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JOURNAL OF ELECTRONIC TESTING: Theory and Applications 20, 389–396, 2004c© 2004 Kluwer Academic Publishers. Manufactured in The United States.

Oscillation Test Strategy: A Case Study∗

EDUARDO ROMERO AND GABRIELA PERETTIElectronics and Control Research Group, Villa Marıa Regional School, National University of Technology,

Avda. Universidad 450, Villa Marıa (5900), [email protected]

CARLOS MARQUESElectronics and Instrumentation Development Group, Mathematics, Astronomy and Physics Faculty,

National University of Cordoba, Medina Allende y Haya de Torre, Cordoba (5000), [email protected]

Received September 9, 2002; Revised May 12, 2003

Editors: F. Vargas and V. Champac

Abstract. In this paper is proposed as a case study the test of a folded cascode operational amplifier using theOscillation Test Strategy (OTS). This Operational Amplifier (OPA) is chosen in order to evaluate the ability of OTSto test a more complex amplifier than those previously reported. To obtain comparative results, three different typesof single-OPA oscillators are employed.

A catastrophic-fault injection procedure is carried out using SPICE. In all oscillators, simulation results show thatthe fault coverage obtained is lower than those previously obtained by many researchers for simpler amplifiers. Thisfact suggests that OTS might be inconvenient for applications using the OPA targeted in this work and requiringhigh fault coverage.

Keywords: oscillation test strategy, operational amplifier, design for test, testing

1. Introduction

The developments in CMOS process technologies havemade possible to combine digital and analog circuits ina single chip, requiring that CMOS circuits perform allthe digital process functions and the necessary analoginterfaces. Due to the monolithic realizations and thecomplex nature of analog signals, testing the analogsections of this kind of systems is one of the most chal-lenging tasks in analog design. Usually, these analogsections require a very small portion of the integratedcircuit area, but their test and the test development pro-

∗This work is based on a Latin American Test Workshop 2002 pre-sentation.

cess are very time consuming. Consequently, the over-all cost is increased [4, 6, 7, 12, 13].

Analog and mixed signal integrated circuits test hasbeen motivated by the increasing importance of thiskind of circuits and many researchers have devotedtheir efforts in the development of new Design for Test(DfT) techniques with the goal of both increasing faultcoverage and reducing test time [5, 14].

The OPAs are the most common analog modulesused in analog and mixed signal integrated systems. Astheir performances depend on the associated externalfeedback circuit, it is very difficult to propose a gener-alized DfT strategy to be applied to all kind of OPA-based analog circuits. The measurement of parameterssuch as gain, unity-gain bandwidth, input bias current,

390 Romero, Peretti and Marques

etc., allows a functional test, but the main drawbackof this approach is that is extremely time-consuming[16]. Due to this reason, other approaches have beensuggested in the literature.

In recent past years Oscillation Test Strategy (OTS)has been proposed as a DfT methodology for analogand mixed-signal circuits. OTS applied to embeddedOPAs converts them (in test mode) in an oscillator cir-cuit, by means of both reconfiguring and adding someextra circuitry. The core idea in this approach is that afault in the OPA will result in either an oscillator fre-quency deviation or a loss of oscillation. Obviously, theobservability of a fault in one of the parameters of theOPA being tested will increase as the sensitivity of theoscillation frequency with this parameter is increased.In [1] a unified treatment of the single amplifier os-cillators useful for testing embedded OPAs has beenpresented.

Several works have been reported in this researcharea. In [18], the authors presented OTS transforma-tions of active RC filter configurations that can be usedas the basis of a simple and effective go-no go test.

OTS has also been applied to test Analog to DigitalConverters (ADC). This strategy converts the ADC intoan oscillator and the oscillation frequency monitors pa-rameters like the ADC conversion rate, differential andintegral non linearity, etc. [2].

Test and diagnosis of catastrophic faults in a thresh-old detector circuit has been suggested as other appli-cation of this strategy [10].

In a very recent work [8], Huertas et al. proposea universal biquad filter as OTS validation vehicle.They inject soft and hard faults that produce devia-tions in the filter transfer function. Their experimen-tal results allow concluding that small deviations attransfer function level are detected by the OTS schemeproposed.

In [3], the authors propose different configurationsof sinusoidal oscillators to test embedded OPAs. Thehighest fault coverage reaches 99% for a single OPAoscillator. It should be mentioned that the Circuit Un-der Test (CUT) is a two-stage CMOS Miller OPA. In[17] OTS is applied to an active low-pass filter, config-uring the embedded OPAs as square-wave oscillators.Although different oscillating structures are proposed,the CUT is also a CMOS Miller OPA as in [3]. Thehighest fault coverage reaches 98.39% for a single OPAoscillator.

In this work, a CMOS folded cascode OPA is pro-posed as a case study, in order to evaluate the ability of

Table 1. OPA specification obtained bysimulation (CL = 10 pf).

Gain at dc Av(0) 81.1 dB

Unity-gain bandwidth 8.9 MHz

Dominant pole 776.6 Hz

Offset voltage 268 µV

Slew rate 10 V/µsec

OTS to test an amplifier with a more complex structurethan those previously reported in the literature.

The organization of the rest of the paper is as fol-lows: in the next section the schematic of the CUTand its main specifications are described. In Section3 the oscillating configurations used in this work andthe main considerations to apply OTS to the proposedCUT are presented. Section 4 details the fault modelused. The simulation results are presented in Section 5and finally Section 6 concludes the paper.

2. Circuit Under Test

The CUT is a typical folded cascode OPA that has beenwidely studied in the literature [9, 11]. This amplifieris designed in CMOS CNM25 2.5 microns technologyand its main design specifications can be found in [15].Fig. 1 depicts the schematic of this OPA and Table 1summarizes its main performance specifications, ob-tained by simulations at VSS = −2.5 V, VDD = 2.5 V.

3. Application of OTS to the CUT

To apply OTS to the CUT we have chosen the singleOPA sinusoidal-wave configurations (named version 1and version 2 in our work) proposed in [3] and thesquare-wave scheme evaluated in [17]. The high faultcoverage previously reported for these oscillators havemotivated this choice. Our goal is to evaluate this teststrategy when it is applied to a more complex amplifiertopology than CMOS Miller OPA.

It should be mentioned that the oscillation frequencyof these oscillators depends strongly on the internalOPA parameters, such as gain and frequency response,as has been pointed out by several authors.

The schematic circuit shown in Fig. 2 is the same forthe sinusoidal oscillator version 1 and the square waveone. It is possible to obtain either square or sinusoidaloutput waves from this structure by a proper selection

Oscillation Test Strategy: A Case Study 391

Fig. 1. Schematic of CUT.

Fig. 2. Schematic of the oscillator, square wave and sinusoidalversion 1.

of the component values. Fig. 3 depicts the schematicof the sinusoidal-wave oscillator version 2.

The operating frequencies are chosen in order to ob-tain acceptable values of resistors and capacitors. Thecomponent values and the oscillation frequencies forthe three configurations can be seen in Table 2.

4. Fault Modeling

Since the layout of the circuit is not available, an ex-haustive fault list from the schematic is generated tak-

Table 2. Component values and oscillation frequencies for sinu-soidal and square wave oscillators.

Configuration

Square-wave Sine-wave Sine waveValues oscillator oscillator version 1 oscillator version 2

R1 50 K� 20 K� 9 k�

R2 50 K� 200 K� 470 k�

R 100 K� 28 K� 250 k�

C 50 pF 50 pF 100 pF

Oscillation 98.2 KHz 824.7 KHz 203.3 KHzfrequency

Fig. 3. Schematic of the sinusoidal-wave oscillator version 2.


Table 3. Schematically redundant nodes.

Reference node Schematically redundant nodes

1 8, 15, 38, 44, 55, 56

2 4, 7, 5, 3, 25, 26, 63

6 13, 24, 28, 50, 65

9 10, 11, 12, 14, 66

16 17, 18

19 –

20 23, 29

21 –

22 27, 30

31 32, 49, 57, 58, 59, 60, 61

33 34, 35, 39, 40

36 64

37 41, 45, 51, 52, 53, 54, 62

42 43

47 46, 48

ing into account only catastrophic faults. A total num-ber of 2033 faults is used to obtain the fault coverage.

This fault list includes all possible open and shortfaults of transistors, with only the exception of gatecontact open fault. We consider short faults at the cir-cuit level rather than only shorts at the transistor levelin order to take into account all probable faults. Addi-tionally, this procedure allows comparisons with resultsreported by other authors.

The OPA has 66 nodes and the schematically redun-dant ones are listed in Table 3. The reference node andits schematically redundant nodes are connected at thesame potential and obviously, it is impossible to detecta short fault between them. Due to this fact, for shortfault simulations only the reference nodes of Table 3are taking into account.

It should be mentioned that schematically redundantnodes are not physically redundant nodes, in the sensethat different behaviors of the CUT could be obtainedif an open fault is located in different schematicallyredundant nodes. Then, for open faults all nodes in thistable are considered, excluding the gate open contactfault as is pointed out above.

Fault simulations are carried out using SPICE con-sidering only single fault injection. Open faults aremodeled by a 10 M� resistor while shorts are mod-eled by a 10 � resistor. It is assumed here that a fault isdetected when the oscillation frequency is out of a tol-erance band, defined as ±5% of the free-fault oscillatorfrequency.

Table 4. Short faults simulation results for square-waveoscillator.

Oscillation frequency OutputShort faults deviation (%) amplitude (V)

Without fault 0 [2.42; −2.42]

N1-N6 0.4 [2.42; −2.42]

N1-N39 −30.8 [−368m; −2.44]

N1-N36 −64.1 [−365m; −2.44]

N1-N31 −0.8 [2.4; −2.4]

N1-N37 −29.0 [−371m; −2.44]

N2-N9 72.1 [2.42; −2.46]

N2-N20 −83.7 [−4m; −2.44]

N2-N22 −85.0 [2.45; 0.01]

N2-N39 −6.2 [2.33; −2.08]

N2-N31 −0.91 [2.27; −2.26]

N2-N37 7.5 [1.76; −1.84]

N9-N16 23.6 [2.44; −2.43]

N9-N39 −3.6 [2.26; −2.20]

N6-N37 −1.7 [2.34; −2.43]

N16-N36 28.0 [2.43; −2.41]

N16-N31 1.3 [2.41; −2.43]

N16-N43 2547.6 [−990m; −1.60]

N16-N37 89.9 [2.41; −2.43]

N19-N20 941.7 [−1.12; −2.31]

N19-N22 −39.3 [2.41; −2.42]

N19-N47 −51.2 [2.37; −2.41]

N19-N37 66.6 [2.25; −2.38]

N20-N39 51.0 [2.43; −2.39]

N20-N36 14.5 [2.41; −2.41]

N20-N37 −8.6 [1.95; −2.43]

N22-N43 1366.4 [−412m; −969m]

N22-N47 1.3 [2.26; −2.44]

N39-N36 0.2 [2.41; −2.43]

N39-N31 −0.45 [2.43; −2.41]

N39-N43 −28.4 [−366m; −2.45]

N39-N37 1.0 [2.42; −2.42]

N36-N31 0.6 [2.23; −2.44]

N36-N43 2.0 [2.25; −2.43]

N36-N37 −27.4 [−461m; −2.44]

N31-N43 2.3 [2.26; −2.43]

N31-N47 4.4 [2.37; −1.35]

N31-N37 2.9 [2.42; −2.43]

N43-N47 1.0 [2.48; −2.43]

N43-N37 −28.3 [−455m; −2.44]

N47-N37 3.1 [1.19; −1.80]

Any other fault No oscillation –


Table 5. Open faults simulation results for square-waveoscillator.

Oscillation frequency OutputOpen fault deviation (%) amplitude (V)

Without fault 0% [2.42; −2.42 ]

N7 −5.3 [2.42; −2.42]

N11 26.8 [−187m; 360m]

N14 −0.2 [2.41; −2.42]

N29 −28.2 [−362m; −2.43]

N33 −27.3 [−356m; −2.43]

N38 −2.08 [2.42; −2.41]

N42 −29.0 [−362m; −2.43]

N43 −29.2 [−362m; −2.43]

N44 −29.2 [−362m; −2.43]

N45 −7.84 [2.20; −2.41]

N47 −28.9 [−355m; −2.43]

N48 −33.1 [7m; −5m]

N49 −0.25 [2.23; −2.43]

N50 −19.5 [190m; −2.43]

N52 −17.2 [579m; −2.43]

N53 −1.1 [2.43; −2.43]

N55 −1.1 [2.43; −2.42]

N56 −15.2 [2.34; −2.15]

N59 −8.0 [2.42; −2.44]

N60 1.0 [2.42; −2.44]

N61 1.0 [2.44; −2.44]

N62 1.0 [2.44; −2.45]

N63 −0.6 [2.41; −2.41]

N65 1.1 [2.46; −2.46]


5. Simulation Results

Short faults simulation results for square-wave os-cillator are shown in Table 4, where only one faultof every schematically redundant fault set is pre-sented. Open faults simulation results are listed inTable 5.

As can be seen from these tables, the main injectedfaults result in a loss of oscillation and a significantnumber of faults remain undetected. The fault coverageobtained was 74.1%, which is meaningful lower thanobtained in [17].

It should be mentioned that in these tables the unde-tected faults have been highlighted. This kind of faultshas been also highlighted in Tables 6–9. In this way, the

Table 6. Short faults simulation results for sinusoidal-waveoscillator version 1.



N1-N6 0.21 [2.25; −2.21]

N1-N31 −3.8 [1.71; −1.59]

N9-N37 −2.2 [2.26; −2.29]

N16-N36 −19.2 [2.33; −2.31]

N16-N31 −5.3 [1.60; −1.47]

N19-N20 94.0 [−1.18; −2.27]

N19-N22 −74.0 [2.26; −2.28]

N19-N43 154.6 [1.33; 840m]

N19-N47 −82.6 [1.95; −2.24]

N19-N37 1112.6 [462m; 60m]

N20-N39 0.21 [2.19; −1.86]

N20-N36 −41.1 [2.10; −2.21]

N21-N36 −3.0 [357m; −1.85]

N21-N31 1.0 [2.26; −2.03]

N22-N43 163.1 [−332m; −471m]

N22-N47 7.3 [1.21; −1.32]

N39-N36 −1.4 [2.12; −2.17]

N39-N31 −3.0 [2.18; −1.83]

N39-N37 −0.6 [2.21; −2.17]

N36-N31 −3.8 [1.87; −1.54]

N36-N43 5.4 [2.15; −2.02]

N36-N47 81.9 [2.31; 658m]

N31-N43 −16.9 [2.31; −2.20]

N31-N37 1.1 [2.20; −2.15]

N43-N47 −38.8 [2.43; −2.30]


breakdown of detected and undetected faults is easilyobserved.

For the sinusoidal oscillator version 1, short and openfault simulation results are reported in Tables 6 and 7respectively. As in the square-wave oscillator case, themain injected faults result in loss of oscillation, and asignificant number of faults remain undetectable. Thefault coverage obtained is 82.8%, which is higher thanthe square-wave oscillator one.

For the sinusoidal oscillator version 2, the faultinjection results are shown in Tables 8 and 9. Thisconfiguration obtained the highest fault coverage,89.6%.


Table 7. Open faults simulation results for sinusoidal-waveoscillator version 1.

Oscillation frequency OutputOpen faults deviation (%) amplitude (V)


N7 −1.4 [2.31; −2.33]

N14 −1.4 [2.33; −2.18]

N34 −24.7 [169m; −145m]

N38 −3.0 [2.31; −2.15]

N53 −7.4 [2.03; −2.20]

N55 −6.7 [2.03; −2.19]

N60 −4.5 [2.23; −2.16]

N61 0.2 [2.22; −2.12]

N62 −4.5 [2.22; −2.25]

N63 −3.8 [2.31; −2.22]

N65 −3.0 [2.01; −2.10]


Table 8. Open faults simulation results for sinusoidal-waveoscillator version 2.

Oscillation frequency OutputOpen faults deviation (%) amplitude (V)


N1 1.8 [1.29; −1.69]

N7 1.9 [1.31; −1.67]

N14 9.1 [1.29; −1.48]

N34 −36.3 [1.17; −1.97]

N38 −0.2 [1.29; −1.58]

N45 −29.3 [860.9m; −1.94]

N48 −24.6 [21.4m; −18.4m]

N49 2.5 [1.29; −1.63]

N51 1.4 [1.31; −1.86]

N52 −1.6 [1.27; −1.70]

N55 −1.7 [1.27; −1.63]

N60 5.5 [1.27; −1.40]

N61 3.9 [1.30; −1.67]

N63 1.6 [1.30; −1.67]

N65 5.5 [1.27; −1.45]


Tables 10 and 11 summarize undetected faults,shorts and opens respectively, for the three configu-rations targeted in this work. An “X” symbol meansthat the fault is undetected in the respective oscil-lator. Additionally, hard-to-detect faults have beenhighlighted.

Table 9. Short faults simulation results for sinusoidal-waveoscillator version 2.



N1-N6 1.85 [1.31; −1.59]

N2-N19 25.9 [0; −1.1]

N9-N16 −68.1 [965m; −982m]

N9-N39 −7.5 [1.24; −1.30]

N9-N37 1.7 [1.30; −1.68]

N16-N21 −61.7 [2.30; −2.27]

N16-N36 15.6 [1.30; −424m]

N16-N31 1.2 [1.24; −1.57]

N20-N39 −2.9 [211m; −113m]

N20-N37 −19.24 [739m; −1.59]

N21-N31 −40.9 [1.43; −1.52]

N21-N37 −79.1 [1.12; −2.30]

N39-N36 1.6 [1.18; −1.40]

N39-N31 −55.3 [1.70; −2.10]

N39-N37 3.6 [1.23; −1.37]

N36-N31 −3.5 [1.03; −844m]

N36-N47 356.8 [2.42; −577m]

N31-N37 5.9 [1.29; −1.50]

N43-N47 −66.9 [2.46; −2.46]


From the results shown in these tables, it is pos-sible to observe some complementarities. This sug-gests that the fault coverage could be improvedusing two configurations in two successive testsessions.

The fault coverage can be also increased to 83.32%for the square wave oscillator, 89.40% for the sinu-soidal wave oscillator version 1 and 94% for the sinu-soidal wave oscillator version 2, if voltage level mea-surement technique is applied. In this work, a toleranceband of ±5% is adopted. Consequently, if the outputlevel is within this band, the fault is considered unde-tected [10].

It should be mentioned that it is necessary to imple-ment a more complicated checker in order to improvethe fault coverage by means of output voltage mea-surements, but the simplicity of OTS is lost. There-fore, this alternative has to be carefully considered, be-cause the fault coverage improvements could be notenough.


Table 10. Undetected short faults.

Short Square Sinusoidal Sinusoidalfaults wave version 1 version 2

N1-N6 X X X

N1-N31 X X

N2-N31 X

N9-N39 X

N9-N37 X X

N6-N37 X

N16-N31 X X

N20-N39 X X

N21-N36 X

N21-N31 X

N22-N47 X

N39-N36 X X X

N39-N31 X X

N39-N37 X X X

N36-N31 X X X

N36-N43 X

N31-N43 X

N31-N47 X

N31-N37 X X

N43-N47 X

N47-N37 X

Table 11. Undetected open faults.

Open Square Sinusoidal Sinusoidalfaults wave version 1 version 2

N1 X

N7 X X

N14 X

N38 X X X

N49 X X

N51 X

N52 X

N53 X

N55 X X

N60 X X

N61 X X X

N62 X X

N63 X X X

N65 X X

6. Conclusion

In this paper, the problem of applying OTS to test amore complex OPA than those previously reported inthe literature has been addressed. As a case study thetest of a folded cascode OPA is proposed. Sinusoidaland square wave oscillators are used in order to obtaincomparative results.

The fault coverage obtained for the CUT in all test-ing configurations was meaningful lower than thosereported by many authors for simpler OPAs. This factsuggests that OTS might become inconvenient for ap-plications that use this kind of amplifier and requirehigh fault coverage.

These results can be slightly improved by means ofvoltage level measurement technique, but the simplic-ity of OTS is lost. Therefore, its application should becarefully considered.

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Eduardo Romero was born in Chaco, Argentina in 1964. He re-ceived the Electronic Engineering degree from the UniversidadCatolica de Cordoba, Argentina in 1987. He is currently an AssociateProfessor at Universidad Tecnologica Nacional, Facultad RegionalVilla Marıa. His main research interests are analog and mixed-signaltest and design for testability.

Gabriela Peretti was born in Cordoba, Argentina in 1973. She re-ceived the Electronic Engineering degree from the Universidad Tec-nologica Nacional, Facultad Regional Villa Marıa in 1998. Currentlyshe is pursuing the PhD at the Universidad Tecnologica Nacional.Her main research interests are analog and mixed-signal test andbuilt-in self-test.

Carlos Marques was born in Cordoba, Argentina in 1947. He re-ceived the Electronic Engineering degree from Universidad Catolicade Cordoba, Argentina in 1970. Currently he is a Professor at Uni-versidad Nacional de Cordoba, Facultad de Matematica, Astronomıay Fısica. His main research interests are scientific instrumentation,digital signal processing and integrated circuit design.