ADAF1080BCPZ datasheet(22/36 Pages) AD

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Data Sheet

ADAF1080

THEORY OF OPERATION

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OPERATING MAGNETIC FIELD RANGE

The ADAF1080 AMR field sensor is capable of accurately measur-

ing magnetic fields in the ±8 mT range along the sense field axis.

The ADAF1080 has a defined operating window dependent on

the combination of the sense field and the cross field detected

by the sensor. The valid operating region for the ADAF1080 over

temperature is shown in Figure 45. The lines plotted within Figure

45 define the worst-case limits for a combination of the sense

field and the cross field that the sensor can be subjected to while

maintaining the sensitivity coefficient in the Specifications section.

Outside this operating region, sensitivity of the sensor reduces from

the specified values. If the combination of the sense field and the

cross field is too far outside the defined operating region shown

in Figure 45, the sensitivity coefficient of the sensor can change

significantly, or in extreme cases, become zero. If the sensitivity

becomes zero, the output voltage of the ADAF1080 does not

change with a changing magnetic field. The diagnostic coil can be

used to determine if such an event has occurred as described in the

Diagnostic Coil section.

When such an event occurs, the functionality of the sensor can be

restored by flipping the sensor by driving the FLIP_DRV pin with

a rising or falling edge provided that the combination of the sense

field and the cross field is within the operating region. For more

information on the flipping functionality, refer to the Flip Coil and

Flip Coil Driver section.

Figure 45. Magnetic Field Operating Region of the ADAF1080

INTEGRATED SIGNAL-CHAIN CONDITIONING

Figure 46 shows the internal sensing signal chain, consisting of

the AMR bridge, flip coil, and the supporting functions for signal

amplification, control, filtering, and buffering.

The sensor sensitivity of an AMR field sensor is inversely propor-

tional to the temperature. To improve the stability of output sensitiv-

ity over temperature, a bridge driver circuit is used to generate

a bridge supply voltage that is proportional to temperature. Full

factory calibration of the bridge driver circuit enables the ADAF1080

to deliver precision stable magnetic field measurements over the

full temperature range.

The precision instrumentation amplifier has programmable gains (G

= 20, 40, or 80) that are set by using the A0 and A1 pins. The

amplified output is then biased around a common-mode voltage of

VSET/2 and buffered to drive the input of an external ADC that can

be referenced to the supply voltage to improve PSRR. This biasing

of the output around a configurable common-mode voltage leads to

optimal use of the input range of the ADC by ensuring that the input

voltage of the ADC tracks the reference voltage of the ADC in a

ratiometric manner.

Figure 46. A Detailed Internal Block Diagram of the ADAF1080

PRECISION INSTRUMENTATION AMPLIFIER

The architecture of the integrated instrumentation amplifier con-

sists of a precision, low-noise, zero-drift amplifier that features

a proprietary chopping technique. This chopping technique offers

a low input-offset voltage (VOFFSET) and input-offset voltage drift

(VOFFSET_LT and TCOFFSET). The zero-drift design also features

ripple suppression circuitry that removes glitches and other artifacts

caused by chopping.

Offset-voltage errors caused by common-mode voltage swings are

corrected by the chopping technique, resulting in a high CMRR of

the static magnetic field. The amplifier features low-input broadband

noise of 15 nV/√Hz with a low flicker noise component due to the

use of chopping. These features are ideal for amplification of the

small AMR bridge signals for high-precision sensing applications.

The gain of the programmable gain instrumentation amplifier

(PGIA) is set through the digital pins (A1 and A0). There are three

possible gain settings that can be used based on the magnetic

field ranges that are used in the application. For example, if the

application only needs a magnetic field range of ±2 mT, using a

PGIA gain = 80 delivers the largest output signal. However, if the

magnetic field range is ±8 mT, using a PGIA gain = 20 enables

the entire magnetic field range to be measured without potentially

overranging an ADC. The gain can be fixed using pull-up and pull-

down resistors or controlled dynamically during the application by a

microcontroller GPIO. The digital inputs (A0 and A1) are internally

pulled up to VDD through a 300 kΩ pull-up resistor. Therefore, the

default gain is 80 if no external voltage is applied, and the A0 and

A1 pins are left no connect. Table 14 outlines the truth table logic

for the three gain setting of the ADAF1080.

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