A1140/41/42/43
Sensitive Two-Wire Chopper-Stabilized Unipolar Hall Effect Switches
Chopper Stabilization Technique
When using Hall-effect technology, a limiting factor for switch-
point accuracy is the small signal voltage developed across the
Hall element. This voltage is disproportionally small relative to
the offset that can be produced at the output of the Hall sensor.
This makes it difficult to process the signal while maintaining an
accurate, reliable output over the specified operating temperature
and voltage ranges.
Chopper stabilization is a unique approach used to minimize
Hall offset on the chip. The patented Allegro technique, namely
dynamic quadrature offset cancellation, removes key sources
of the output drift induced by thermal and mechanical stresses.
This offset reduction technique is based on a signal modulation-
demodulation process. The undesired offset signal is separated
from the magnetic
fi
eld-induced signal in the frequency domain,
through modulation. The subsequent demodulation acts as a
modulation process for the offset, causing the magnetic
fi
eld-
induced signal to recover its original spectrum at baseband, while
the dc offset becomes a high-frequency signal. The magnetic-
sourced signal then can pass through a low-pass
fi
lter, while the
modulated dc offset is suppressed.
The chopper stabilization technique uses a 200 kHz high fre-
quency clock. The chopping occurs on each clock edge, result-
ing in a 400 kHz chop frequency. This high-frequency opera-
tion allows a greater sampling rate, which results in higher
accuracy and faster signal-processing capability. This approach
desensitizes the chip to the effects of thermal and mechani-
cal stresses, and produces devices that have extremely stable
quiescent Hall output voltages and precise recoverability after
temperature cycling.
This technique is made possible through the use of a BiCMOS
process, which allows the use of low-offset, low-noise ampli-
fi
ers in combination with high-density logic integration and
sample-and-hold circuits. This process is illustrated in
fi
gure 2.
The repeatability of magnetic
fi
eld-induced switching is
affected slightly by a chopper technique. However, the Allegro
high-frequency chopping approach minimizes the affect of
jitter and makes it imperceptible in most applications. Applica-
tions that are more likely to be sensitive to such degradation
are those requiring precise sensing of alternating magnetic
fi
elds; for example, speed sensing of ring-magnet targets. For
such applications, Allegro recommends its digital sensor fami-
lies with lower sensitivity to jitter. For more information on
those devices, contact your Allegro sales representative.
Regulator
Clock/Logic
Hall Element
Amp
Low-Pass
Filter
Figure 2. Chopper stabilization circuit (dynamic quadrature offset cancellation)
Sample and
Hold
A1140-DS
Allegro MicroSystems, Inc.
115 Northeast Cutoff, Box 15036
Worcester, Massachusetts 01615-0036 (508) 853-5000
www.allegromicro.com
7
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
