Current Sensor FAQ

The Hall Effect is the presence of a voltage when an external, perpendicular magnetic field is applied to a current carrying conductor. The conductor, or the Hall Element, is biased with a constant current. As magnetic field changes, a change in voltage across the hall element occurs. This voltage can then be amplified and conditioned to provide an output that is related to the magnetic field. Using this principle, magnetic field can be concentrated perpendicular to the hall element using integrated packaging, ferromagnetic cores, or coreless busbars. Hall effect current sensors have the advantage of inherent isolation, low power loss, and stability across temperature while providing an analog output voltage that can be monitored by a microcontroller.   

Parts can be ratiometric or non-ratiometric. Ratiometric indicates that the device sensitivity is proportional to the device supply voltage, VCC. In addition, the device output at 0 A, also called V_IOUT(Q), is nominally equal to vcc / 2. Non-ratiometric devices will have VIOUT(Q) and sensitivity values stable over V_CC variations within the specified input voltage range. Ratiometry can be useful when the input voltage of the sensor is on the same line as the ADC reference voltage. Non-ratiometric parts are useful in applications where the sensor input voltage is noisy or unstable. Unstable V_CC will produce a noisy output if the part is ratiometric.

The three main types of error in current sensors are defined as:

Sensitivity Error: E_SENS = (((Measured Sensitivity) / Sens) -1) × 100(%)

Offset Error: V_OE  = Measured QVO – QVO

Total Output Error: E_TOT = ((V_OUT  – V_{OUT IDEAL} )) / (Sens_ideal× I_P ))× 100(%)

This application note goes more in depth on sources and definitions of error.

On the Allegro Current Sensor device homepage, navigate to the “Part Number Specifications and Availability”. Select “View Data” on the desired gain option. The MSL rating is included in the “Materials Declaration Report”. 

While MSL rating are specifically for surface mount parts, Allegro does qualify and provide a through hole equivalent (THD) for non-surface mount parts that directly correlates to the standard MSL ratings. 

One of the key benefits of Hall Effect current sensors is their inherent galvanic isolation. Because there is no electrical connection between the primary current path and the signal circuitry, much higher working voltages are available. There are several isolation parameters or tests included in the current sensor device datasheets such as:

Dielectric Surge Strength- The amount of voltage that can be handled of a pulse of known rise time, width, and amplitude.

Dielectric Strength- The amount of voltage and time that can be withstood before electrical breakdown occurs. This is tested for a set amount of time (usually 60 seconds) while measuring leakage current to ensure breakdown has not occurred.

Working Voltage- The maximum voltage that can be continuously applied to the device. It usually has a specified value for DC, peak-to-peak, and RMS voltages.

Isolation characteristics are specific to the package of the device. Below is a table summarizing the various packaging types offered by Allegro and their isolation characteristics as well as other important package information.

 

Package Descriptor	SOICW-16 LA	SOICW-16 MA	SOICW-16 MC	SOIC-8 LC1	SOIC-8 LC2	QFN-12 EXB	7-pin PSOF LR	5-pin CB
Picture
Dimension	10.3x10.3mm	10.3x10.3mm	11.3x13mm	4.9x6mm	4.9x6mm	3x3mm	6.4x6.4mm	14x22mm
Conductor Resistance	0.85 mΩ	0.85 mΩ	0.27 mΩ	1.2 mΩ	0.65 mΩ	0.6 mΩ	0.2 mΩ	0.1 mΩ
Dielectric Strength	RMS3600 V	RMS5000 V	RMS5000 V	RMS2400 V	RMS2400 V	NA	NA	RMS4800 V
Working Voltage	DC870 V RMS616 V	DC1550V RMS1097 V	DC1618 V RMS1144 V	DC420 V RMS297 V	DC420 V RMS297 V	DC100 V RMS70 V	DC100 V RMS70 V	DC1358 V RMS700 V

 

Allegro also offers core and coreless field sensors. These devices can sense currents >1000A and achieve >5000VRMS of dielectric strength isolation.

The part output will continue to increase or decrease until it reaches a high (current > IPR) or low (current < I_PR) saturation point, where I_PR is the current sensing range of the part. Voltage Output High/Low (V_OH/V_OL), sometimes referred to as Output Saturation Voltage (V_SAT(H/L)), is defined as the voltage that sensor output, V_IOUT, does not pass as a result to an increasing/decreasing magnitude of current. This can be seen in the figure below. Note that changing the sensitivity does not change the saturation voltage.

 

The functional range for the linear performance of V_IOUT, and its related datasheet parameters, is valid from -I_PR to +I_PR.  It is possible for the output to report voltages beyond the full-scale measurement  until the saturation point, but  parameters are not guaranteed beyond the full scale measurement.

Every Allegro current sensor will have a power on time specified in the datasheet such as the example below:

Power-On Time, t_PO , is defined as the time interval between a) the power supply has reached its minimum specified operating voltage (V_CC(min)), and b) when the sensor output has settled within ±10% of its steady-state value under an applied magnetic field. An example of the output and supply voltage can be seen in the scope capture below:

The Magnet, Concentrator and Magnetic Shield Suppliers page on the Allegro website provides a vendor list and overview of recommended magnets. This list includes information on core/concentrator and shielding suppliers.

Allegro current sensors come in two range variants, bidirectional and unidirectional, which are identified with the part suffix B and U respectively.  Bidirectional parts can sense positive and negative currents while unidirectional parts only sense in one direction. The output of a bidirectional parts will be at a minimum when current is full scale in the negative direction and at a maximum when current is full scale in the positive direction. The output of a unidirectional part will be at a minimum if current is less than or equal to zero and at a maximum when current is at full scale in the positive direction. Bidirectional parts are useful in detecting negative undershoot or beneficial to systems where current flows in both directions. Unidirectional devices can be used to increase sensitivity when there is no need to sense current in the negative direction.

Every Allegro current sensor includes pins for V_CC, Ground (GND), V_OUT, and a path for the primary current being sensed, if it is an integrated sensor. Some parts include additional pins for enhanced features. A list of these pins includes:

VREF/VZCR- supplies the Zero-Current Output Voltage (V_IOUT(Q)) to a reference pin. This allows for differential measurement and the user to know the zero-current voltage for the output channel V_IOUT. (ACS37002, ACS730)

FAULT /Over Current Fault (OCF)- open drain output that will pull low when a current threshold has been met. The fault output may be latched or unlatched in operation (ACS37002, ACS71240, ACS720, ACS732, ACS733, A1365)

V_OC- Some parts can select the over current fault threshold using an external voltage. This is done with a resistor ladder connected to the V_CC pin. (ACS37002,ACS720, ACS732, ACS733)

Gain Select- Some parts can change the gain depending on the logic applied to the gain select pin which looks for a high or low input (ACS37002).

FILTER- By attaching an external capacitor, the V_OUT filter pole location can be set (ACS720, ACS724/5)

Noise

Noise (input referred [mA_RMS] or output referred [mV_RMS]) is the root mean square value of the noise on the output evaluated at the specified bandwidth.

Noise Density 

Noise Density (input referred [(µA_RMS)/√Hz] or output referred [(µV_RMS)/√Hz]) is noise as a function of frequency. To approximately convert from noise density to noise, multiply noise density by √(bandwidth*π/2)  (note that at lower frequencies, around <1kHz, flicker noise, or 1/f noise, plays a factor and will affect the overall noise performance, i.e. not all noise is removed with a DC input).

The resolution of the device is equivalent to the noise input referred [mA_RMS] at the desired bandwidth. If a device is specified with noise density, convert to noise by multiplying noise density by √(bandwidth*π/2) . If the device is specified as output referred, divide by sensitivity to get input referred. 

Another factor to consider when calculating resolution is the capability of the ADC in which the current sensor output is connected. The ADC resolution of the sensor output in amps is equal to: 

As an example, calculate the resolution of the ACS732KLATR-20AB-T at 1MHz bandwidth using a 5 V ADC with 11.5 effective number of bits. The ACS732 is specified with a noise density of 55 (µA_RMS)/√Hz.

Multiply this value by √(1MHz * π/2)  to get a noise of 69 mA_RMS, which gives the resolution of the sensor output.

Next, calculate the ADC resolution where:

This results in an ADC resolution of 17 mA. When calculating the total resolution of the system, take the maximum of these two calculations, or in this case, 69 mA.

Low pass filtering of the Allegro current sensor output will decrease the noise but at the cost of device bandwidth. If a specific noise level or resolution is desired, solve for bandwidth (BW) in the following equation: desired noise = noise density * √(BW*π/2). 

Next, pick R and C values that generate the desired bandwidth. The bandwidth of an RC filter is equal to 1 / (2*π*R*C). It is important to use an R value that is low enough to not affect the ADC reading. Because ADC’s generally have high input impedance, a value of around 1Kohm or less is typically acceptable. 

• Verify how noise is specified in the datasheet. For example, noise may be dependent on the capacitor on V_OUT, like the specification below.

• Increase the capacitance on V_OUT. The datasheet will include a value for the maximum output capacitance that can be connected to V_OUT, like the specification below. 

• If changing the capacitance does not fix the problem, layout should be examined. If the V_OUT signal has a long trace to the ADC or measurement instrument, there may be other signals interacting with the output signal. Attach an oscilloscope as close as possible to the output of the current sensor and monitor the noise directly at the part output. 
• Another potential issue is an unstable input voltage to the sensor. Ratiometric parts will transfer a noisy input voltage to the device output signal. Monitor the V_CC pin of the current sensor to check for an unstable input. Make sure that the correct bypass capacitor value is being used and is placed as closely as possible to the part on the PCB. 
• Another source of noise could be from stray magnetic fields. Refer to Question 1 of the Design Support section to learn about mitigating stray fields.

Listed in the datasheet for each device is a Selection Guide, typically located on page 2 or 3. While there is some variation from device to device in what is included in the device selection guide, some primary attributes of the selection guide are the part number, sensitivity (Sens), optimized current range (only applicable to integrated conductor sensors), operating temperature (T_A),  package type, and supply voltage (if the device has 5 V and 3.3 V variants). This table can be used as a guide when selection the current sensor for an application. 

Examples: 

Core based (ACS70310) Selection Guide from device datasheet:  

There are two basic Allegro current sensor naming schemes, one for integrated (ACS71240, ACS724, ACS37002, etc.) and one for core-based sensors (ACS70310, A1365, etc). 

Common naming components to integrated and core-based sensors: Allegro current sensors begin with ACS (with the exception of legacy A1363/5/6/7), followed by a three to five digit part number. The part number is followed by a letter to indicate the operating temperature range of the sensor. The temperature range designation is followed by the package designator, which can be two/three digits. Following the package designation, integrated sensors then have a two letter designation for available packaging/shipping options and core-based sensors will have a two letter designation for the leadform option. Next, integrated sensors have a two/three digit current range value and the core-based sensors have the trimmed sensitivity value. This is then followed by the sensor’s directionality, bidirectional (B) or unidirectional (U). Next is the device’s nominal supply voltage level. Included at the end of the part name are custom features (custom fault level, set polarity, customer programmable, etc.). See the device specific datasheet for more information about the device’s part number. Note that legacy devices, like the ACS722/ACS723, ACS724/ACS725, and ACS732/ACS733, do not have a bidirectional or unidirectional designation in the name nor do they have a designation for nominal supply voltage. Different part numbers were made for 3.3 V and 5 V variants (i.e.,  the ACS724 is a 5 V device while the ACS725 is a 3.3 V device but these parts have identical functionality). 

Examples of Naming Schemes: 

Core Based (ACS70310):  

 
Integrated (ACS71240):  

 

Legacy Integrated (ACS724 vs. ACS725, note no supply voltage designation in the Selection Guide): 

 

A min/max limit guarantees that no devices will be above or below the min/max value when leaving the Allegro factory. Typical values are mean ± 3 sigma. This means that 99.7% of devices will fall within the typical values and none will fall outside the min/max limits within the specified operating temperature range, input voltage, or any other test conditions. 

It is also important to note that Sensitivity Error (ESENS) and Total Error (ETOT) are specified at a given current (typically the full-scale current, or half-scale current). Error results may vary with different applied currents. The main example of this is Total Output Error at lower currents. For example, if the full-scale range of a part is 20A and there is a 5% maximum Sensitivity Error and 1A maximum Offset Error, maximum Total Output Error = 20 A * (5% / 100) + 1 A = 2 A or 10% of the full-scale applied current of 20 A. At 5 A applied with the same sensitivity error and offset, Total Output Error = 5*(5% / 100)  + 1 A = 1.25 A or 25% of the full-scale applied current.

The following application note provides characterization data for the Allegro current sensor packages. This application note includes data taken on Allegro demo boards. This document is useful when deciding the correct Allegro current sensor package for a given application and current requirement.

The absolute maximum amount of current that can flow through the package is different than the range of current a device can sense. The maximum allowable current is dependent on package and PCB layout and is a function of ambient temperature. Refer to Question 1  of the Thermal Section of the FAQ for information on determining the maximum allowable current of Allegro current sensors. The output of the device will saturate when current flowing through is greater than optimized current sensing range. Refer to Question 7 of the General Section of the FAQ for a further explanation on output saturation. 

This application note describes the best approach to evaluate junction temperature of an Allegro integrated conductor current sensor. The implementation section of the referenced application note describes how to ensure maximum junction temperature is not exceeded, as well as how to determine the maximum current level of a system.

The Allegro integrated current sensors have a copper conduction path. The temperature drift on copper can be used to approximate the temperature drift of the IP path. This value is +0.393 percent per degree C.

A – Allegro 
S – Sensor 
E – Evaluation 
K – Kit

Navigate to the Allegro Microsystems homepage. Allegro current sensor demo boards begin with the “ASEK” designation. For example, if a ASEK37800KMAC‐015B5‐SPI demo board is required to evaluate the ACS37800KMACTR-015B5-SPI, search ASEK37800 in “Check Stock” search bar on the top right corner of the Allegro homepage. 

The search for “ASEK37800” will provide results for all available ASEK37800 demo boards. Click the Cart icon to be routed to the Digikey website for purchase. 

On the device home page, click the link for “Design Support Tools” as shown in the picture below: 

 

If the demo board has a user guide it will be available in the Design Support Tools with a downloadable link as shown below:

All components on the demo board will be rated at or above the max temperature rating of the current sensor under test. The current rating of the demo board will depend on the package of the current sensor and ambient temperature. The following application note provides characterization data for current sensor packages on Allegro demo boards at various ambient temperatures.

On the device homepage of each Allegro current sensor is a Design Support section, located near the bottom of the web page. Here, there is a zip file containing the Gerber files of the ASEK demo board for the device.  Gerber files are files that contain information on each board layer of a PCB design. 

After unzipping the Gerber files folder, there will be a FAB document. This FAB document contains information about the demo board layout as well as information about copper thickness, PCB layer count, among of the demo board attributes. 

 

In each device datasheet, there is a PCB layout and thermal application section that is specific to that device and package. 

Related Application Notes: 

1. Managing External Magnetic Field Interference When Using ACS71x Current Sensor ICs 
2. Techniques to Minimize Common-Mode Field Interference When Using Allegro current sensor ICs (ACS724 and ACS780) 
3. Common Mode Field Rejection in Coreless Hall-Effect Current Sensor ICs 

Refer to Question 1 of the Design Support FAQ section to learn about mitigating stray fields.

On the specific Allegro current sensor device homepage, navigate to the “Part Number Specifications and Availability”. Select “View Data” on the desired gain option. The package weight is included in the “Materials Declaration Report”. 

On the device homepage of the desired Allegro current sensor, located near the bottom of the web page, is a Packaging section. In the Packaging section, there is typically an image of the package the device is housed in (more than one if the device is offered in multiple packages). Step files for the device package can be found here.  

If the step file is not located on the device homepage, please refer to the packaging homepage on the Allegro website.

Acquiring a UL certification indicates that Allegro current sensors have been tested to applicable standards; UL is globally recognized in their ability to provided accreditation to productions in the industry. Allegro current sensors housed in the MA, LC, MC, LA, and CB packages have been certified to related UL standards 60950-1, 2nd Edition and 62368-1, 1st Edition (MA only). 

Located on the device homepage of Allegro Current sensors housed in the MA, LC, MC, LA, and CB packages are the UL Certificates of Compliance and UL CB Test Certificates. 

Certificates of Compliance verify that devices have been tested by UL in accordance with UL standard 60950-1 and UL standard 62368-1 (MA only). CB Test Certificates provide the UL certified working voltage for basic and reinforced insultation numbers as well as the maximum rated isolation voltage. 

Refer to the “Soldering Methods for Allegro Products” application note on the Allegro website that can be downloaded here. 

• LA has the highest sensitivity with the Hall element closest to the conductor
  • Fill-chip locates top of die closest to the leadframe
• MA has high internal isolation 
  • 2 layers of polyimide insulation and 3 layers of insulating adhesive
• MC has the lowest internal conductor resistance
  • Longer creepage for better working voltage 

Also refer to Question 6 of the General Question section of the FAQ for more package information.

There are several ways to begin the product selection flow. The first would be based off the required isolation or package size. The following table provides an overview of the available packages (not including field sensors).

Package Descriptor	SOICW-16 LA	SOICW-16 MA	SOICW-16 MC	SOIC-8 LC1	SOIC-8 LC2	QFN-12 EXB	7-pin PSOF LR	5-pin CB
Picture
Dimension	10.3x10.3mm	10.3x10.3mm	11.3x13mm	4.9x6mm	4.9x6mm	3x3mm	6.4x6.4mm	14x22mm
Conductor Resistance	0.85 mΩ	0.85 mΩ	0.27 mΩ	1.2 mΩ	0.65 mΩ	0.6 mΩ	0.2 mΩ	0.1 mΩ
Dielectric Strength	RMS3600 V	RMS5000 V	RMS5000 V	RMS2400 V	RMS2400 V	NA	NA	RMS4800 V
Working Voltage	DC870 V RMS616 V	DC1550V RMS1097 V	DC1618 V RMS1144 V	DC420 V RMS297 V	DC420 V RMS297 V	DC100 V RMS70 V	DC100 V RMS70 V	DC1358 V RMS700 V

 

The other product selection flow may begin with the required current sensing level. Allegro has landing pages for various current sensing levels including:

• 0-50A Sensors
• 0-400A Current Sensors
• 0 to >1000A Field Sensors

The Current Sensors Innovations also highlights the benefits of the different product families.

When debugging stray magnetic fields, check to see if the sensor is single or dual hall technology by checking the functional block diagram on the device specific datasheet.

Single Hall and Stray Fields:

Because Allegro current sensors use the Hall Effect to measure current, any additional magnetic field seen at the hall element outside of the current being measured will affect the output of the sensor. These additional magnetic fields are generally called stray or common magnetic fields. The most common cause for stray magnetic fields is the presence of a high current trace or wire near the current sensor. To approximate error caused by a current carrying wire, model the magnetic field of the trace as an infinite wire where B = µ* (I / (2π×d)).

B is the magnetic field in Gauss (G), µ is the permeability of free space in G equal to 4π*0.001, I is the current in amps, and d is the distance in meters of the line from a point on the wire to the hall element perpendicular to the wire. Once the magnetic field is known, multiply by the magnetic coupling factor [G/A] (provided in most datasheets)  which will result in an absolute error in amps. Once error is estimated, testing can be performed by removing the stray field generating wire or trace and retesting the sensor output. Another solution if the trace or wire cannot be removed is to pull the sensor off the board and wire the part off the PCB away from the suspected current carrying wire. Lastly, shielding can be utilized by placing a ferrous material around the sensor to block the stray field.

This application note describes in more detail the effects of magnetic field interference and shielding.

Dual Hall and Stray Fields:

Allegro also offers sensors with dual hall elements to mitigate stray field error. Two Hall elements are used differentially and placed on opposite sides of the current loop. This allows the common magnetic field to be removed allowing output voltage to be significantly unaffected by the common field. Although dual hall elements minimize stray field error, they do not entirely eliminate the potential of error induced by stray field. The same testing/mitigation techniques in the previous paragraph can also be used when debugging dual hall sensors.

This application note explains in more detail how to estimate and mitigate common magnetic field when using sensors with dual hall elements. 

Allegro provides LTSpice models for most Allegro current sensors. The following link will download a zip folder with the entire library of spice models for Allegro current sensors. Read “ALLEGRO_ACS_LT_Guide.pdf” included within the zip folder for a detailed walk through on how to get started and how to use Allegro parts in LTSpice.

This note gives an overview, guidelines, and simulation results for designing with a bus bar. 

Allegro also offers an online interactive modeling tool to help design bus bars for coreless solutions.

This note provides an overview and guidelines for designing a core/concentrator to pair with Allegro field sensors. 

The following is a check list to follow to verify the proper output of an Allegro current sensor:

• Is the input voltage above the minimum and below the maximum specified in the datasheet? Adjust the input voltage to match the typical VCC value in the datasheet.
• Does the input current match the typical on the datasheet? If the current is lower than expected, there may be an open circuit between the input supply and the sensor. If current is higher than expected, there may be something else on the input sinking current and preventing the device from biasing properly. Verify the supply hookup to the part.
• When the device is biased on correctly, but no current is applied, what is the output of the device? Ensure that when in this state, the device output matches the zero-current output voltage (V_IOUT(Q)) that is specified in the datasheet. If this does not match, measure the resistance from the output to ground and make sure nothing is pulling the output low. Also try to resolder the sensor or replace with another part to see if the problem persists. This will verify if the issue is part or application related.
• If V_IOUT(Q) is normal, is the sensitivity of the device within the datasheet specification? To quickly test sensitivity, apply 0 A and measure V_IOUT, then apply a known current and remeasure V_IOUT. The slope of these two points is the sensitivity in mV/A. To troubleshoot issues with sensitivity, ensure that the resistance of the current path being sensed is as expected by measuring with an ohmmeter. Faulty soldering or a stray trace could lower the resistance through the conductor being measured and introduce error.

Other potential issues include noise (refer to the Noise section of the FAQ) and stray magnetic field (refer to the Design Support Section of the FAQ). 

Essentially every shunt solution can be replaced with an integrated Hall Effect sensor simply by routing the current trace through the integrated current sensor instead of through an external shunt. The few shunt solutions that may not be practical for an integrated Hall Effect sensor include ultra-low current resolution (in the uA’s) or ultra-high speed (>1Mhz).

The key benefits of switching from a shunt solution to an integrated Hall Effect solution are increased isolation, decreased layout size, and decreased design complexity. Most shunt solutions cannot exceed more than 100 V common mode voltage without the use of an isolation amplifier that requires external isolation circuitry. Compare that to Hall Effect current sensors, which offer inherent isolation from the current path to the signal pins. Switching to a Hall Effect sensor also eliminates the need for an external shunt and input filtering. This lowers the layout space as well as the design complexity.

There are many ways to measure current in a system, but the following table highlights and compares the main current sensing solutions:

Located on the Allegro Customer Portal are programmer GUIs/DLLs for Allegro customer programmable sensors. In addition to programming software for customer programmable devices, the Allegro Customer Portal has helpful design tools including user guides and interactive design tools.

The ASEK20 is a device used to program and evaluate customer programable Allegro current sensors (the ASEK20 can be used for angle position, linear position, and digital position sensors). The ASEK20 is used in combination with the device specific daughterboard (which are available separately from the ASEK20). The ASEK20 is a benchtop validation and programming tool useful in characterizing and understanding the performance of Allegro current sensors. The ASEK20 is also useful in calibrating Allegro current sensors in the field. Device specific software applications can be found on Allegro’s Software Portal. 

Customer Programable Allegro Current Sensors that can be used with the ASEK20: 

1. ACS70310
2. ACS70311
3. ACS71020
4. ACS37800
5. A1363
6. A1365
7. A1367 

Please refer to “What does ‘RoHS’ mean?” found in the Quality and Environment FAQ.

Please refer to the Quality and Environment FAQ. 

Navigate to the Quality Standards and Environmental Certifications homepage on the Allegro website. In the section titled “Policies and Declarations”, there is a section titled Declarations and Statements. Here, the RoHS compliance declaration can be found.