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High-Performance Power ICs and Hall-Effect Sensors
FAQs - GTS/ATS 
1. What does GTS stand for?
Gear Tooth Sensor. It applies to Allegro's family of products that have integrated magnets and allow for the sensing of steel gears and targets.
2. What does ATS stand for?
Adaptive Threshold Sensing. The term applies to the general principles used in Allegro's GTS product family. Various features of ATS provide for intelligent learning of the rotating target and optimized operation during a variety of transient events.
3. What is AGC?
AGC stands for Automatic Gain Control. It is an ATS feature that, upon power-up, rapidly adjusts the sensitivity of the sensor to provide an optimum and consistent signal level independent of air gap. Most ATS devices utilize AGC.
4. What principles are used for switching in ATS devices?
Two basic methods are used to interpret the analog signal into a digital output: peak detection and threshold detection. Peak detection devices track and hold the analog signal and switch on its peaks. Threshold detection applies levels through which the analog signal must pass to induce switching.
5. How do the sensors adapt to changing operating conditions?
Various sensors apply different algorithms, though two basic principles are utilized: conservative update and continuous update. Conservative update allows the device to adjust switching thresholds only after the sensor observes a trend over a number of teeth. Alternatively, continuous update provides for the instantaneous adjustment of switching thresholds on each edge.
6. Is the ATS family of products application specific?
In general the ATS product family members have been designed for specific applications. For example, the requirements of ABS (Anti- Lock Brake System) differ greatly from those of cam sensing. However, there is an overlap of application needs and some devices can offer a solution to more than a single application type.
7. What is the typical functional air gap range for Hall-effect sensors?
Back-biased GTS sensors are specified from 0 to 2.5 mm, though operation can exceed the specified maximum air gap limits.
8. What do the terms TPOS and VPOS stand for?
TPOS and VPOS stand for True Power On State and Valid Power On State, respectively. The terms are synonyms and apply to the operating principle that allows the device, on power-up, to know whether it is facing a tooth or a valley without requiring target rotation.
9. What type of sensors provide TPOS?
Magnetic principles require a single-element Hall device to provide the TPOS function. However, not all Allegro single-element devices provide the TPOS function.
10. What is the difference between a single element and differential element Hall-effect sensor?
A single-element sensor contains a single Hall element that is used to generate a signal proportional to the magnetic field generated by the proximity of a steel object. A differential sensor has two Hall elements; the difference in the output of the two signals generates a signal that has peaks corresponding to edges of steel features. These principles of operation make a differential sensor an edge detector and a single element sensor a tooth/valley detector.
11. What is the output polarity of Allegro devices?
Single-element devices are designed to have a fixed output polarity (high over a tooth or high over a valley, depending on the specific device). Refer to data sheets to determine the actual polarity of a given single-element device. Differential devices can be high over a tooth or high over a valley depending solely on target rotation direction. Refer to data sheets to determine the rotation/polarity convention for a given differential device.
12. What are advantages/disadvantages of the single-element and differential sensor configurations?
If TPOS (VPOS) is desired, then a single-element sensor is required. If operation that is independent of the installed sensor rotation angle is desired, then a single-element sensor is required. Because of the special magnet design required by single-element sensors, they tend to use larger packages (SA and SE) and be more expensive than differential sensors.
13. What are SA, SB, SE, SG, SH, and SJ?
These are all Allegro packages. The SA and SB are more mature than the others and are mechanically assembled with several components which include a Hall-effect SIP, magnet and plastic housing. The SA is larger than the SB and is used predominately for single-element sensors. The SE, SG and SJ are the newest Allegro packages that are comprised of a Hall-effect IC, lead frame and magnet molded in one step into an integrated package. The SH is a two-wire version of the SG that has wide leads for easy soldering or welding. Both the SH and SG are used predominately for differential devices. The SE is larger than the SG/ SH and is used predominately for single-element devices.
14. Are Hall-effect sensors adversely affected by fluids, dirt, etc.?
The presence of fluids or dirt in the space between the sensor and the target/magnet do not affect the sensor operation. The presence of ferrous material (i.e. gear shavings), however, can affect the sensor operation. Intrusion of contaminates into the package can cause catastrophic failure. It is strongly recommended that the sensor be sealed for exposure to harsh environments.
15. What does the term "zero-speed operation" mean?
This refers to the ability of a sensor to provide valid output down to 0 rpm. Zero-speed typically refers to running mode and it is expected that a zero-speed sensor may require an initial power-up calibration cycle before the zero-speed output is correct. AC-coupled sensors cannot provide zero-speed sensing because they track analog signals with a capacitive circuit.
16. How fine of pitch can the teeth of a target be?
At some point, the tooth size becomes too small to generate a large enough magnetic field to allow switching. The actual size limitation is dependent upon the sensor type (single versus differential), the Hall- plate spacing (of differential sensors) and magnet size. Typically, teeth should be no smaller than 5 mm for single-element sensors. Standard differential sensors require teeth of at least 2 mm while special fine- pitch differential sensors can accommodate smaller teeth.
17. What is the maximum allowable rotational speed of targets?
Hall-effect sensors typically come equipped with on-board low-pass filters. Depending on the specific sensor, the filter bandwidth is usually the limiting factor for speed and is specified on the data sheet.
18. What happens if a gear tooth sensor is used beyond its maximum air gap specification?
If the operating air gap of a given device is exceeded, the output becomes unpredictable and may not meet specifications or may miss edges. However, depending on target geometry, a gear tooth sensor may function beyond the maximum air gap specified. The specifications are established using a reference target that is application appropriate to provide full compliance to all pertinent specifications over all valid operating conditions.
19. What if a two-wire interface is required instead of an open-collector, three wire interface?
There are both interface types available in both gear tooth sensors and non-biased sensors. There is also an application note on the Allegro website explaining how a three wire sensor interface can be made in to a two-wire interface. True two-wire sensors require no additional external circuitry to provide two-wire interfacing.
20. Will all open-collector, three wire devices and two-wire, current source devices have the same output polarity?
A two-wire sensor has a high or low current level output that, when measured with the use of a sense resistor to ground or supply, could be translated to a voltage. A three-wire sensor has a high or low voltage level output, so it is difficult to compare the polarities. However, three-wire and two-wire sensors do have a standard output polarity for each interface type for a defined direction of rotation. The polarity is defined in the data sheet.
21. Can I get ATS devices in SIP form for ring magnet sensing?
Several of the back-biased ATS devices are offered in SIP form. Please see your local Sales Representative or Field Application Engineer.
22. Why can't I find your new devices on the Allegro web site?
Allegro's newest products may not have been released to the general public via the Internet. Contact your local sales representative for news on the latest product offerings.
23. What is the typical ESD rating on your devices?
Human Body Model capabilities range from about 2 kV to greater than 9 kV. Contact factory for specific device ratings.
24. Do I need to add any external components for the device to operate properly?
Except for the AC-coupled devices that require an external tracking capacitor, the only required components are a bypass capacitor (between Vcc and ground) and a sense resistor. For three-wire devices, a pull-resistor is needed and for two-wire devices, a series sense resistor is needed. Additional components may be required to provide compliant operation during certain electrical and electro- magnetic transients. These need to be established for a given device, application and transient type.
25. Can I allow the output to go below ground?
No, the device output structure cannot handle significant reverse output voltages.
26. Is the output short circuit protection continuous, or is there a time duration limit for survival of the device?
Typically a shorted output has a limited time before damage occurs. However, there are particular sensor types that have been designed to handle a continuous short circuit condition. Refer to the data sheet for device specific information. Continuous capability will be stated, if it is not, then maximum junction temperature must not be exceeded.