Chopper-Stabilized Hall Effect Bipolar Switch
A3230
9
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Figure 2. Chopper Stabilization Circuit (Dynamic Quadrature Offset Cancellation) 
Amp
Regulator
Clock/Logic
Hall Element
Chopper Stabilization Technique
When using Hall-effect technology, a limiting factor for 
switchpoint 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 element. This makes it dif cult to process the signal while 
maintaining an accurate, reliable output over the speci ed oper-
ating 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- eld-induced signal in the frequency domain, 
through modulation. The subsequent demodulation acts as a 
modulation process for the offset, causing the magnetic- eld-
induced signal to recover its original spectrum at baseband, 
while the dc offset becomes a high-frequency signal. The 
magnetic- eld-induced signal then can pass through a low-pass 
 lter, while the modulated dc offset is suppressed. This con gu-
ration is illustrated in  gure 2.
The chopper stabilization technique uses a 200 kHz high-fre-
quency clock. For demodulation process, a sample and hold 
technique is used, where the sampling is performed at twice the 
chopper frequency (400 kHz). This high-frequency operation 
allows a greater sampling rate, which results in higher accuracy 
and faster signal-processing capability. This approach desensi-
tizes the chip to the effects of thermal and mechanical 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 ers in combination with high-density logic integration and 
sample-and-hold circuits.
The repeatability of magnetic- 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. Applications that 
are more likely to be sensitive to such degradation are those 
requiring precise sensing of alternating magnetic  elds; for 
example, speed sensing of ring-magnet targets. For such applica-
tions, Allegro recommends its digital device families with lower 
sensitivity to jitter. For more information on those devices, 
contact your Allegro sales representative.