ADXL105 Просмотр технического описания (PDF) - Analog Devices

Номер в каталоге

Компоненты Описание

производитель

ADXL105

High Accuracy 61 g to 65 g Single Axis iMEMS® Accelerometer with Analog Input

Analog Devices 

ADXL105

THEORY OF OPERATION

The ADXL105 is a complete acceleration measurement system

on a single monolithic IC. It contains a polysilicon surface-

micromachined sensor and BiMOS signal conditioning circuitry

to implement an open loop acceleration measurement architec-

ture. The ADXL105 is capable of measuring both positive and

negative accelerations to a maximum level of ± 5 g. The acceler-

ometer also measures static acceleration such as gravity, allow-

ing it to be used as a tilt sensor.

The sensor is a surface micromachined polysilicon structure

built on top of the silicon wafer. Polysilicon springs suspend the

structure over the surface of the wafer and provide a resistance

against acceleration-induced forces. Deflection of the structure

is measured with a differential capacitor structure that consists

of two independent fixed plates and a central plate attached to

the moving mass. A 180° out-of-phase square wave drives the

fixed plates. An acceleration causing the beam to deflect, will

unbalance the differential capacitor resulting in an output square

wave whose amplitude is proportional to acceleration. Phase sensi-

tive demodulation techniques are then used to rectify the signal

and determine the direction of the acceleration.

An uncommitted amplifier is supplied for setting the output

scale factor, filtering and other analog signal processing.

A ratiometric voltage output temperature sensor measures the

exact die temperature and can be used for optional calibration

of the accelerometer over temperature.

VDD

The ADXL105 has two power supply (VDD) pins, 13 and 14.

The two pins should be connected directly together. The output

of the ADXL105 is ratiometric to the power supply. Therefore a

0.22 µF decoupling capacitor between VDD and COM is re-

quired to reduce power supply noise. To further reduce noise,

insert a resistor (and/or a ferrite bead) in series with the VDD

pin. See the EMC and Electrical Noise section for more details.

COM

The ADXL105 has two common (COM) pins, 4 and 7. These

two pins should be connected directly together and Pin 7

grounded.

ST

The ST pin (Pin 6) controls the self-test feature. When this pin

is set to VDD, an electrostatic force is exerted on the beam of the

accelerometer causing the beam to move. The change in output

resulting from movement of the beam allows the user to test for

mechanical and electrical functionality. This pin may be left

open-circuit or connected to common in normal use. The self-

test input is CMOS and TTL compatible.

AOUT

The accelerometer output (Pin 8) is set to a nominal scale fac-

tor of 250 mV/g (for VDD = 5 V). Note that AOUT is guaranteed

to source/sink a minimum of 50 µA (approximately 50 kΩ out-

put impedance). So a buffer may be required between AOUT and

some A-to-D converter inputs.

VMID

VMID is nominally VDD/2. It is primarily intended for use as a

reference output for the on board uncommitted amplifier (UCA)

as shown in Figures 14a and 14b. Its output impedance is ap-

proximately 10 kΩ.

+V

0.22␮F

TOUT

ST

VDD VDD

TEMP

SENSOR

X SENSOR

ADXL105

UNCOMMITTED

AMPLIFIER

COM

COM AOUT VMID

VNIN VIN

R1

R2

UCAOUT

OUTPUT

GAIN

1

2

3

4

SCALE – mV/g

250

500

750

1000

R1 R2

50k⍀

50k⍀

50k⍀

50k⍀

50k⍀

100k⍀

150k⍀

200k⍀

a. Using the UCA to Change the Scale Factor

+V

0.22␮F

TOUT

ST

VDD VDD

TEMP

SENSOR

X SENSOR

ADXL105

UNCOMMITTED

AMPLIFIER

COM

COM AOUT VMID

VNIN VIN

UCAOUT

(250) R2

SCALE = R1

mV/g

R1

+V

R3

10k⍀

R2

OUTPUT

R3 = 5R1

R1 > 20k⍀

b. Using the UCA to Change the Scale Factor

and Zero g Bias

Figure 14. Application Circuit for Increasing Scale Factor

TOUT

The temperature sensor output is nominally 2.5 V at +25°C and

typically changes 8 mV/°C, and is optimized for repeatability

rather than accuracy. The output is ratiometric with supply

voltage.

Uncommitted Amplifier (UCA)

The uncommitted amplifier has a low noise, low drift bipolar

front end design. The UCA can be used to change the scale

factor of the ADXL105 as shown in Figure 14. The UCA may

also be used to add a 1- or 2-pole active filter as shown in Fig-

ures 15a through 15d.

–6–

REV. A

Share Link: