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ACS712 Datasheet(PDF) 11 Page - Allegro MicroSystems |
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ACS712 Datasheet(HTML) 11 Page - Allegro MicroSystems |
11 / 12 page Fully Integrated, Hall Effect-Based Linear Current Sensor with 2.1 kVRMS Voltage Isolation and a Low-Resistance Current Conductor ACS712 11 Allegro MicroSystems, Inc. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (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 R INTFC R F + RINTFC 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, 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 RF Amp Out VCC +5 V Pin 8 Pin 7 VIOUT Pin 6 N.C. Input GND Pin 5 IP+ IP+ 0.1 F 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 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 CF 1 nF CF 1 nF |
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