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over the temperature range (i.e. the full-scale). In our case, it is
equal to about 10%.
11-13
May 2011, Aix-en-Provence, France
noise floor is evaluated over the 1-30 Hz range and is equal to
2.59m°C/Hz.
Vdd
S 0
S 1
R fb
(4)
T3
T4
T2 C
R fb
S 1
S 0
IV.
T1
Out+
Fig. 5 Implementation of the temperature sensor based
on a Σ∆ modulator topology.
EXPERIMENTAL RESULTS
Comparator
D Q
CK
Fig. 6 presents both simulated and experimental output of
the modulator as a function of temperature.
The simulated output represents the ratio of ‘ones’ in the
bit-stream, within 1024 clock periods for each temperature,
extracted from a transient analysis. Experimentally, the same
ratio is calculated over a 10s time window.
°
Fig. 6 Normalized digital output versus temperature
In order to determine the resolution of the sensor, a spectral
analysis of the output bit-steam is made at 16 kHz clock
frequency (Fig. 7). The classical noise shaping of a 1 st
order Σ∆ modulator is well observed with an increase of the
noise at high frequencies. From this spectrum, the average
Fig. 7 Bit-stream spectral density (F clk=16 kHz)
Now, let us focus on the linearity performance obtained
with the Active Bridge in a Σ∆ modulator configuration, and
compare it to the one of the Wheatstone bridge (Fig. 8). The
Wheatstone bridge remains the most common approach to
condition resistive sensors [4]. This architecture introduces a
major tradeoff between resolution and power consumption [5].
On the one hand, the smaller the sensor resistor is, the higher
the current in the bridge will be. On the other hand, the higher
the sensor resistor is, the higher the noise floor will be. The
performances are compared with identical resistors and thus
similar silicon surface (MOS transistor surfaces are
negligible). Note that in our case, temperature sensor is highly
sensitive; therefore the SNR is not really an issue.
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Fig. 8 Wheatstone bridge topology with Rpolyh and Rpoly2
Fig. 9 presents the simulated non linearity for both
architectures. These results are compared to the non linearity
experimentally observed for the Σ∆ modulator. It shows that
the calculated nonlinearities (as a percentage of the full-scale
temperature range, i.e. 140°C) are identical for both
architectures. It can be deduced that this non linearity comes
from the sensing elements themselves and not from the
architectures. Besides, the maximum is reached at both
extremities of the temperature range and the shape of the nonlinearity
reasonably allows suspecting a quadratic effect of the
temperature on the resistance. Regarding experimental non
linearity, we can conclude that the experimental set-up of
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