Data Sheet

ADA4571-2

Rev. 0 | Page 9 of 13

THEORY OF OPERATION

The ADA4571-2 is an AMR sensor with integrated signal

conditioning amplifiers and Σ-Δ ADC drivers. The ADA4571-2

produces two analog outputs, sine and cosine, that indicate the

angular position of the surrounding magnetic field.

Sensitec GmbH developed the ADA4571-2 AMR technology.

Figure 14 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 by the device

and 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, gain control mode

disabled) and buffered. The single-ended outputs are biased

driving the 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

the 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.

+

–

+

–

–

+

–

+

VDDx

VDDx/2

3.3kΩ

3.3kΩ

50Ω

20pF

62.7pF

VSINx

62.7pF

AMR

BRIDGE

VDDx

VDDx

Figure 14. Detailed Internal Diagram of the ADA4571-2, Single Sine Channel