Sensitivity

Philips Semiconductors

Magnetoresistive sensors for

magnetic ﬁeld measurement

General

BARBER-POLE SENSORS

A number of Philips’ magnetoresistive sensors use a ‘barber-pole’ construction to linearize the R-H relationship, incorporating slanted strips of a good conductor to rotate the current. This type of sensor has the widest range of linearity, smaller resistance and the least associated distortion than any other form of linearization, and is well suited to medium and high ﬁelds.

equal widths. The characteristic is plotted in Fig 20 and it can be seen that for small values of Hy relative to H0, the R-H dependence is linear. In fact this equation gives the same linear R-H dependence as the planar Hall-effect sensor, but it has the magnitude of the magnetoresistive sensor.

handbook, halfpage	Permalloy	Barber pole
y + Ι		Ι	−
			−
	Ι

handbook,R halfpage

ΔR

MBH615

Magnetization

MBH614

Fig.19 Linearization of the magnetoresistive effect with barber-poles (current and magnetization shown in quiescent state).

The current takes the shortest route in the high-resistivity gaps which, as shown in Fig 19, is perpendicular to the barber-poles. Barber-poles inclined in the opposite direction will result in the opposite sign for the R-H characteristic, making it extremely simple to realize a Wheatstone bridge set-up.

The signal voltage of a Barber-pole sensor may be calculated from the basic equation (1) with Θ = φ + 45˚ (θ = + 45˚):

0

	−1	−0.5	0	0.5	1	HY
	−1	−0.5	0	0.5	1

						H0

Fig.20 Calculated R-H characteristic of a barber-pole sensor.

Barber-pole sensors require a certain magnetization state. A bias ﬁeld of several hundred A/m can be generated by the sensing current alone, but this is not sufﬁcient for sensor stabilization, so can be neglected. In most applications, an external ﬁeld is applied for this purpose.

Due to the high demagnetization, in most applications ﬁeld components in the z-direction (perpendicular to the layer plane) can be ignored. Nearly all sensors are most sensitive to ﬁelds in the y-direction, with Hx only having a limited or even negligible inﬂuence.

UBP = ρ⊥l

⎛

⎞

⎛

Δρ⎞

Δρ Hy

⎛ Hy⎞

2⎞

⎝

-----

⎠

α⎜1

+ --

⎝

------

±------ ------

–

------

⎟

⎝

⎠

ρ H0

⎠

⎝ H

(9)

Deﬁnition of the sensitivity S contains the signal and ﬁeld variations (DU and DH), as well as the operating voltage U0 (as DU is proportional to U0):

So =	ΔU⎛ 1	⎞	=	⎛	ΔU ⎞	(10)
	------- ------			⎝ ---------------U0ΔH⎠
where a is a constant arising from the partial shorting of the	ΔH⎝ U0⎠

resistor, amounting to 0.25 if barber-poles and gaps have

2000 Sep 06