Analog Integrated Circuits and Signal Processing, 32, 187–190, 2002©C 2002 Kluwer Academic Publishers. Manufactured in The Netherlands.

A Simple and Secure Start-Up Circuitry for Oscillation-Based-Test Application

DIEGO VAZQUEZ, GLORIA HUERTAS, ADORACION RUEDA AND JOSE L. HUERTASInstituto de Microelectronica de Sevilla, Centro Nacional de Microelectronica (IMSE-CNM)

Avda Reina Mercedes s/n, Edificio CICA-CNM, 41012, Sevilla, SPAINE-mail: dgarcia@imse.cnm.es; gloria@imse.cnm.es; rueda@imse.cnm.es; huertas@imse.cnm.es

Abstract. A simple start-up strategy specially suitable for the oscillation-based-test application of opamp-basedcircuits is presented. The proposed approach not only ensures that the oscillator will start to run (safe start-up) butalso the steady-state (SS) can be reached very fast (short transient-time).

Key Words: oscillation-based-test, design-for-test, active filters testing

1. Introduction

One of the structural test solutions for analog and mixedsignal-circuits that have focused the interest during thelast years has been the so-called oscillation-based-test(OBT) [1,2]. It is based on converting the circuit un-der test (CUT) into an oscillator during the test mode.Oscillation parameters are then closely related to thebehavior of the circuit in the normal mode and thus,faulty circuits can be detected in this way. Even more,complex circuits can be split into different blocks towhat the OBT can be applied separately. This approachseems very attractive because it is conceptually sim-ple, does not requires strong circuit modifications dur-ing testing and can handle built-in-self-testing (BIST)without the penalty of dedicated, additional on-chipgeneration hardware.

But there is an important issue whenever an oscilla-tor is involved: the start-up. Since the basis of OBT isthe conversion of the analog circuitry into an oscillatorto measure the main oscillation parameters, it shouldbe obvious that its feasibility can be seriously compro-mised if a fast and safe start-up is not ensured. First, itmust be ensured that the configured oscillator duringthe test mode will start to run (safe start-up). Secondly,the measurement of oscillation parameters has to beperformed once the steady-state (SS) has been reached.For this, it is convenient that the configured oscillatorwill be provided with a proper mechanism to shortenits transient time because the test time is also of vitalimportance.

It is well known that the start-up problem of oscilla-tors can be overcome if it is forced to start to run froma proper initial state. Even more, the closer the initialthe state from a point of the SS is, shorter the transienttime until both frequency and amplitude of oscillationsare stabilized is. For that, it is necessary to store, previ-ously to run, the appropriated initial conditions in thepassive-memory elements that compose the oscillator.

Based on the assumption above, we aim to show astart-up approach that not only save the problems ofsecure and short transient start-up, but also ensures avery low impact on the performance of the CUT duringthe normal operation. Even more, its application is notrestricted to the context of the OBT, but also can be usedin any other context where an oscillator is involved.

2. Basic Technique

The proposed approach makes use of a modifiedopamp, called ‘sw-opamp’, that has been shown to bevery effective and useful for the application of the OBTtechnique [2], and the test of analog and mixed-signalcircuits [3] in general, while its impact on the perfor-mance can be made negligible if it is properly designed[4]. It is conceptually illustrated as shown in Fig. 1. Itcan be described as a functional block with two operat-ing modes depending on a digital control (�C). In onemode, the circuit operates as an opamp (opamp mode),where terminals V + and V − are the inputs and Vout theoutput. In the other mode, the circuit acts as a voltage

188 Vazquez et al.

VoutVout

Vout

VT

�C = L

�C = H

VT

(a) (b) (c)

(�C = L) (�C = H)

�C

Fig. 1. Sw-opamp concept. (a) Symbol. (b) Opamp mode. (c) Buffer mode.

follower or buffer (Buffer mode), where the signalpresent at terminal VT appears at output Vout.

Let us make use of the circuit in Fig. 2 as the base forthe explanation of the approach. This circuit is based ona second order switched-capacitor (SC) Band-Pass fil-ter that have been converted into an oscillator by addinga zero crossing detector as the feedback loop. The ob-jective is then to be able to set a proper initial conditionfor the state variables represented by the outputs (Vo1

and Vo2) of the integrators. For that, the correspondingcharge must be stored in the memory elements (C3 and

C2

C1

C3C4

C5

C6

C8

C7

C9

�1 �1

�2 �2

�2

�2

�1

�1

�1

�2

�2

Vo1

Vo2

Vinput

�C

sw-opamp

DigitalControl: �C

�C

�C�C

Vinit

NormalInput

sw-st

sw-st

sw-opamp

Vinit

VT

VT

SC Band-Pass Filter

Zero-Crossing detector

Fig. 2. SC oscillator with the proposed start-up approach.

C4). Concerning the filter block, it differs from a con-ventional implementation on the fact that the opampshave been substituted by sw-opamps and that a coupleof switches (named sw st), controlled also by �C , havebeen placed between ground and the top terminals ofthe integration capacitors C3 and C4. The terminal VT

of both sw-opamps are connected to a voltage refer-ence of value Vinit. In fact, the reference voltage maybe different for each sw-opamp, but it is not needed ingeneral if the value Vinit is selected as the correspond-ing point where the oscillation waves in the SS at Vo1

A Simple and Secure Start-Up Circuitry 189

and Vo2 cross the other. It can be previously calculated,for example, by performing a transient simulation ofthe circuit.

The start-up process begins by setting the digitalcontrol signal �C = High. The sw-opamp is then work-ing as a buffer and switches named sw st short the topplate of capacitors C3 and C4 to ground. The circuitconfiguration forces the voltage at Vo1 and Vo2 to beequal to Vinit following approximately the equation,

Vo1,o2(t) = Vinit(1 − e−t/GB) + Vo1,o2(0)e−t/GB

where GB is the gain bandwidth of the amplifier in-volved in the buffer and Vo1,o2(0) represents the valueof the corresponding output just before the start-up pro-cess begins. In addition, the rest of elements (capacitorsand zero-crossing detector) are then initialized accord-ingly to the voltages at Vo1 and Vo2.

Because the GB of the amplifier is normally muchlarger than the maximum operating frequency of thecircuit, the speed of loading process is very high. Moreconcretely, it would only take about five times the timeconstant (∼5/GB) to reach the final value Vinit. How-ever, it should be notice that, in the SC case, the loadingprocess should be synchronized with the clock phases(�1 and �2) and maintained during an integer numberof clock periods.

In fact, the offset voltage of the sw-opamps and otherfactors will introduce small errors in the charge storedin the capacitors. However, it is not a major problemwhenever these errors be kept smalls.

Once the capacitors have been loaded, the controlsignal �C returns to Low. Then, switches named sw stare opened and the sw-opamps return to the normalmode (opamp mode). Taking into account the chargeconservation law, and the fact that the top plate of ca-pacitors C3 and C4 switch between ground and virtualground, the oscillator start to run but from the pointdefined by Vo1 = Vo2 = Vinit. Because this point corre-sponds to a point in the SS of the oscillator, it is ensurednot only that the oscillator will run, but also the SS willbe reached very fast.

3. Example of Application

A circuit corresponding to the schematic in Fig. 2 hasbeen electrically simulated using SPECTRE. The sw-opamp has been designed using the pdup st approachreported in [4]. Signal Vo1 FREE in Fig. 3(a) has beenobtained by leaving the circuit to start to run free from

0 1 2 3 40

1

2

3

Vo1_ST

�C

2.5

1.5(a)

Vol

ts

0.5

0

1

2

3

2.5

1.5(b)

0.5

0.5 1.5

time (ms)

2.5 3.5

0 1 2 30.5 1.5

vo1 (Volts)

vo2

(Vol

ts)

2.5

Vo1_FREE

Fig. 3. Simulation results. (a) Time domain responses. (b) Dynamicstate diagrams.

an arbitrary point. It can be seen that after 5 ms, thecircuit is still far away from the SS, also confirmedbecause no limit cycle is reached in the correspondingdynamic state diagram of Fig. 3(b). However, when theproposed strategy is applied, signal Vo1 ST in Fig. 3(a)has been obtained. The value of Vinit has been chosento be 1.2 Volts, which corresponds to a point closeto the SS. The pulse in signal �C at the beginningdenotes the activation of the strategy. As expected, theoscillation is stabilized very fast as can also be deducedfrom Fig. 3(b), where the limit cycle is reached almostimmediately.

4. Conclusions

A simple start-up circuitry for opamp-based oscilla-tors based on the use of a modified opamp, calledsw-opamp, has been presented. It not only ensures thatthe oscillator will start to run (safe start-up) but also thesteady-state can be reached very fast (short transient-time). Moreover, the extra circuitry has a negligibleimpact on the normal operation of the circuit. In these

190 Vazquez et al.

terms, its use in the context of the OBT technique isclear. Electrical simulations demonstrate the feasibilityof the approach.

References

1. Arabi, K. and Kaminska, B., “Testing analog and mixed-signalintegrated circuits using oscillation-test method.” Trans. onComputer-Aided Design of Integrated Circuits and Systems 16(7),July 1997.

2. Huertas, G., Vazquez, D., Rueda, A. and Huertas, J. L.,“Effective oscillation-based test for application to a DTMF filterbank.” Proc. IEEE International Test Conference, pp. 549–555,September 1999.

3. Bratt, A. H., Harvey, R. J., Dorey, A. P. and Richardson, A. M. D.,“Design-for-test structure to facilitate test vector application withlow performance loss in non-test mode.” IEE Electronic Letters29(16), pp. 1438–1440, August 1993.

4. Vazquez, D., Huertas, J. L. and Rueda, A., “Reducing the im-pact of DfT on the performance of analog integrated circuits: Im-proved SW-OPAMP design.” Proc. 14th VLSI Test Symposium,pp. 42–47, 1996.