Datasheet
AD8622/AD8624 Data Sheet
Rev. D | Page 16 of 20
HALL SENSOR SIGNAL CONDITIONING
The AD8622/AD8624 is also highly suitable for high accuracy,
low power signal conditioning circuits. One such use is in Hall
sensor signal conditioning (see Figure 57). The magnetic
sensitivity of a Hall element is proportional to the bias voltage
applied across it. With 1 V bias voltage, the Hall element
consumes about 2.5 mA of supply current and has a sensitivity
of 5.5 mV/mT typical. To reduce power consumption, bias
voltage must be reduced, but at the risk of lower sensitivity. The
only way to achieve higher sensitivity is by introducing a gain
using a precision micropower amplifier. The AD8622/AD8624,
with all its features, is well suited to amplify the sensitivity of the
Hall element.
The ADR121 is a precision micropower 2.5 V voltage reference.
A precision voltage reference is required to hold a constant current
so that the Hall voltage only depends on the intensity of the
magnetic field. Using the 4.12k:98.8k resistive divider, the bias
voltage of the Hall element is reduced to 100 mV, leading to only
250 µA of power consumption. The 3-op-amp in-amp
configuration of the AD8622/AD8624 then increases the
sensitivity to 55 mV/mT. The key to high CMRR for this in-amp
configuration are resistors that are well matched (where R1/R2
= R3/R4) from both the resistive ratio and relative drift. The
resistors are important in determining the performance over
manufacturing tolerances, time and temperature. At least 1% or
better resistors are recommended. Using the AD8622/AD8624 to
amplify the sensor signal can reduce power while also achieving
higher sensitivity. The total current consumed is just 1.2 mA,
resulting in 21× improvement in sensitivity/power.
V
SY
V
SY
V
SY
R6
9.9kΩ
R3
9.9kΩ
R5
9.9kΩ
R1
9.9kΩ
R2
9.9kΩ
R7
200Ω
R8
4.12kΩ
R9
98.8kΩ
R4
9.9kΩ
ADR121 – 2.5V
400Ω
×4
HALL
ELEMENT
V
OUT
= 2.5V + × MAGNETIC FIELD (mT)
55mV
mT
07527-052
+
–
+
–
V
SY
–
+
V
SY
AD862x
AD862x
AD862x
AD862x
–
+
+
C3
0.1µF
TO 10µF
C2
0.1µF
C1
1µF TO 10µF
NOTES
1. USE MATCHED RESISTORS FOR IN-AMP.
2. FOR INFORMATION ON C1, C2, AND C3, REFER TO ADR121 DATA SHEET.
Figure 57. Hall Sensor Signal Conditioning