A1425(2005) 查看數據表（PDF） - Allegro MicroSystems

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A1425
(Rev.:2005)

High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal

Allegro MicroSystems 

A1425

High Accuracy Analog Speed Sensor with Integrated Filter Capacitor and Dual Zero-Crossing Output Signal

Functional Description

The A1425 is a versatile high-precision differential sensing

device that can be used in a wide range of applications. Proper

choice of the target material and shape, and assembly techniques

enables large working air gaps and high switchpoint accuracy

over the device operating temperature range.

Sensor Operation

The A1425 sensor IC contains two integrated Hall transducers

that are used to differentially sense a magnetic field across the

surface of the IC. Referring to figure 1, the trigger switches the

output off (output high) when the differential magnetic field

crosses zero while increasing in strength (referred to as the posi-

tive direction), and switches the output on (output low) when

the differential magnetic field crosses zero while decreasing (the

negative direction).

The operation is achieved through the use of two separate

comparators. Both comparators use the same reference point, 0

G, to provide high accuracy, but one comparator has a positive

hysteresis, BHYS1, and the other a negative hysteresis, BHYS2.

Therefore, one comparator switches (BOP) at the zero crossing on

an increasing differential signal and the other switches (BRP) at

the zero crossing on a decreasing differential signal. The hyster-

esis on each comparator precludes false switching on noise or

target jitter.

Start-up

During power-on time, tPO, the output signal, VOUT, is high.

Beyond this time, if the applied magnetic field, Bdiff, is absent or

less than 50 G peak-to-peak, the switching state and VOUT polar-

ity are indeterminate. VOUT will be valid for Bdiff > 50 Gp-p,

after the additional settling time, tSettle, has also elapsed.

Also during tPO, a circuit in the A1425 is briefly enabled that

charges the on-chip capacitor. This feature reduces tPO, relative

to the long RC time constant of a high-pass filter.

Delay

The on-chip band-pass filter induces delay in the output signal,

VOUT, relative to the applied magnetic field, Bdiff. Simulation

data shown in the Characteristic Data section quantify the effect

of the input signal amplitude on the phase shift of the output.

AC-Coupled Operation

Steady-state magnet and system offsets are eliminated using an

on-chip differential band-pass filter. The low and high frequency

poles of this band-pass filter are set using internal integrated

capacitors and resistors. The differential structure of this filter

improves the ability of the IC to reject single-ended noise on

the ground (GND pin) or supply line (VCC pin) and, as a result,

makes it more resistant to electromagnetic interference typically

seen in hostile remote-sensing environments.

11.0

Applied Magnetic

Field, Bdiff 0.0

–11.0

Comparator 1

Comparator 2

BOP(typ) BHYS1

BRP(typ)

A BHYS2

BOP(max) / BRP(max)

BOP(min) / BRP(min)

Switching State

Off

On

Off

Output Signal, VOUT

t+

Figure 1. Typical output characteristics with dual comparator operation. Characteristics shown without delay, see characteristic

data charts for delay and phase shift contributions.

A1425-DS

Allegro MicroSystems, Inc.

10

115 Northeast Cutoff, Box 15036

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

www.allegromicro.com

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