REV. A

–6–

AD22151

Knowing the values of R3 and R4 and noting Equation 2, the

parallel combination of R2 and R4 required is:

85

15 1

6 071

k

k

Ω

Ω

–

.

()

=

Thus:

R

kk

k

2

1

1

6 071

1

141 666

6 342

=









































=

.

–

.

.

ΩΩ

Ω

NOISE

The principal 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, as shown in Figure 13. The

PSD indicates an rms noise voltage of 2.8 mV within the 3 dB

bandwidth of the sensor. 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 a

100 Hz LP filter, for example, 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 were taken with a 0.1

µF decoupling

°C.

LOGMAG

5 dB/div

100 H

1 H

START: 64Hz

NOISE: PSD (8mV/GAUSS)

STOP: 25.6kHz

RMS: 64

B MARKER

64Hz

Y: 3.351 H

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

GAIN – mV/Gauss

23

4

56

0

2

3

4

5

1

6

7

1

3dB FREQUENCY (kHz)

Figure 13. Small Signal Gain Bandwidth vs. Gain

[

[

T

3ACQS

CH2 p-p

19.2mV

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

[

[

T

7ACQS

CH2 p-p

4.4mV

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

(10 mV/Division)