Data Sheet

Fully Integrated, Hall Effect-Based Linear Current Sensor IC
with
2.1 kVRMS Isolation and a Low-Resistance Current Conductor
ACS712
13
Allegro MicroSystems, LLC
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 device. Such a low-
pass filter improves the signal-to-noise ratio, and therefore the
resolution, of the device output signal. However, the addition of
an RC filter to the output of a sensor IC can result in undesirable
device output attenuation — even for DC signals.
Signal attenuation, ∆V
ATT
, is a result of the resistive divider
effect between the resistance of the external filter, R
F
(see
Application 6), and the input impedance and resistance of the
customer interface circuit, R
INTFC
. The transfer function of this
resistive divider is given by:
Even if R
F
and R
INTFC
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,
R
INTFC
, 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, C
F
(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 ∆V
ATT
. 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
AT T
R
INTFC
R
F
+ R
INTFC
V
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, R
F,
and
the resistance of the customer interface circuit, R
INTFC
.
This resistive divider will cause excessive attenuation,
as given by the transfer function for ∆V
ATT
.
Application 7. Using the FILTER pin
provided on the ACS712 eliminates the
attenuation effects of the resistor divider
between R
F
and R
INTFC
, shown in Appli-
cation 6.
Application
Interface
Circuit
Resistive Divider
R
INTFC
Low Pass Filter
R
F
Amp
Out
VCC
+5 V
Pin 8
Pin 7
VIOUT
Pin 6
N.C.
Input
GND
Pin 5
Filter
Dynamic Offset
Cancellation
IP+ IP+
0.1 µF
Pin 1 Pin 2
IP– 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
Dynamic Offset
Cancellation
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
R
INTFC
Allegro ACS712
Allegro ACS706
C
F
1 nF
C
F
1 nF