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ACS754 Current Sensor Frequently Asked Questions 
- What are the differences between the various members of the ACS754 family?
- How is the CB package different from the CA package used in the ACS750 and 752 families?
- Can the ACS754 family sense both dc and ac currents?
- Can the ACS754 sense 0 to 400 A as well as ±200 A?
- What does "ratiometric" mean?
- What external components are required?
- Is there any way to adjust the gain of the ACS754?
- How small of a current can the ACS754 resolve?
- What is the ESD tolerance of the ACS754?
- Can Allegro bend the leads under the package or outward, so that I can surface-mount the ACS754?
- How should I solder the ACS754 onto my board?
- How should I connect to the straight-leaded package configurations?
- Do you have a recommended footprint for the ACS754 (CB package)?
- Can I get the Gerber files for your evaluation board?
- I can't use Gerber files; is any other format available?
- How thick are the copper traces on your evaluation board?
- Are there any other design guidelines for applying the ACS754?
- What is the inductance of the ACS754 (CB package)?
- Does the ACS754 contain lead?
- What is the high-current leadframe made of?
- How susceptible is the ACS754 to stray magnetic fields?
- What safety certifications does the ACS754 have?
- What is the behavior of the ACS754 output during a slow ramp-up of VCC?
- How soon after the application of power will a valid signal be available from the ACS754?
- What happens if I try to drive more than the specified 10 nF maximum capacitance with the output of the sensor?
- What happens if I try to drive less than the specified 4.7 kΩ minimum resistance with the output of the sensor?
- What is the overcurrent tolerance of the ACS754 devices?
- How will capacitive coupling between the current path in the ACS754 and the Hall element effect the sensor output?
What are the differences between the various members of the ACS754 family?
The only functional differences are the mV/A sensitivities of each version.
Although the datasheet for the 200 A version does not show the -PFF package configuration (bent signal and primary conductor leads), which is shown on the other datasheets in the family, it can be provided if desired. The -PFF configuration was not included because currents that high are typically not carried in PC board traces. Contact your local Allegro Sales office for more information.
How is the CB package different from the CA package used in the ACS750 and 752 families?
Externally the packages are identical and have the same footprint. Internally, the CB package leadframe has greater cross-sectional area where it passes the Hall effect element. As a result, the CB package has a resistance of only 100 µΩ, versus 130 µΩ for the CA package.
Can the ACS754 family sense both dc and ac currents?
The ACS754 family uses Hall effect technology, which is capable of sensing electrical currents having both dc and ac components. As the datasheet states, the bandwidth of the ACS754 is 35 kHz typical. There is usually a phase lag of at least 45º on the output for ac currents at frequencies exceeding 35 kHz. For transient current signals, the response time can be expected to be ≈10 µs.
Can the ACS754 sense 0 to 400 A as well as ±200 A?
No, the maximum current in the sensing range of the ACS754 is an absolute current of 200 A. The magnetic circuit within the ACS754 package will not provide a linear output above field levels generated by 200 A.
This feature is particularly valuable when using the ACS754 with an analog-to-digital converter. A/D converters typically derive their LSB from a reference voltage input. If the reference voltage varies, the LSB will vary proportionally. The ratiometric feature of the ACS754 means its gain and offsets are proportional to its supply voltage, VCC. If the reference voltage and the supply voltage for the ACS754 are derived from the same source, the ACS754 and the A/D converter will both track those variations, and such variations will not be a source of error in the analog-to-digital conversion of the ACS754’s output.
What external components are required?
Allegro recommends the use of a 0.1 µF bypass capacitor between the VCC pin and the GND pin. The capacitor should be located as close as practical to the ACS754 package body.
Is there any way to adjust the gain of the ACS754?
No, the ACS754 sensitivity and 0-ampere quiescent voltage level are programmed at the factory.
How small of a current can the ACS754 resolve?
Without filtering, the ACS754's 50 A version can resolve a change in current level of ≈2.5% of full scale, at 25ºC, through its primary conductor leads. The 200A version can resolve ≈1.6%. At these levels, the amount of magnetic field coupled into the linear Hall Effect IC is just above its noise floor. However, depending on the temperature, 0-ampere offset error, and the dc current level, the percentage error of the current sensor output will often be greater than this minimum resolvable current. Resolution can be improved by filtering the output of the ACS754. Up to a threshold value, this can be done without limiting the response time. See the application note "Managing Noise Levels in the ACS75X Current Sensors," at www.allegromicro.com/techpub2/an/acs75xan1.pdf.
What is the ESD tolerance of the ACS754?
Typical ESD tolerance is 6 kV human body model, 600 V machine model.
Can Allegro bend the leads under the package or outward, so that I can surface-mount the ACS754?
In order to safely conduct 200 A currents, the power leadframe in the ACS754 has been constructed with a relatively heavy gauge. Because of this heavy gauge, the terminal leads are not very flexible. If the ACS754 is surface-mounted, small amounts of board flex, or the action over time of thermal expansion and contraction, would break the IC off the board. Allegro does not believe surface-mounting is practical for this device.
How should I solder the ACS754 onto my board?
To ensure a robust joint to the board, Allegro recommends adding a ring of through-holes in the solder pad area around each of the two broad primary conductor leads that carry the current being sensed. These holes are shown in the diagram in the FAQ "Do you have a recommended footprint for the ACS754 (CB package)." General soldering recommendations for the CA and CB packages have been added to the application note "Soldering Methods for Allegro's Products," which is on the Allegro website at www.allegromicro.com/en/Products/Design/an/an26009.pdf.
How should I connect to the straight-leaded package configurations?
Allegro recommends a tin-fusing welding technique. This method is described in our application note "Hall-Effect Devices: Gluing, Potting, Encapsulating, Lead Welding and Lead Forming" at www.allegromicro.com/en/Products/Design/an/an277031.pdf.
Do you have a recommended footprint for the ACS754 (CB package)?
Yes, the following shows the recommended footprint for the –PFF configuration.
The portion (A) of the above footprint, the three small through holes for the signal pins, also applies to the –PSF configuration (which has straight primary conductor leads).
Can I get the Gerber files for your evaluation board?
Yes, download from:
http://www.allegromicro.com/en/Products/Part_Numbers/0754/Allegro_CA_CB_EvalBoard.zip.
I can't use Gerber files; is any other format available?
Yes, an AutoCAD 2004 .DXF file can be downloaded from:
http://www.allegromicro.com/en/Products/Part_Numbers/0754/Allegro_CA_CB_EvalBoardDXF.zip.
The copper areas are defined as "regions" in these files.
How thick are the copper traces on your evaluation board?
The evaluation board uses 4 oz. copper.
Are there any other design guidelines for applying the ACS754?
Care should be taken to minimize the inductance of the current path to be measured. Also, attention should be paid to minimizing the contact/connection resistance of any connections in that path.
What is the inductance of the ACS754 (CB package)?
Typical measured inductance versus test signal frequency are:
- 32 nH at 10 kHz
- 24 nH at 100 kHz
- 21 nH at 200 kHz
No, the ACS754 family is lead-free. All leads are plated with 100% matte tin, and there is no lead inside the package.
What is the high-current leadframe made of?
The heavy gauge leadframe is made of oxygen-free copper.
How susceptible is the ACS754 to stray magnetic fields?
The ACS754 contains a concentrator core that acts not only as a concentrator of the flux lines generated by IPRIMARY, but also as a shield to protect the sensor from ambient common-mode fields (typical rejection of common mode fields is –41 dB). The results are detailed in the following chart, which compares the output voltages, VOUT, of an unshielded linear Hall effect sensor and that of the ACS754. The devices have the same gain, and are exposed to the same magnetic field, applied in an air core through the top of the package.
What safety certifications does the ACS754 have?
The ACS754 family also has been certified by TÜV America to the following standards:
- UL 60950-1:2003
- EN 60950-1:2001
- CAN/CSA C22.2 No. 60950-1:2003
The mold compound is UL recognized to UL94V-0.
What is the behavior of the ACS754 output during a slow ramp-up of VCC?
The typical output behavior of the ACS754xCB-050 during a 500 ms ramp-up of VCC is shown for both 0 A and 50 A in the following two charts:
How soon after the application of power will a valid signal be available from the ACS754?
The typical time to valid output is given in the following table and charts.
IP = 0 A |
IP = 50 A |
18 μs |
30 μs |
Startup of ACS754 with 0 amperes applied, then a VCC step from 0 to 5 V
Startup of ACS754 with 50 amperes applied, then a VCC step from 0 to 5 V
What happens if I try to drive more than the specified 10 nF maximum capacitance with the output of the sensor?
The output of the sensor may oscillate.
What happens if I try to drive less than the specified 4.7 kΩ minimum resistance with the output of the sensor?
The sensor may not produce an output, as its output driver will not be able to supply sufficient current.
What is the overcurrent tolerance of the ACS754 devices?
Because of its low, 100 µΩ internal resistance, the overcurrent capability of the CB package of the ACS754 sensor is highly dependent on the characteristics of the bus bar or printed circuit board on which it is mounted. In the case of a PCB mounting, trace width and thickness, the number of layers, the presence or absence of ground and/or power planes, and the gauge of the cables that carry the current on and off the board are all significant factors. It is also dependent on the maximum operating temperature of your application and the frequency with which the overcurrent occurs.
By way of example, we have characterized the ACS754 devices on the Allegro ACS754 evaluation board, connected to the current source with 2 AWG cables. This a 2-layer board with 4-oz. copper. (For drawings and Gerber files, see the FAQs: Can I get the Gerber files for your evaluation board? and I can't use Gerber files; is any other format available? on this page.) The results are provided in the following table.
| Tested Maximum ACS754 Overcurrent Levels and Durations Applicable to devices on Allegro ASEK 754 evaluation boards connected with 2 AWG cables |
|
|---|---|
| Ambient Temperature (°C) |
Maximum Current (A) |
| 10 s, 10% Duty Cycle, 100 pulses applied | |
| 25 | 350 |
| 85 | 350 |
| 150 | 260 |
| 3 s, 3% Duty Cycle, 100 pulses applied | |
| 25 | 450 |
| 85 | 425 |
| 150 | 375 |
| 1 s, 1% Duty Cycle, 100 pulses applied | |
| 25 | 1200 |
| 85 | 900 |
| 150 | 600 |
How will capacitive coupling between the current path in the ACS754 and the Hall element effect the sensor output?
The power supply rejection test is conducted by injecting a high-frequency sinusoidal frequency onto the high-current leads. The signal coupling onto the output of the Hall effect device is then measured. The ACS754 family devices exhibit a high level of power supply rejection as the following table, of typical capacitive coupling of a 20 V peak-to-peak signal on the current path, reveals. In addition, the chart following the table indicates performance as a function of frequency.
| Noise Frequency (MHz) |
Passed-Through Signal Magnitude (mv/20 V) |
Attenuation (dB) |
|---|---|---|
| 10 | 720 | –28.9 |
| 5 | 430 | –33.3 |
| 1 | 50 | –52.0 |
| 0.5 | 48 | –52.4 |
