REV. A

–7–

ADXL105

Output Scaling

250 mV/g. This scale factor may not be appropriate for all appli-

cations. The UCA may be used to increase the scale factor. The

simplest implementation would be as shown in Figure 14a.

Since the 0 g offset of the ADXL105 is 2.5 V

± 625 mV, using a

gain of greater than 4 could result in having the UCA output at

0 V or 5 V at 0 g. The solution is to add R3 and VR1, as shown

in Figure 14b, turning the UCA into a summing amplifier. VR1

is adjusted such that the UCA output is V

C

R1

OUT

R2

IN

1

2 CR1

GAIN = –

R1

R2

a. 1-Pole Low-Pass Filter

0.22 F

OUT

20k

IN

20k

0.18 F

b. 2-Pole Bessel Low-Pass Filter

R1

OUT

R2

IN

1

2 CR2

GAIN = –

R1

R2

C

R3

R3

2.5 R1

~

~

c. 1-Pole High-Pass Filter

44.2k

OUT

IN

59k

0.39 F

0.39 F

d. 2-Pole Bessel High-Pass Filter

Figure 15. UCA Used as Active Filters*

Device Bandwidth vs. Resolution

In general the bandwidth selected will determine the noise floor

and hence, the measurement resolution (smallest detectable

acceleration) of the ADXL105. Since the noise of the ADXL105

has the characteristic of white Gaussian noise that contributes

equally at all frequencies, the noise amplitude may be reduced

by simply reducing the bandwidth. So the typical noise of the

ADXL105 is:

Noise (rms) = (225

µg/√Hz) × (√Bandwidth × K)

Where

K

≈ 1.6 for a single-pole filter

K

≈ 1.4 for a 2-pole filter

So given a bandwidth of 1000 Hz, the typical rms noise floor of

an ADLX105 will be:

Noise = (225

µg/√Hz) × (√1000 × 1.6)

= 9 mg rms for a single-pole filter

and

Noise = (225

µg/√Hz) × (√1000 × 1.4)

= 8.4 mg rms for 2-pole filter

Often the peak value of the noise is desired. Peak-to-peak noise

can only be estimated by statistical means. Table I may be used

for estimating the probabilities of exceeding various peak values

given the rms value. The peak-to-peak noise value will give the

best estimate of the uncertainty in a single measurement.

Table I. Estimation of Peak-to-Peak Noise

Nominal Peak-to-

% of Time that Noise Will

Peak Value

Exceed Peak-to-Peak Value

2

× rms

32%

3

× rms

13%

4

× rms

4.6%

5

× rms

1.2%

6

× rms

0.27%

7

× rms

0.047%

8

× rms

0.0063%

The UCA may be configured to act as an active filter with gain

and 0 g offset control as shown in Figure 16.

0.1 F

OUT

IN

GAIN = 2

0.1 F

100k

47k

47k

10k

47k

Figure 16. UCA Configured as an Active Low-Pass Filter

with Gain and Offset

EMC and Electrical Noise

The design of the ADXL105 is such that EMI or magnetic

fields do not normally affect it. Since the ADXL105 is ratiomet-

This is particularly true for noise at the ADXL105’s internal

clock frequency (200 kHz) and its odd harmonics. So maintain-

ing a clean supply voltage is key in preserving the low noise and

high resolution properties of the ADXL105.

Figure 17. Using the component values shown in Figure 17,

noise at 200 kHz is attenuated by approximately –23 dB. As-

suming the ADXL105 consumes 2 mA, there will be a 100 mV

drop across R1. This can be neglected simply by using the

shown in Figure 17.

*For other corner frequencies, consult an active filter handbook.