ADA4571

Data Sheet

Rev. 0 | Page 14 of 21

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THEORY OF OPERATION

The ADA4571 is an AMR sensor with integrated signal

conditioning amplifiers and ADC drivers. The ADA4571

produces two analog outputs, sine and cosine, which indicate

the angular position of the surrounding magnetic field.

The AMR sensing element is designed and manufactured by

Sensitec GmbH.

Figure 27 shows the sine channel, consisting of an AMR sensor

element and the supporting functions for control, filtering,

buffering, and signal amplification. A reference voltage that is

proportional to the supply voltage is generated and it controls

the supply voltage of the sensor bridges. For noise and

electromagnetic compatibility (EMC) suppression purposes, the

bridge supply is low-pass filtered. The bridge output voltages

are amplified by a constant factor (G = 40, GC mode disabled)

and buffered. The single-ended outputs are biased around a

inputs of an external ADC referenced to the supply voltage.

For optimum use of the ADC input range, the cosine and sine

output voltages track the supply voltage ensuring a ratiometric

configuration. To achieve high signal performance both output

signals are carefully matched in both amplitude and phase. The

amplifier bandwidth is sufficient to ensure low phase delay at

maximum specified rotation speed.

Electromagnetic interference (EMI) filters at the sensor outputs

and between the first and second stages reject unwanted noise

and interference from appearing in the signal band.

The architecture of the instrumentation amplifier consists of

precision, low noise, zero drift amplifiers that feature a proprietary

chopping technique. This chopping technique offers a low input

offset voltage of 0.3 μV typical and an input offset voltage drift

of 0.02 μV/°C typical. The zero drift design also features

chopping ripple suppression circuitry, which removes glitches

and other artifacts caused by chopping.

Offset voltage errors caused by common-mode voltage swings

and power supply variations are also corrected by the chopping

technique, resulting in a dc common-mode rejection ratio that

is greater than 150 dB. The amplifiers feature low broadband

noise of 22 nV/√Hz and no 1/f noise component. These features

are ideal for amplification of the low level AMR bridge signals

for high precision sensing applications.

In addition, extensive diagnostics are integrated on-chip to self

check sensor and IC conditions.

Figure 26. Direction of Homogeneous Magnetic Field for α = 0°

Figure 27. Detailed Internal Diagram of the ADA4571 Sine Channel

1

2

3

4

8

7

6

5

ADA4571

(Not to Scale)

TOP VIEW

+

–

+

–

–

+

–

+

VDD

VDD/2

VDD

VDD

VTEMP

3.3kΩ

3.3kΩ

50Ω

20pF

62.7pF

VSIN

AMR

BRIDGE

ADA4571

62.7pF