MICROPOWER,

ULTRA-SENSITIVE

HALL-EFFECT SWITCHES

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

7

FUNCTIONAL DESCRIPTION

Low Average Power. Internal timing circuitry activates the

sensor for 60

µs and deactivates it for the remainder of the

period (240

µs for the A3213xx and 60 ms for the A3214xx).

A short "awake" time allows for stabilization prior to the

sensor sampling and data latching on the falling edge of the

timing pulse. The output during the "sleep" time is latched in

the last sampled state. The supply current is not affected by

the output state.

Chopper-Stabilized Technique. The Hall element can be

considered as a resistor array similar to a Wheatstone bridge.

A large portion of the offset is a result of the mismatching of

these resistors. These devices use a proprietary dynamic offset

cancellation technique, with an internal high-frequency clock

to reduce the residual offset voltage of the Hall element that is

normally caused by device overmolding, temperature depen-

dencies, and thermal stress. The chopper-stabilizing technique

cancels the mismatching of the resistor circuit by changing the

direction of the current flowing through the Hall plate using

CMOS switches and Hall voltage measurement taps, while

maintaing the Hall-voltage signal that is induced by the

external magnetic flux. The signal is then captured by a

sample-and-hold circuit and further processed using low-offset

bipolar circuitry. This technique produces devices that have an

extremely stable quiescent Hall output voltage, are immune to

thermal stress, and have precise recoverability after tempera-

ture cycling. This technique will also slightly degrade the

device output repeatability. A relatively high sampling

frequency is used in order that faster signals can be processed.

More detailed descriptions of the circuit operation can be

found in Technical Paper STP 97-10, Monolithic Magnetic

Hall Sensor Using Dynamic Quadrature Offset Cancellation

and Technical Paper STP 99-1, Chopper-Stabilized Amplifiers

With A Track-and-Hold Signal Demodulator.

Dwg. WH-017-1

0

PERIOD

60

µs - "AWAKE"

"SLEEP"

SAMPLE &

OUTPUT LATCHED

Dwg. EH-012-1

X

+V

+V

HALL

VOLTAGE

B

+

—

Dwg. AH-011-2

Operation. The output of this device switches low (turns on)

when a magnetic field perpendicular to the Hall sensor exceeds

the output is capable of sinking up to 1 mA and the output

output switches high (turns off). The difference in the mag-

device. This built-in hysteresis allows clean switching of the

output even in the presence of external mechanical vibration

and electrical noise.

As used here, negative flux densities are defined as less

than zero (algebraic convention) and -50 G is less than +10 G.