Two-Wire, True Zero-Speed, High Accuracy

Sensor IC with Speed and Direction Output

A1698

8

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

FUNCTIONAL DESCRIPTION

The sensor IC contains a single-chip Hall-effect circuit that

supports a trio of Hall elements. These elements are used in

differential pairs to provide electrical signals containing informa-

tion regarding edge position and direction of target rotation. The

A1698 is intended for use with ring magnet targets, or, when

back-biased with an appropriate magnet, with ferromagnetic

targets (gears). The IC detects the peaks of the magnetic signals

and sets dynamic thresholds based on these detected signals.

Data Protocol Description

When a target passes in front of the device (opposite the branded

face of the package case), the A1698 generates an output pulse

for each magnetic pole, or each tooth and valley, of the target.

Speed information is provided by the output pulse rate, while

direction of target rotation is provided by the duration of the

output pulses. The sensor IC can sense target movement in both

the forward and reverse directions. The translation of magnetic

input to the output is shown in Figure 6.

FORWARD ROTATION

For the –F variant, when the target is rotating such that a target

feature passes from pin 1 to pin 2, this is referred to as forward

rotation. This direction of rotation is indicated on the output by

indicated for target rotation from pin 2 to 1 (see Figure 4).

REVERSE ROTATION

For the –F variant, when the target is rotating such that a target

feature passes from pin 2 to pin 1, this is referred to as reverse

rotation. This direction of rotation is indicated on the output by

indicated for target rotation from pin 1 to 2.

The IC is always capable of properly detecting input signals up to

the defined operating frequency. However, the end user will note

output edge, but, at high frequencies, potentially truncating the

Figure 4: Target Orientation Relative to Device

(ring magnet shown).

N

N

N

S

S

S

N

N

N

S

S

S

Rotation from pin 1 to pin 2

Pin 1

Pin 2

Branded Face

of Package

Rotatin

(Ring magnet or

ferromagnetic)

(Ring magnet or

ferromagnetic)

g Target

Rotation from pin 2 to pin 1

Pin 1

Pin 2

Branded Face

of Package

Rotating Target

Figure 5: Output Timing Example

S

N

t

w(FWD)

t

w(FWD)

t

w(Pre)

t

w(Pre)

I

CC(LOW)

I

CC(HIGH)