtained from the first release of analog and mixed-signal benchmark circuits [16]and it has been implemented in a 0.35m m technology.A short and an open at the drain/source of each MOS transistor have been simulated,as well as at each component of the compensating network.The hard open is simulated by adding a high resistance (R s ¼100M V )in series with the faulty component.The hard short is simulated as a very small resistor(R p ¼1V )in parallel with the component to be faulted.The total number of hard faults in our two-stage compensated OpAmp is 21:two times the total number of internal components (9MOS devices plus the resistor and the compensating capacitor)minus one,because the open at R c is the same one as that at C c :Moreover,MOS floating gate defects modelled with a zero value current source have been introduced (Fig.6)for HSPICE simulations,as proposed in Ref.[17].We get HSPICE convergence and starting-up DC continuity by imposing an initial gate voltage of 0V.This fault model fulfils two necessary conditions for this defect:first,we force current to be zero,and second,we assure voltage discontinuity between the terminals of the null-current source.We simulate the presence of Gate Oxide Shorts (GOS)defects by using the electrical model des-cribed in Ref.[18].In this unidimensional model two parameters are used to describe the defect.The first parameter models the oxide short position withinTable 1Gate voltage for the transistors MS2and MS1for the four possible control states S 2S 1Gate voltage Sensor stateMS2MS100Vdd Vdd OFF 01Gnd Vg ON 10Vg Gnd ON 11VgVgON Fig.6.Floating gate model,with zero initial gatevoltage.Fig.5.Schematics of the OTA under test.J.Font et al./Microelectronics Journal 34(2003)919–926922the device along the transistor length,since it was shown that the width positions for a given length can be modelled through the second parameter.The GOS position is modelled through the parameter a;that takes a value between0(a gate-source oxide short)and 1(gate-drain oxide short).Any value of a between0 and1corresponds to a short between the gate and the channel.The second parameter of the model in Ref.[18]describes the effective resistance of the oxide short and is physically related to the defect size and conductive properties.It was also shown that this parameter can be used to model the defect position along the transistor width,thus compacting the defect model parameters.This second parameter is called R s as it is an effective resistance.Fig.7shows the equivalent electrical model for a MOS transistor with a GOS defect.It is composed of two transistors connected in series,with a resistor R s between the gate and their connection.The size of each device is modified by means of the parameter a;such that if the original fault free device has a length L;then in the faulty device the original transistor is splitted into two minor devices Na and Nb of length a L andð12aÞL for Na and Nb, respectively.Note that if the defect is a gate-source oxide short,then a¼0and the defect is simply a resistance between the gate and the source.In this paper,we have modelled the presence of a GOS with a¼0:5and R s¼1V in each device.Since we have been using the actual layout,some additional assumptions should be made for each kind of the previously listed defects.Actually,since each transistor from the schematics(Fig.5)is divided into smaller transistors in the layout(hereafter referred to as minor transistors),the real number of potential faults increases dramatically.For instance,M9is composed by a number of minor transistors in series,while all the others consist of a number of minor transistors in parallel.As far as opens and floats are concerned,we are only considering hard faults in the lines between transistor blocks,not considering probable defects for each minor transistor.In that way,we are opening all the minor transistors or injecting zero current to all the gates of the transistor block.On the other hand, the shorts at the transistor and the shorts at the minor transistors are equivalent.Regarding GOS defects,we also have to assume that the transistors are splitted into smaller ones.In this case,we simulated only a GOS in one of the minor transistors by using the model described before.It can be shown that,as all minor transistors are equivalent,an a¼0:5and R s¼1V GOS in a minor transistor has the same effect that an a¼0:5GOS in any other minor transistor.5.2.Description of the CUTResults have been generated by using a CUT composed by three blocks of three OTAs each,as shown in Fig.3.The control signals S1and S2select one of the four possibilities,as has been specified in Table2.The operational amplifiers have been configured as a multiple OpAmp phase-shift oscillator based on 3,6or9OpAmps,depending on the control signals. The oscillation frequency of the fault-free CUT depends on the number of OpAmps and the feedback network time constant RC.This frequency decreases when either the number of OpAmps or the product RC increases.For a resistor R¼2K V and a capacitor C¼0:5pF, the negative supply current(indicated as power consump-tion in Table2and Fig.8)for a CUT of three OpAmps has been found to present a mean of0.92mA(0.18mA peak to peak).For a CUT of six OpAmps,the mean current was 1.7mA(1.04mA peak to peak),and 2.7mA(0.8mA peak to peak)for a CUT of nine OpAmps.The degradation induced by the sensor has been made smaller than5%by taking length L¼0:5m m and widths of W1¼300and W2¼600m m for the transistors MS1and MS2,respectively.As far as the ground voltage errorðV inÞdue to the sensor is concerned,we obtain an oscillating voltage of margin[16mV,25mV](Fig.9),which represents a maximum virtual ground level raise(respect to ground level)of less than1%with respect to the supply voltage(3.3V).If we compute the equivalent sensing resistance of both MS1and MS2transistors,working in the linear zone,for the three different CUT configurations,we obtain that the maximum input voltage(virtual ground level V in of the CUT)of our sensor through the above transistors isalways Fig.7.GOS electrical model.Table2Current consumption for the three blocks of OTAs without sensorS2S1#of OpAmps Includedin the CUT Power consumption of each CUT without sensor(mA)00Sensor off–0130.92mA106 1.70mA119 2.7mAJ.Font et al./Microelectronics Journal34(2003)919–926923the same.For a CUT of three,six and nine OpAmps,the equivalent resistance are 62,31and 21V ,for S 1S 2¼01,10,and 11,respectively.5.3.Fault coverageTo evaluate the fault coverage of the proposed sensor when using the oscillation-test technique for a set of OpAmps configured as a phase-shift oscillator,an exhaustive set of faults (opens,shorts,floating gates and GOS defects)has been analyzed.The faults have been injected in the extracted layout of each OTA under test,considering the assumptions made in Section 5.1.We have verified that any open,short,or floating gate defect in any of the OpAmps configured as followers (in the OpAmp-chain,all the OpAmps except the first one)impacts our test observable in a similar way.In other words,except in the first OpAmp of the chain (set up as a comparator),the effect of the same fault is similar in any OTA.So,for our purposes,it should be sufficient to simulate defects in the first OpAmp and in any other one of the chain.Once this assumption is made,the results of testing the three-OpAmp CUT can be extended to the other CUT schemes of six and nine OTAs.For the sake of simplicity and comprehension upon calculating the fault coverage,we have only counted nine GOS defects for each OpAmp,one GOS defect at only one minor transistor of each transistor.The output voltage of the sensor ðV out Þis shown in Fig.10for the fault-free case and for some faults.The fault simulation results for our CUT are presented in Table 3.The only undetected fault has been the short at the compensating resistor R c :Indeed,it seems reasonable as the short at the compensating resistor hardly changes the effect of the compensating network on the supply current.The other faults can be detected by using either peak-to-peak value of the observable or its oscillating frequency.By simultaneously observing these two parameters,a fault coverage greater than 97%is achieved.Otherwise,a 93%is obtained by observing only the peak-to-peak value and a 95%by observing the oscillatingfrequency.Fig.8.Current Consumption for Phase-Shift Oscillator in the cases with three,six and nine OpAmps.J.Font et al./Microelectronics Journal 34(2003)919–926924Fig.10.Sensor Output Voltage for the fault-free,open at M9located at OTA3,floating gate of M7in OTA2,short at M1in OTA1and short at M3inOTA1.Fig.9.Virtual Ground Level (Vin)raise for the CUT with the sensor activated in the cases with three,six and nine OpAmps.J.Font et al./Microelectronics Journal 34(2003)919–9269256.ConclusionsAn innovative BICS adaptable to different current ranges has been presented in this work with very good performance.This BICS,composed by an OTA and a controllable sensing resistance,has shown itself as a reliable I-V converter for our test observable (negative power supply current,I ss ).The output voltage of the sensor provides us a scaled copy of I ss ;where the scale factor is a function of the design parameters of the sensor and two control signals.A fault coverage greater than 97%can be achieved by applying OTS to a chain of three operational amplifiers configured as a phase-shift oscillator.Also,the area overhead is small and depends upon the number of OTAs in the oscillator:the greater the number of OTAs is,the smaller the area overhead will be.As it has been stated before,the fault coverage of opens,shorts and floating gates can be extended to chains of more OpAmps.By contrast,for three OpAmps,it has been found out that the behaviour of a GOS defect is different depending at which OTA the defect is located,so results obtained for three OpAmps cannot be extended.This way,further work should be devoted to GOS defects for chains longer than three OpAmps.References[1]C.Chalk,M.Zwolinski,Design for test technique for increasing theresolution of supply current monitoring in analog circuits,Electron.Lett.33(21)(1997)1746–1747.[2]D.Papakostas,A.Hatzopulos,Supply current testing in linear bipolarICs,Electron.Lett.30(2)(1994)128–130.[3]Eckersall,Erighton,et al.,Testing mixed signal ASICs through theuse of supply current monitoring,Proc.Eur.Test Conf.(1993)385–391.[4]K.Arabi,B.Kaminska,Design for testability of embedded integratedoperational amplifiers,IEEE J.Solid-State Circuits 33(4)(1998)573–581.[5]G.Huertas,D.Va´zquez,A.Rueda,J.L.Huertas,Oscillation-based test,Proc.IEEE VLSI India January (2000).[6]G.Huertas,D.Va´zquez,A.Rueda,J.L.Huertas,A practical method for reading test outcomes in oscillation-based 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defects in the three-OTA phase-shift oscillator FaultsPeak-to-peak Frequency Peak-to-peak &frequency Faultcoverage (%)Shorts (11£3)30303091Opens (10£3)303030100Floats(9£3)272727100GOS (9£3)222427100Fault coverage (%)939597J.Font et al./Microelectronics Journal 34(2003)919–926926。