AD22151

–6–

REV. 0

Knowing the values of R3 and R4 from above, and noting Equa-

tion 2, the parallel combination of R2 and R4 required is:

85 k

Ω

15 –1

()

= 6.071 kΩ

Thus:

R2

=

1

1

6.071 k

Ω









–

1

141.666 k

Ω

































= 6.342 kΩ

NOISE

The principle noise component in the sensor is thermal noise

from the Hall cell. Clock feedthrough into the output signal is

largely suppressed with application of a supply bypass capacitor.

Figure 12 shows the power spectral density (PSD) of the output

signal for a gain of 5 mV/Gauss. The effective bandwidth of the

sensor is approximately 5.7 kHz. This is shown by Figure 13

small signal bandwidth vs. gain. The PSD indicates an rms

noise voltage of 2.8 mV within the 3 dB bandwidth of the sen-

sor. A wideband measurement of 250 MHz indicates 3.2 mV

rms (see Figure 14a).

In many position sensing applications bandwidth requirements

can be as low as 100 Hz. Passing the output signal through an

LP filter, for example 100 Hz, would reduce the rms noise volt-

age to

≈1 mV. A dominant pole may be introduced into the

output amplifier response by connection of a capacitor across

feedback resistor R3, as a simple means of reducing noise at the

expense of bandwidth. Figure 14b indicates the output signal of

a 5 mV/G sensor bandwidth limited to 180 Hz with a 0.01

µF

feedback capacitor.

Note: Measurements taken with 0.1

µF decoupling capacitor

°C.

LOGMAG

5 dB

/div

100

H

1

H

START: 64 Hz

NOISE: PSD (8 mV/GAUSS)

STOP: 25.6 kHz

RMS: 64

B MARKER X 64Hz

Y: 3.351

H

Figure 12. Power Spectral Density (5 mV/G)

3dB FREQ. (kHz)

GAIN – mV/GAUSS

7

6

2

5

3

4

12

3

1

0

45

6

Figure 13. Small-Signal Gain Bandwidth

2

[

[

T

3ACQS

T

CH2 PK-PK

19.2mV

Figure 14a. Peak-to-Peak Full Bandwidth (10 mV/Division)

2

[

[

T

7ACQS

T

CH2 PK-PK

4.4mV

Figure 14b. Peak-to-Peak 180 Hz Bandwidth

(10 mV/Division)