Dynamic Self-Calibrating Peak-Detecting Differential

Hall Effect Gear Tooth Sensor IC

ATS616LSG

10

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

AGC circuit sets the gain of the device after power-on. Up to a

0.25 mm air gap change can occur after calibration is complete

without significant performance impact.

Superior Performance.

The ATS616 has several advantages

over conventional Hall-effect devices. The signal-processing

techniques used in the ATS616 solve the catastrophic issues that

affect the functionality of conventional digital gear-tooth sen-

sors, such as the following:

• Temperature drift. Changes in temperature do not greatly

affect this device due to the stable amplifier design and the

offset rejection circuitry.

• Timing accuracy variation due to air gap. The accuracy varia-

tion caused by air gap changes is minimized by the self-cali-

bration circuitry. A 2×-to-3× improvement can be seen.

• Dual edge detection. Because this device switches based on

the positive and negative peaks of the signal, dual edge detec-

tion is guaranteed.

• Tilted or off-center installation. Traditional differential sensor

ICs can switch incorrectly due to baseline changes versus air

gap caused by tilted or off-center installation. The peak detec-

tor circuitry references the switchpoint from the peak and is

immune to this failure mode. There may be a timing accuracy

shift caused by this condition.

• Large operating air gaps. Large operating air gaps are achiev-

able with this device due to the sensitive switchpoints after

power-on (dependent on target dimensions, material, and

speed).

• Immunity to magnetic overshoot. The patented adjustable

hysteresis circuit makes the ATS616 immune to switching on

magnetic overshoot within the specified air gap range.

• Response to surface defects in the target. The gain-adjust

circuitry reduces the effect of minor gear anomalies that would

normally cause false switching.

• Immunity to vibration and backlash. The gain-adjust circuitry

keeps the hysteresis of the device roughly proportional to the

peak-to-peak signal. This allows the device to have good im-

munity to vibration even when operating at close air gaps.

• Immunity to gear run out. The differential chip configuration

eliminates the baseline variations caused by gear run out.

Differential vs. Single-Element Design.

The differential chip

configuration is superior in most applications to the classical

single-element design. The single-element configuration com-

monly used (Hall-effect element mounted on the face of a simple

permanent magnet) requires the detection of a small signal (often

<100 G) that is superimposed on a large back-biased field, often

1500 G to 3500 G. For most gear/target configurations, the back-

biased field values change due to concentration effects, resulting

in a varying baseline with air gap, valley widths, eccentricities,

and vibration (figure 4). The differential configuration (figure 5)

cancels the effects of the back-biased field and avoids many of

the issues presented by the single Hall element design.

Peak Detecting vs. AC-Coupled Filters.

High-pass filtering

Figure 4. Affect of varying valley widths on single-element circuits.

Figure 4. Affect of varying air gaps on differential circuits.