ACS712
Fully Integrated, Hall Effect-Based Linear Current Sensor IC
with 2.1 kVRMS Isolation and a Low-Resistance Current Conductor
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, ∆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:
∆VATT
=
VIOUT ⎜⎜⎝⎛RFR+INRTIFNCTFC
⎞
⎟⎠
.
Even if RF and RINTFC are designed to match, the two individual
resistance values will most likely drift by different amounts over
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.
+5 V
Pin 3 Pin 4
IP– IP–
0.1 MF
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.
VCC
Pin 8
Allegro ACS706
Voltage
Regulator
To all subcircuits
Amp
Out
Gain
Temperature
Coefficient
Trim Control
Offset
VIOUT
Pin 7
N.C. RF
Pin 6
Resistive Divider
Input
Application
Interface
Circuit
Low Pass Filter
CF
1 nF
RINTFC
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.
+5 V
IP+ IP+
Pin 1 Pin 2
VCC
Pin 8
GND
Pin 5
IP+
Pin 1
IP+
Pin 2
Hall Current
Drive
IP–
Pin 3
IP–
Pin 4
Sense Temperature
Coefficient Trim
Sense
Trim
Signal
Recovery
0 Ampere
Offset Adjust
GND
Pin 5
Allegro ACS712
Buffer Amplifier
and Resistor
FILTER
Pin 6
CF
1 nF
VIOUT
Pin 7
Input
Application
Interface
Circuit
RINTFC
Allegro MicroSystems, Inc.
13
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com