ACS712 datasheet(13/14 Pages) ALLEGRO

13 / 14 page

Fully Integrated, Hall Effect-Based Linear Current Sensor with

2.1 kVRMS Voltage Isolation and a Low-Resistance Current Conductor

ACS712

Allegro MicroSystems, Inc.

115 Northeast Cutoff

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

1.508.853.5000; www.allegromicro.com

Improving Sensing System Accuracy Using the FILTER Pin

In low-frequency sensing applications, it is often advantageous

to add a simple RC filter to the output of the sensor. Such a low-

pass filter improves the signal-to-noise ratio, and therefore the

resolution, of the sensor output signal. However, the addition of

an RC filter to the output of a sensor IC can result in undesirable

sensor output attenuation — even for dc signals.

Signal attenuation, ∆VATT, is a result of the resistive divider

effect between the resistance of the external filter, RF (see

Application 6), and the input impedance and resistance of the

customer interface circuit, RINTFC. The transfer function of this

resistive divider is given by:

Even if RF and RINTFC are designed to match, the two individual

resistance values will most likely drift by different amounts over

temperature. Therefore, signal attenuation will vary as a function

of temperature. Note that, in many cases, the input impedance,

RINTFC, of a typical analog-to-digital converter (ADC) can be as

low as 10 kΩ.

The ACS712 contains an internal resistor, a FILTER pin connec-

tion to the printed circuit board, and an internal buffer amplifier.

With this circuit architecture, users can implement a simple

RC filter via the addition of a capacitor, CF (see Application 7)

from the FILTER pin to ground. The buffer amplifier inside of

the ACS712 (located after the internal resistor and FILTER pin

connection) eliminates the attenuation caused by the resistive

divider effect described in the equation for ∆VATT. Therefore, the

ACS712 device is ideal for use in high-accuracy applications

that cannot afford the signal attenuation associated with the use

of an external RC low-pass filter.

∆V

ATT

INTFC

F + RINTFC

IOUT

⎟

⎠

⎞

⎜⎜

⎝

⎛

Application 6. When a low pass filter is constructed

externally to a standard Hall effect device, a resistive

divider may exist between the filter resistor, RF, and

the resistance of the customer interface circuit, RINTFC.

This resistive divider will cause excessive attenuation,

as given by the transfer function for ∆VATT.

Application 7. Using the FILTER pin

provided on the ACS712 eliminates the

attenuation effects of the resistor divider

between RF and RINTFC, shown in Appli-

cation 6.

Application

Interface

Circuit

Resistive Divider

RINTFC

Low Pass Filter

Amp

Out

VCC

+5 V

Pin 8

Pin 7

VIOUT

Pin 6

N.C.

Input

GND

Pin 5

IP+

0.1 F

Pin 1 Pin 2

IP–

Pin 3 Pin 4

Gain

Temperature

Coefficient

Offset

Voltage

Regulator

Trim Control

To all subcircuits

Input

VCC

Pin 8

Pin 7

VIOUT

GND

Pin 5

FILTER

Pin 6

IP+

Pin 1

IP+

Pin 2

IP–

Pin 3

IP–

Pin 4

Sense

Trim

Signal

Recovery

Sense Temperature

Coefficient Trim

0 Ampere

Offset Adjust

Hall Current

Drive

+5 V

Application

Interface

Circuit

Buffer Amplifier

and Resistor

RINTFC

Allegro ACS712

Allegro ACS706

1 nF

ACS712 Datasheet(PDF) 13 Page - Allegro MicroSystems

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