Hall sensors

F. Formenti - J. Alozy 22/11/07

1

Hall sensors

Vhall = KBI + ZFO;

KSY44 / KSY46 (Siemens)

K = 150 … 265T/VA

ZFO = ±15mV Max

Rin = 600 … 900Ω

Rout = 1000 … 1500Ω

ΔK/K = -300ppm/oC

ΔZFO/ZFO = -3000ppm/oC

ΔRin/Rin = +3000ppm/oC

ΔRout/Rout = +3000ppm/oC

HGT 2100 (Lake Shore)

K = 110 … 280T/VA

ZFO = ±2.8mV Max

Rin = 400 … 900Ω

Rout = 550 … 1350Ω

ΔK/K = -600ppm/oC

ΔZFO/ZFO = ±360ppm/oC

ΔRin/Rin = +1500ppm/oC

ΔRout/Rout = +1500ppm/oC

Remarks

Highest K, smallest spread

Less important for FS measurement

Not too high, Vdc pileup

Not important, high ADC Zin

Smallest, affects accuracy

Less important for FS measurement

Not important, current source driver

Not important, high ADC Zin

Hall probes have very poor characteristic matching.

K=Hall sensitivity [T/VA]; ZFO=Zero Field Offset [V];


2

Hall sensors

• Best I bias is when getting closer to ADC full scale, at measured B:

Ex. HGT 2100, B=1T, FS=100mV => I = 0.36mA … 0.9mA

Ex. HGT 2100, B=1T, FS=1V => I = 3.6mA … 9mA

• Using common I bias => I bias is set for worst case, i.e. highest K, loosing accuracy for lowest K

Do select Hall probes? Possible from manufacturer? Cost?

• Using independent I biases => I bias can always be set for optimal case, i.e. FS range

Easy to equalize probes (i.e. large dispersion of K parameter) through I bias adjust

• Additional constraint for probes in series is Vdc pileup vs. Rin and I bias:

Ex. 3 x HGT 2100, Rin=400…900Ω, I = 0.36mA … 0.9mA => Vpileup=0.43V…2.4V

BUT: ADC Max IN Common Mode Voltage = 0.95V on FS=100mV

IRin RinRin

Vdc Vdc Vdc

Vcm=Vdc Vcm=2Vdc Vcm=3Vdc

• I bias shall not be too low, to make sure that effect of ZFO is negligible vs. KBI:

Ex. HGT 2100, B=1T, FS=100mV => fix ZFO < KBI /10 => I > 0.1mA … 0.25mA

Compatible with above cases of FS=100mV and FS=1V

• I bias shall not be too high, to limit temperature effect => case of FS=1V and I=9mA may be excessive

FS 100mV looks a good compromise for the case of B=1T


3

Current source

• Use of ZeroDrift series AmpOp => OPA335

• Auto-zeroing technique to provide very low offset and zero drift over time and temperature

Vofs = 5µV Max

ΔVofs/Vofs = 100ppm Max

• Vout margin from supply rails = 100mV Max

• Very good dispersion characteristic vs. production

Ibias = (Vref-Vofs)/R

ΔIbias/Ibias = Vofs/Vref = 2ppm Max

• Very low impact of Vofs over Ibias

• Ibias matching assured by resistor value tolerance => 0.1% (0.01% also possible on request)

This is much better than Hall probe sensitivity matching (ΔK = 155%)

• Resistor thermal stability = Vref thermal stability = 5ppm/oC => to compare with ΔK/K = 600ppm/oC

No problems to use three independent current sources

+

-

Vofs

+

-

Load

Vref = 2.5V

Ibias

5µV Max

R


4

50µV

(Channel with inverted polarity)

Current source

• B-Field card with three equal Ibias and Rload = 249Ω (NOTE: it also includes temp variation of Rload)

50µV

Three ADC channels behave in equal manner with equal resistors:good characteristic matching among channels.

Slope differences correspond to different resistor temperature coefficients:

optimal resistor temperature coefficient can be adopted.

• B-Field card with three different Ibias and Rload = 249Ω (NOTE: it also includes temp variation of Rload)


5

ADC

• Analog Input Common Mode:1.85V 2.65V

Low Ranges(25mV … 100mV)

0V 5VHigh Ranges (1V … 5V)NBV at GND =>

0V 5VHigh Ranges (1V … 5V)

- 0.15V 0.95V

Low Ranges(25mV … 100mV)

NBV = -1.8V … -2.5V =>

0.8V

1.1V

• For best noise performance, it is better to Ground NBV (no charge pump)

=> Set IN Common Mode = 2.5V to assure compatibility for Low and High Ranges.

=> V Hall probes pileup = 0.43V … 2.4V

Vcm=2.5V

0.8V not enough to safely accommodate 3 probes in series and have FS measurement.

• System channel calibration => It is more effective than self calibration

Sel

f C

alib

ratio

n

Sys

tem

Cal

ibra

tion

System calibration will be implemented


6

ADC

Slo

win

g ou

tpu

t ra

te (

long

er

conv

ersi

on

time)

=>

impr

oves

S/N

Incr

easi

ng F

S r

ange

=>

mod

era

tely

impr

oves

S/N

100mV and 1V FS ranges behave in similar way w.r.t S/N


7

ADC

• DC Vin acquisition statistics (1000 samples) at 1V range, without hall probes and three different I bias

Mean = -252.421mV; σ = 4.93µV Mean = -123.374mV; σ = 4.34µV Mean = -632.574mV; σ = 5.00µV

Measured ADC S/N performance – for minimal relative error, measurement has to be done closest to FS

• Example of 1000 samples acquisition

LSB = 2V / 224 0.12µV(bipolar mode, 1V range)

Accuracy 310-5 of FS(single acquisition, 15Hz)

σ 5µV

6σ

= 3

0µ

V

No

ise

fre

e r

eso

luti

on

(16

bit

s)

28 L

SB

co

un

ts

10 LSB 1.2µV

Recording the average, the accuracy improves of √1000 30 times (30µV -> 1µV, i.e. 10-6)


8

ADC

• Offset measurement at constant Temperature

Channel calibration (offset and gain) must be done by averaging samples, not on single acquisition

Average fit = best calibration value


9

Voltage reference

• LT1019A Linear Technology.

B-field circuit architecture is conceived to be only dependent on Vref accuracy and stability.

A stable voltage reference is important for the performance

2.5V nominal voltage

0.05% Max accuracy (i.e. 1.25mV Max)(compensation of relative variations of ADC LSB amplitude and Hall probe voltage gain)

5ppm/oC Max, over 0oC … 70oC (i.e. 0.875mV Max over 70oC) (same as precision resistors)

9µV/ oCi.e. 10ppm/ oC

• Tests with a 5ppm/oC to 20ppm/oC reference and external Vin source.


10

Temperature sensor

• Test with two temperature sensors: inside the Voltage reference (LT1019) and on board (DS60)

LT1019 => Temp coeff 2.1mV/oC

NOTE: absolute temperature depends from process => applicable only for relative temperature change measurement

DS60 => Temp coeff 6.25mV/oC (6.0mV/oC … 6.5mV/oC) and 424mV offset at 0oC

Accuracy = ±2oC (guaranteed stable on long term)

A precise temperature sensor for absolute temperature measurement is necessary

Both sensors have good linearity

2.15mV/oC

6.44mV/o C


11

Conclusions

• Sensor card (electronics) works according datasheet expectations.

• For best performance select very good Vref, some stable resistors and a precise temp sensor.

• Accuracy of 10-4 Max (electronics) can be guaranteed through single data acquisition (FS measurement).

• ADC system calibration gives better results than self calibration.

• Hall probes need equalization for best channel accuracy.

Performance vs. temperature has not yet been checked w.r.t. datasheet

• Sensor card (electronics) temperature test resulted in 10ppm/oC drift (range 20oC to 65oC).

• Better (<10-4) accuracy (electronics) can be achieved either:

by lowering ADC output rate (higher S/N, slower readout)

through over-sampling and averaging technique (can extend limitation of min out rate)

Hall sensors

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