A CMOS Readout Circuit for Resistive Transducers Based on Algorithmic Resistance and Power Measurement
Summary (3 min read)
Introduction
- Most integrated readout circuits for resistive transducers only measure resistance, without measuring or stabilizing power dissipation [1, 2, 5, 6].
- As a result, the previously-reported constant power circuits, based on translinear loops or other feedback loops, still rely on the accuracy of external voltage and current (or resistance) references.
- Using precision circuit design techniques as well as appropriate calibration and correction schemes, bandgap references can achieve high accuracy over a wide temperature range with low chip-to-chip variations [12-15].
A. Algorithmic resistance and power measurement
- Measuring resistance and/or power involves both voltage and current measurements.
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- These results depend on the ADC’s reference voltage Vref and on accurate knowledge of the value of Rref.
- As detailed below, to eliminate the dependence on Vref, the authors algorithmically construct an accurate bandgap voltage reference, by digitizing several base-emitter voltages.
C. Algorithmic temperature measurement
- Expressions (3) and (11) still depend on Rref, which will generally be subject to process tolerances and temperature drift: Copyright (c) 2017 IEEE.
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- To compensate for the resistor’s temperature drift, information about the die temperature T is needed.
- Fortunately, this can readily be obtained from the PTAT voltage given by (6): (13) where the relation (9) between Vref and Vbg is again used to obtain an expression independent of Vref.
- The result only depends on the current ratio p, the bandgap scale factors a1,2, the bandgap voltage Vbg and physical constants k and q.
D. Compensation for BJT non-idealities
- As mentioned, expression (5) for the base-emitter voltage ignores various non-idealities of the BJT [15].
- Second, the transistor’s finite current gain causes the collector current to deviate from the bias current, which is applied to the transistor’s emitter.
- Leakage current and series resistance lead to errors in the bandgap reference and the temperature measurement that cannot be corrected based on a single-temperature calibration [14].
- The conventional approach to dealing with this is to choose the current level and transistor size such that these errors are sufficiently small.
- The authors algorithmic approach offers the unique possibility to correct for leakage and series resistance by combining more than two base-emitter voltages digitally.
III. CIRCUIT IMPLEMENTATION
- The block diagram of the readout circuit is shown in Fig.
- For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
- Since the algorithmic readout relies on the accuracy of the ADC, the non-idealities of the ADC must be taken into account when selecting or designing the ADC.
- Similarly, CMRR also plays an important role due to the different common-mode voltage levels to be measured.
A. Circuit Implementation of the Transducer Front-End
- The transducer front-end circuit for resistance and power measurements is shown in Fig.
- The voltage-to-current converter includes a chopped operational transconductance amplifier (OTA) in a feedback loop.
- The voltage across the transducer is thus stabilized to Vbias.
- The added cascode transistor M0b decreases the drain-source voltage of the main transistor M0a, effectively reducing this leakage current.
- For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org.
B. Circuit Implementation of the BJT Front-End
- The BJT front-end circuit shown in Fig. 8 generates the base-emitter voltages needed for the construction of the voltage reference and temperature sensor.
- The OTA used in the BJT front-end is the same as the one used in the transducer front-end (as shown in Fig. 7).
- The amplifier needs to have low offset and high open-loop gain to minimize errors in the bias current [25].
- This is achieved by applying dynamic element matching (DEM) in the current mirror in Fig.
- Thus, the mismatch of the current sources is modulated by the DEM clock, and the resulting average current is close to p times the average unit current.
IV. EXPERIMENTAL RESULTS AND DISCUSSION
- A chip photograph and a plot of the chip layout with the main circuit blocks are shown in Fig. 10.
- The chip (DUT), mounted on a PCB (PCB1), is placed inside a climate chamber (Vötch VTM 7004) to perform measurements at temperatures ranging from -40°C to 125°C.
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- This is improved to about 24 ppm/°C after compensation for series resistance and leakage using the method described in Section II-D. After compensation for the systematic quadratic curvature, the temperature coefficient is further improved to 18 ppm/°C (Fig. 13(b)).
B. Temperature Measurement
- Fig. 14 shows the error in the temperature measured using the method described in Section II-C, relative to the reference temperature sensor, for 5 samples of the chip.
- By applying the algorithmic approach described by (13) and (14), a leakage-free PTAT voltage Vbe,ideal can be obtained, which can then be converted to temperature by linear scaling.
- These errors are relatively large compared to the state of the art [25], which can be attributed to the large initial errors due to the leakage currents in the multiplexer switches, which can be reduced in a re-design by reducing the transistor sizes.
- Nevertheless, the accuracy currently obtained is sufficient to Copyright (c) 2017 IEEE.
C. Resistance Measurement
- As described in II-A, the resistance of the transducer is measured relative to an on-chip reference resistor Rref in series with the transducer (Fig. 6).
- The error in the measurement of the precision resistor then reduces significantly, as shown in Fig. 15(b).
- Second, the temperature coefficient Rref is determined by a two-point batch correction (at 27C and 100C) in the calibration result of Fig. 15(a).
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D. Power Measurement
- The power consumption of the transducer was measured using the method described in Section II, eq. (11).
- As described above, the accuracy of the on-chip reference resistance can be further improved by a room-temperature individual trim and a two-point batch calibration (27°C and 100°C) to find Rref0 and Rref.
- This reduces the errors in the power dissipation measurement to ±0.8% as shown in Fig. 17(c).
V. CONCLUSIONS
- The authors have reported a readout architecture for resistive transducers, which is capable of accurately measuring their resistance and power dissipation.
- The key idea behind the readout architecture is to avoid analog signal processing as much as possible, by first digitizing the analog signals and then combining the results in the digital domain.
- This algorithmic approach greatly improves the flexibility of the signal processing and facilitates the removal of errors such as leakage current, series resistance, and systematic nonlinearity in the digital domain.
- In addition, the accuracy of the analog reference voltage of the ADC in this system does not impact the measurement accuracy, as this reference voltage is replaced by the constructed bandgap reference voltage in further data processing.
- Experimental results have shown that the resistance and power dissipation of a Pt100 resistor can be measured with an inaccuracy of ±0.55 Ω and less than ±0.8% respectively over the military temperature range of -40°C and 125°C, showing the effectiveness of the applied techniques.
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