ADXL250AQC 查看數據表（PDF） - Analog Devices

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ADXL250AQC

±5 g to ±50 g, Low Noise, Low Power, Single/Dual Axis iMEMS® Accelerometers

Analog Devices 

ADXL150/ADXL250

DEVICE BANDWIDTH VS. MEASUREMENT

RESOLUTION

Although an accelerometer is usually specified according to its

full-scale g level, the limiting resolution of the device, i.e., its

minimum discernible input level, is extremely important when

measuring low g accelerations.

100mg

660mg

10mg

66mg

1mg

10

100

3dB BANDWIDTH – Hz

6.6mg

1k

Figure 19.␣ ADXL150/ADXL250 Noise Level vs. 3 dB

Bandwidth (Using a “Brickwall” Filter)

The limiting resolution is predominantly set by the measure-

ment noise “floor,” which includes the ambient background

noise and the noise of the ADXL150/ADXL250 itself. The level

of the noise floor varies directly with the bandwidth of the mea-

surement. As the measurement bandwidth is reduced, the noise

floor drops, improving the signal-to-noise ratio of the measure-

ment and increasing its resolution.

The bandwidth of the accelerometer can be easily reduced by

adding low-pass or bandpass filtering. Figure 19 shows the

typical noise vs. bandwidth characteristic of the ADXL150/

ADXL250.

The output noise of the ADXL150/ADXL250 scales with the

square root of the measurement bandwidth. With a single pole

roll-off, the equivalent rms noise bandwidth is π divided by 2 or

approximately 1.6 times the 3 dB bandwidth. For example, the

typical rms noise of the ADXL150 using a 100 Hz one pole post

filter is:

( ) ( ) Noise rms =1mg/ Hz × 100 1.6 =12.25 mg

Because the ADXL150/ADXL250’s noise is, for all practical

purposes, Gaussian 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 I is useful for esti-

mating the probabilities of exceeding various peak values, given

the rms value.

Table I.

Nominal Peak-to-

Peak Value

2.0 × rms

4.0 × rms

6.0 × rms

6.6 × rms

8.0 × rms

% of Time that Noise Will Exceed

Nominal Peak-to-Peak Value

32%

4.6%

0.27%

0.1%

0.006%

RMS and peak-to-peak noise (for 0.1% uncertainty) for various

bandwidths are estimated in Figure 19. As shown by the figure,

device noise drops dramatically as the operating bandwidth is

reduced. For example, when operated in a 1 kHz bandwidth,

the ADXL150/ADXL250 typically have an rms noise level of

32 mg. When the device bandwidth is rolled off to 100 Hz, the

noise level is reduced to approximately 10 mg.

Alternatively, the signal-to-noise ratio may be improved consid-

erably by using a microprocessor to perform multiple measure-

ments and then to compute the average signal level.

Low-Pass Filtering

The bandwidth of the accelerometer can easily be reduced by using

post filtering. Figure 20 shows how the buffer amplifier can be

connected to provide 1-pole post filtering, zero g offset trimming,

and output scaling. The table provides practical component values

+VS

C1

0.1␮F

TP

(DO NOT CONNECT)

+VS

RT

200k⍀

0g TRIM

R2

1M⍀

5

14

ADXL150

GAIN

AMP

SENSOR

CLOCK

+VS

2

DEMODULATOR

25k⍀

5k⍀

R1a R1b

75k⍀ 50k⍀

10

BUFFER

AMP

SCALE

FACTOR

TRIM

(OPTIONAL)

9

SELF-TEST

7

COM

OFFSET

NULL

8

+VS

2

0.1␮F

Cf

R3

100k⍀

+VS

0.1␮F

2

7

OP196 6

3

4

VOUT

DESIRED

F.S.

OUTPUT

RANGE

SCALE FACTOR

76mV/g

؎25g

100mV/g

؎20g

200mV/g

؎10g

400mV/g

؎5g

EXT

AMP

GAIN

2.0

2.6

5.3

10.5

R3 Cf (␮F) Cf (␮F) Cf (␮F)

VALUE 100Hz 30Hz 10Hz

200k⍀

261k⍀

536k⍀

1M⍀

0.0082

0.0056

0.0033

0.0015

0.027 0.082

0.022 0.056

0.010 0.033

0.0056 0.015

Figure 20.␣ One-Pole Post Filter Circuit with SF and Zero g Offset Trims

–10–

REV. 0

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