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1410 IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 23, NO. 6, DECEMBER 1988 TABLE I supply voltage supply current temperature range accuracy error due to supply voltage changes reference frequency frequency ratio p 2.5 - 3.1 V 200 IIA 32°C to 44°G ? O.l °C, see text o.04°c [for the voltage range 2. 5v-3. IV ] O.ol”c [for the voltage range 2. 6v-3.3V] 30 kHz 12(R4/R2) (T - Tz)/T z Analysis showed that the maximum error is within ~ O.l”C, provided that perfect trimming is performed at a wafer tempera- ture of about 38”C. The error is mainly caused by internal power dissipation, spreading in the base–emitter voltage and finite current gain of the n-p-n transistors. The main measurement results are listed in Table I. The factor of 12 in the frequency ratio p represents the current-mirror transfer ratio of Q16 and Q17. The greatest sensitivity of the frequency ratio to drift in the supply voltage is found at its lowest value (2.5 V). The changes in 1~ and I,,f partially compensate for each other. For changes in 1~ and I,,f separately we measured changes of about 1 percent in the nominal voltage range. As mentioned in Section II, with a single-point calibration the required accuracy can only be achieved when a process with low spreading in V~E values is used. In other cases a two-point calibration is necessary unless the circuit is modified. A possible improvement is introduced in [4]: for both the currents 1~ and I.ef the same base–emitter voltage is used. By trimming the emitter area both signals 1~ and I,ef are adjusted to the right value. Because spreading in the PTAT voltage is much less than that in VBE, no special precautions are required. VI. CONCLUSIONS The temperature information of the transducer described in this paper is converted into a frequency ratio. In this way the need for absolute accuracy of the signal transfer is eliminated and inaccurate passive on-chip components can be applied. The temperature transducer is a four-terminal device with complete on-chip microcomputer interfacing. High accuracy and very high resolution is obtained by using fundamental properties of bipolar transistors. Calibration is sim- ple and can be performed in a single step. The transducer contains a low-voltage ultra-linear I–f con- verter and two new accurate low-voltage current mirrors. With these circuits accurate signal transfer at a supply voltage of only 2.5 V is achieved. [1] [2] REFERENCES D. Ch. van Maaren, J. KliJn, aud G. C. M. MeiJer, “An integrated micmpower low-voltage temperature-controlled oscillator,” IEEE J. Solid-State Czrcrats, vol. SC-17, pp. 1197-1201, Dec. 1982. M. J. S. %mth, M. A. Prisbe, and J. D. Meindl, “A micropower IC for a blomedicnl pressure transducer, “ in Tech. Dig., IEEE Iirt. Con/. Solld- Stute Sensors and Actuators (Transducers ’85) (Phdadelphia, PA), June 1985, pp. 42–45.
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