ADXL50

–14–

REV. B

Capacitors C3 and C4 are chosen to provide the desired 3 dB

bandwidth. Component values are specified for bandwidths of

10 Hz, 30 Hz, 100 Hz, and 300 Hz. For other 3 dB bandwidths

simply scale the capacitor values; i.e., for a 3 dB bandwidth of

20 Hz, divide the 10 Hz bandwidth numbers by 2.0. The nomi-

nal buffer amplifier output will be +1.8 V

± 19 mV/g. Note that

the ADXL50’s self-test will be fully functional since the buffer

amplifier is operated at unity gain and resistor R1 is large. The

external op amp offsets and scales the output to provide a +2.5 V

± 2 V output over a wide range of full-scale g levels. The exter-

nal op amp may be omitted in high g, low gain applications.

NOISE CONSIDERATIONS

The output noise of the ADXL50 scales with the square root of

its bandwidth. The noise floor may be reduced by lowering the

bandwidth of the ADXL50 either by increasing the value of the

demodulator capacitor or by adding an external filter.

The typical rms noise of the ADXL50J with a bandwidth of

100 Hz and a noise density of 125

µV/√Hz is estimated as

follows:

Noise (rms) = (125

µV/√Hz) √100 = 1.25 mV rms

Peak-to-peak noise may be estimated with the following

equation:

Noise p-p = (6.6) Noise rms

Peak-to-peak noise is thus estimated at 8.25 mV or approxi-

mately 0.4 g p-p. The ADXL50 noise is characteristic of white

noise. Typical rms and p-p noise for various 3 dB bandwidths is

estimated in Figure 25.

1.7

0

10

100

1k

0.73

0.33

1.0

1.4

3dB BANDWIDTH – Hz

0.26

0

0.11

0.05

0.16

0.21

Figure 25. ADXL50 Noise Level and Resolution vs. –3 dB

Bandwidth

Because the ADXL50’s noise is for all practical purposes Gaus-

sian in amplitude distribution, the highest noise amplitudes have

the smallest (yet nonzero) probability. Peak-to-peak noise is,

therefore, difficult to measure and can only be estimated due to

its statistical nature. Table III is useful for estimating the prob-

abilities of exceeding various peak values, given the rms value.

Table III.

Nominal Peak-to-

% of Time that Noise will Exceed

Peak Value

Nominal Peak-to-Peak Value

2.0

× rms

32%

3.0

× rms

13%

4.0

× rms

4.6%

5.0

× rms

1.2%

6.0

× rms

0.27%

6.6

× rms

0.1%

7.0

× rms

0.046%

8.0

× rms

0.006%

If a dc response is not required, as in applications such as mo-

tion detection or vibration measurement, then ac coupling

should be considered. In low g applications, the output voltage

change due to acceleration is small compared to the 0 g offset

voltage drift. Because ac coupling removes the dc component of

the output, the preamp output signal may be amplified consid-

erably without increasing the 0 g level drift. The most effective

way to ac couple the ADXL50 is between the preamp output at

BUFFER

AMP

ADXL50

PRE-AMP

V

OUT

V

IN–

V

PR

R1

R2

R3

1.8V

C4

V

PR

8

9

10

Using this configuration, the system’s ac response is now rolled

can be seen in Figure 27.

is:

pole of the system which is controlled by either the demodulator

capacitor and its associated time-constant or by a dominant post

filter.

As a consequence of ac coupling, any constant acceleration

component gravity will not be detected (because this too is a dc

OBSOLETE