Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
these preliminary results must be improved but obtained
results are close to the simulated one.
TABLE I: Comparison chart between the
Wheatstone bridge and the Active Bridge performances
Active bridge with
Σ∆ modulator
Wheatstone
bridge
Consumption 1 (µA) 2 5.3
v n (nV/Hz) 101.5
Sensitivity (mV/°C) 2.14
Resolution (°C/Hz) 2.59m 47.42µ
Non linearity 6.9° 7.5°
1
Consumption of the sensor only.
Fig. 9 Linearity study of the two circuits from -40 to 100°C
To confirm the relationship between linearity and the
resistance dependence to temperature, we have studied the
linearity of the resistance of both materials used to implement
resistance temperature sensors (Fig. 10). It confirms that the
non linearity is basically due to the thermal quadratic terms of
both materials (polyh and poly2).
°
Fig. 10 Intrinsic linearity with temperature of resistors.
Finally, Table I summarizes performances obtained at 27°C,
for both studied architectures: Wheatstone bridge and Active
Bridge in a Σ∆ modulator. Due to its one-bit digital output, it
is impossible to calculate output noise or sensitivity for the Σ∆
modulator. These notions are only meaningful after
decimation filtering. Noise that has been determined
previously for the Σ∆ modulator output (Fig. 7.) corresponds
to a quantization noise that allows determining a resolution
proportional to
[6], where f c is the cut-off frequency of
the low-pass filter implementing the integrator and f ck is the
clock frequency of the modulator. It is then possible to freely
adjust these parameters to reach the targeted resolution down
to the limit fixed by the intrinsic noise of the Active Bridge
(Eq. 3).
The main advantage of the Active Bridge is its ability to
reduce the power consumption while providing a digital output
when used in a Σ∆ modulator.
V. CONCLUSION
This paper presents a temperature sensor with an innovative
structure for the signal conditioning of resistance. A
comparison between the traditional Wheatstone bridge and the
Active Bridge has been made that demonstrates the main
advantages of the proposed solution are its very low power
consumption and its capacity to provide directly a one-bit
digital output. In particular, the high output resistance of the
Active Bridge allows implementing a modulator with very
few additional parts.
ACKNOWLEDGMENT
This work was supported by ANR, the French National
Research Agency, under the project MIDISPPI.
REFERENCES
[1] F. Mailly, N. Dumas, N. Pous, L. Latorre, O. Garel, E. Martincic,
F. Verjus, C. Pellet, E. Dufour-Gergam, P. Nouet. Original
Research Article Sensors and Actuators A: Physical, Volume 156,
Issue 1, November 2009, Pages 201-207
[2] Boujamaa E. M., Dumas N., Mailly F., Latorre L., Nouet P. «The
Active Bridge: an Alternative to the Wheatstone Bridge for
Efficient Conditioning of Resistive MEMS Sensors » Design, Test,
Integration of MEMS/MOEMS (DTIP’09), Avril 2009.
[3] Boujamaa E. M., Alandry B., Hassine S., Mailly F., Latorre L.,
Nouet P. « A Low Power Interface Circuit for Resistive Sensors
with Digital Offset Compensation » IEEE International
Symposium on Circuits and Systems (ISCAS’ 10), Juin 2010.
[4] Luc, Hébrard, Jean-Batiste, Kammerer and Francis, Braun. « A
chopper Stabilized Biasing Circuit Suitable for Cascaded
Wheatstone-Bridge-Like Sensors ». IEEE Transaction on Circuits
and Systems. August 8, 2005, Vol. 52, 8.
[5] Apinunt, Thanachayanont and Suttisak, Sangtong. « Low-Voltage
Current-sensing CMOS Interface Circuit for Piezo-Resistive
Pressure Sensor ». ETRI Journal. February 2007, Vol. 29, 1.
[6] O. Leman, F. Mailly, L. Latorre, P. nouet, «A wide bandwith, wide
dynamic-range thermal Σ∆ architecture for convective
accelerometers», 8 th IEEE Conference on Sensors (SENSORS’09),
October 2009.
323