A CMOS Spinning-Current Hall Effect Sensor
with Integrated Submicrovolt Offset
Instrumentation Amplifier
Anton Bakker and Johan H. Huijsing
Delft University of Technology
Delft Institute of Microelectronics and Submicron technology (DIMES)
Mekelweg 4, 2628 CD Delft, The Netherlands
tel. +31 (0)15-2785747 fax. +31 (0)15-2785755
a.bakker@its.tudelft.nl, http://duteisp.et.tudelft.nl/~anton
Abstract - An eight-contact spinning-current Hall effect
sensor with an integrated chopper amplifier is presented.
This smart Hall effect sensor has a typical offset of 5µT
and noise of 1.2µT (top-top) in a band from 0 to 2 Hz.
Power consumption is only 700µA@5V. A spinning-current cycle is done in 30 ms. It is shown that at this frequency the 1/f noise of the Hall sensor is completely
removed. The measured noise of 1.2µT (top-top) is equivalent to 25nV/sqrt(Hz) and equals the input-referred
noise of the CMOS chopper amplifier. The offset figure is
one order of magnitude better than that of commercially
available 4-contact Hall plates. This work extends the
range of Hall sensors to the µT level, and makes it suitable
in low-cost, high-accuracy applications such as electronic
compasses.
Keywords - Magnetic sensors, Intelligent systems, Smart
sensors, Hall sensors, Spinning-current
I. INTRODUCTION
The main advantage of silicon Hall sensors over other
materials like GaAs or other principles like magneto
resistors or flux gates, is that they are fully compatible
with standard IC technology. They suffer, however,
from large offsets which make them less appropriate in
low-frequency, high-accuracy applications. This offset
has typical values of several mTeslas and can hardly be
calibrated, because it is very sensitive to stress in the
material.
The accuracy of Hall sensors has been improved several
years ago by the introduction of the 4-contact switching
Hall plate[1]. With this technique the offset could be
reduced below 0.5mT. More recent research on spinning-current Hall plates with 8 contacts showed that the
offset can even be reduced to values as low as 2µT.
[2][3]
A commercial version of this 8-contact Hall plate has
however never been implemented. This can be
explained mainly by the difficulties of designing an
appropriate amplifier: since the sensitivity of a silicon
Hall sensor is limited to around 100mV/T, the amplifier
ISBN: 90-73461-18-9
should have an offset less than 200nV to achieve the
2µT magnetic offset level. Also the noise level should
be extremely low.
This paper describes an 8-contact spinning-current Hall
plate with integrated amplifier in CMOS that approximates the theoretical limits of silicon Hall plates regarding offset. It also shows that the 1/f noise of Hall plates
is greatly reduced by the spinning-current principle.
This work extends the range of Hall sensors to the µT
level, and makes it suitable in low-cost, high-accuracy
applications such as electronic compasses.
II. DYNAMIC OFFSET-CANCELLATION
As already mentioned above, the quality of the total system is almost completely depending on the offset and
noise of the integrated amplifier. Although bipolar
amplifiers have better offset and noise performance than
CMOS, a BiCMOS implementation would be much
more expensive. Also the input bias currents of lownoise bipolar amplifiers give problems, because of the
Hall sensor’s impedance of a few kΩ. We decided to
use a standard CMOS technology and reduce the offset
and noise with dynamic offset cancellation techniques.
There are roughly two types of dynamic offset-cancellation techniques: autozeroing and chopping [4]. The
autozero principle first samples the offset and extracts it
in a second phase. By proper design with low-sensitivity
inputs, low-injection switches and large capacitors, the
offset can be reduced to values below 1µV. The 1/f
noise is also reduced. However, since autozeroing needs
a sampling of the offset, the residual noise at low frequencies is significantly higher than the thermal noise
floor. The theory of this in-band noise sampling is very
well described in [4].
Chopping is based on modulation instead of sampling.
In this principle, the input signal is modulated to a certain frequency, amplified and demodulated again. The
principle is shown in Fig. 1. The major advantage of
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A. Bakker and J.H. Huijsing
Fig. 1: Chopper technique
chopping over autozeroing is that the residual noise at
low frequencies is almost equal to the thermal noise
floor. This makes the chopping principle fundamentally
the best choice for our system, where minimal noise is
one of the major requirements.
III. LOW-OFFSET CHOPPER AMPLIFIER
The residual offset of the chopper amplifier, as shown in
Fig. 1, i is determined by the chopping frequency and
the spikes generated by the input chopper [5]. The chopping frequency can however not be reduced too much,
because, for minimal residual noise, it should be higher
than the 1/f noise corner frequency, which is the frequency at which the 1/f noise equals the thermal noise.
The spikes produced by the input chopper can be minimized using a differential approach, i.e. an instrumentation amplifier.
A method to further reduce the residual offset is the use
of a bandpass amplifier as described by Menolfi et
al.[5]. They achieved submicrovolt level, but at the cost
of complex electronics. We reduced the offset by merging the amplifier in the spinning-current principle. This
does not increase the complexity, but still makes it possible to achieve offsets below 1µV.
IV. SPINNING-CURRENT
The spinning-current technique has been elaborately
described by Munter [2] and Bellekom [3]. The principle for an eight-contact Hall plate is shown in Fig. 2.The
bias current is periodically rotated, while the voltage at
the also rotating sense contacts is measured. The offset
reduction is achieved by the fact that the Hall voltage
Fig. 2: The eight different bias current and sense
connections of a spinning-current Hall plate
does not depend on the current direction, but the offset
voltage does. By averaging the output voltages of the
spinning-current Hall plate, the offsets are cancelled,
but the desired Hall voltage stays intact.
The main advantage over 4-contact switching Hall
plates is that with this technique also errors in nonorthogonal directions are cancelled, resulting in a better
offset compensation.
V. IMPLEMENTATION
Fig. 3 shows the simplified schematic of the 8-contact
spinning-current Hall plate. The total system can be
divided in a spinning-current Hall plate, the necessary
switches and control, the ultra-low-offset instrumentation amplifier and clocking circuitry.
The Hall plate is made of a standard N-well-implant in a
p-substrate, which is the material with the highest Hall
sensitivity in a standard CMOS process. Our Hall plate
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A CMOS Spinning-Current Hall Effect Sensor
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chop2a
Ibias
chop1a
chopout
Switches
Hall
&
plate
R2
Control
32 kHz
:2
16 kHz
:64
256 Hz
:8
Vout
R1b
chop1b
chop
ck
R1a
chop2b
32 Hz
Fig. 3: Schematic of the 8-contact spinning-current Hall plate with chopper amplifier
has a sensitivity of approx. 70mV/T for a bias voltage of
4 Volts and a bias current of 500µA. The diameter is
400µm, which is a trade-off between chip area and initial offset.
The instrumentation amplifier consists of two equal
amplifiers. Each amplifier consumes around 100µA, of
which 35µA is flowing through the input stage. The
thermal noise of each amplifier is 16nV/√Hz and the 1/f
noise corner frequency is 1.5 kHz. The chopping frequency is chosen nominally at 16 kHz, which is quite
high, but makes it possible to investigate the residual
offsets for different frequencies, without increasing the
noise. For the final version, this chopping frequency
will be reduced to approximately 2kHz. With the chopping technique, the 1/f noise is completely removed and
the residual noise of the complete instrumentation
amplifier will be around 23nV/√Hz. The amplification
of the instrumentation amplifier is determined by resistors R1a, R1b and R2. With values of 99k, 99k and 2k
resp., the amplification is exactly 100, resulting in an
output sensitivity of our Hall plate of 7V/T.
Fig. 4: Chip micrograph
VI. EXPERIMENTAL RESULTS
The circuit has been implemented in a 1.6µm singlepoly, double metal CMOS process, which is the inhouse test process of the DIMES Technology Center. A
chip micrograph is shown in Fig. 4. The chip measures
6mm2. A commercial version is now being designed in
a regular 0.7µm, 5Volt process and measures only
1.5mm2.
The Hall plate is switched through its 8 states by the use
of 32 switches. A part of these switches is moved to the
output of the instrumentation amplifier to reduce the
effects of spikes of the choppers. It will be shown in the
measurements that this feature results in a residual
amplifier offset below 1µV.
In the left upper corner, you can see the Hall plate. The
lower part shows the instrumentation amplifier.
One period for the spinning-current action takes around
30ms, which limits the sampling frequency of this Hall
plate to 32 Hz. The magnetic input signal of the sensor
should therefore be limited to 16Hz, to avoid aliasing.
The choice for this 32 Hz is a trade-off between offset
and bandwidth. Higher frequencies will result in worse
offset performance, but will not affect functionality.
Eight samples from two different batches have been
tested. No significant differences between the two
batches have been found. Fig. 5 shows the offset of the
Hall plates versus the spinning-current frequency. All
samples show offsets below 5µT for the nominal spinning-current frequency of 32Hz. At higher frequencies,
these offsets increase, but stay below 10µT for frequen-
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A. Bakker and J.H. Huijsing
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20
chip 2a
chip 3a
10
Offset (µT)
Table 1: Measurement results of the spinning-current
Hall plate with integrated amplifier
chip 7a
0
chip 9a
-10
chip 1b
Typ.
Unit
Supply voltage
5.0
V
Supply current
700
µA
Sensitivity
7
V/T
5
10
µT
µT
Noise (0.1-2Hz, top-top)
1.2
µT
Bandwidth
32
samples/sec
chip 2b
-20
chip 4b
-30
chip 5b
-40
30
40
50
60
70
80
90
100
Spinning-current frequency (Hz)
Fig. 5: Offset versus spinning-current frequency
for 8 samples from two different batches
(23oC)
Offset
0-50oC
Fig. 6 also shows large frequency components at 32Hz
and 96Hz. These signals are offset and noise components of the Hall plate that are modulated by the spinning-current principle. Especially the 32Hz component
can be 1000 times larger than the wanted low-frequency
Hall sensor signal. It needs therefore attention that this
component is carefully filtered out.
An overview of the results is shown in table 1.
VII. CONCLUSIONS
Fig. 6: Output noise spectrum of spinning-current Hall plate with external 100-times amplifier
and 3-Hz filter
cies below 60Hz.
Offset has also been tested in a temperature range from
0 to 50oC. Results show that the offset is slightly
increasing, but remains below 10µT for the nominal frequency.
From the results of Fig. 5 can also be derived that with a
Hall plate sensitivity of 70mV/T, the offset of the instrumentation amplifier is below 350nV! This extremely
low value has not been reported before. Menolfi et al.
reported a best value of 500nV in [5].
For noise testing the circuit has been extended with an
external low-noise 100-times amplifier and a first-order
3 Hz low-pass filter. The noise output spectrum for a
band between 1 and 100Hz is shown in Fig. 6. From this
picture it can be derived that the input referred noise at a
frequency of 1Hz is 27nV/√Hz or 0.3µT/√Hz. This
noise figure is very close to the calculated noise of the
amplifier and shows that the noise of the spinning-current Hall plate itself is almost negligible. It proves also
that the 1/f noise of the Hall plate is cancelled by the
spinning-current principle.
For the first time a spinning-current Hall plate with integrated amplifier has been presented that achieves a 5µT
offset level. In a bandwidth of 0.1-2Hz, noise is only
1.2µT top-top, without 1/f noise.
The extreme low-offset level of this Hall sensor is at
least one order of magnitude better than of existing 4contact switching Hall sensors. This work shows that
the application range of Hall-effect sensors can now be
extended to the µT level, which makes it for example
possible to make low-cost compasses based on spinning-current Hall sensors.
REFERENCES
[1] ITT semiconductors, “HAL 400 Linear Hall effect Sensor IC”,
http://www.intermetall.de, August 1995
[2] Munter, P.J.A., A low-offset spinning-current Hall plate, Sensors and Actuators, A21-A23 (1990), pp. 743-746
[3] Bellekom, A.A., “Origins of offset in conventional and spinning-current Hall plates”, Ph.D. Thesis, October 1998
[4] Enz, C.C. and G.C. Temes, “Circuit techniques for reducing the
effects of opamp imperfections: autozeroing, correlated double
sampling and chopper stabilization”, Proc. of the IEEE, vol.84,
no.11, pp. 1584-1614, november 1996
[5] Menolfi, C. and Q. Huang, “A fully integrated CMOS instrumentation amplifier with submicrovolt offset”, IEEE Journal of
Solid-State Circuits, vol.34, pp.415-420, March 1999
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