True Zero-Speed Low-Jitter High Accuracy

Gear Tooth Sensor

ATS625LSG

13

Allegro MicroSystems, Inc.

115 Northeast Cutoff, Box 15036

Worcester, Massachusetts 01615-0036 (508) 853-5000

www.allegromicro.com

AGC (Automatic Gain Control)

The AGC feature is implemented by a unique patented self-

calibrating circuitry. After each power-on, the device measures

the peak-to-peak magnetic signal. The gain of the sensor is then

adjusted, keeping the internal signal amplitude constant over the

air gap range of the device, AG. This feature ensures that opera-

tional characteristics are isolated from the effects of changes in

AG. The effect of AGC is shown in figure 7.

Differential Electrical Signal versus Target Rotation

at Various Air Gaps, Without AGC

Differential Electrical Signal versus Target Rotation

at Various Air Gaps, With AGC

0.25 mm

AG:

0.50 mm

1.00 mm

1.50 mm

2.00 mm

0.25 mm

AG:

0.50 mm

1.00 mm

1.50 mm

2.00 mm

Target Rotation (°)

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

0

3

6

9

12

15

18

21

24

Target Rotation (°)

-1000

-800

-600

-400

-200

0

200

400

600

800

1000

0

3

6

9

1215182124

Offset Adjustment

In addition to normalizing performance over varying AG, the

gain control circuitry also reduces the effect of chip, magnet,

and installation offsets. This is accomplished using two DACs

(D to A converters) that capture the peaks and valleys of the

old Comparator subcircuit, which controls device switching. If

induced offsets bias the absolute signal up or down, AGC and

the dynamic DAC behavior work to normalize and reduce the

impact of the offset on sensor performance.