ADIS16204

Rev. 0 | Page 10 of 24

THEORY OF OPERATION

OVERVIEW

The ADIS16204 integrates a dual-axis ±70 g/±37 g MEMS

acceleration sensor into a complete impact/shock measurement

and recording system. The integrated mixed signal processing

circuit digitizes the sensor data, applies corrections factors,

provides many user-programmable features, and offers a simple

communication conduit: the serial peripheral interface (SPI).

ACCELERATION SENSOR

The ADIS16204 base sensor core provides a fully differential

sensor structure and circuit path, resulting in substantial

rejection of electromagnetic interference (EMI) effects. It

uses electrical feedback with zero-force feedback for improved

accuracy and stability. The sensor’s resonant frequency is well

beyond the cut-off frequency of the filter, which adds further

noise rejection to the sensor signal conditioning circuit.

UNIT

SENSING

CELL

MOVABLE

FRAME

FIXED

PLATES

UNIT

FORCING

CELL

ANCHOR

MOVING

PLATE

PLATE

CAPACITORS

ANCHOR

Figure 17. Simplified View of Sensor Under Acceleration

Figure 17 is a simplified view of one of the differential sensor

elements. Each sensor includes several differential capacitor

unit cells. Each cell is composed of fixed plates attached to the

substrate and movable plates attached to the frame. Displace-

ment of the frame changes the differential capacitance, which

is measured by the on-chip circuitry.

Complementary 200 kHz square waves drive the fixed plates.

Electrical feedback adjusts the amplitudes of the square waves

such that the ac signal on the moving plates is 0 V. The feedback

signal is linearly proportional to the applied acceleration. This

unique feedback technique ensures that there is no net electro-

static force applied to the sensor. The differential feedback control

signal is also applied to the input of the filter, where it is filtered

and converted to a single-ended signal.

TEMPERATURE SENSOR

This sensor reflects the sensor’s junction temperature and

provides a convenient temperature measurement for system-

level characterization and calibration feedback.

IMPACT/SHOCK RESPONSE

The sensor’s mechanical structure provides a linear meas-

urement range that is 8 times that of each axis’ actual output

measurement range. Therefore, when considering the response

to high-g, short duration events, the 2-pole, 400 Hz, low-pass

Bessel filter network influences the output response. Figure 18

provides a frequency response for this signal chain. In Figure 19,

the X-axis accelerometer experiences a 560 g shock event that

lasts 0.1 ms, causing the output response to reach 70 g. For users

that need to avoid output saturation, keeping the integration of

the event’s acceleration response (acceleration-time product in

the case of Figure 19) below 56 g-ms is critical.

10

0

–20

–10

–30

–40

–50

10

100

1k

10k

FREQUENCY (Hz)

X: 418.9

Y: –3.291

Figure 18. ADIS16204 Frequency Response

0

50

100

150

200

250

300

350

400

450

500

550

600

–0.50 –0.25

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

0

TIME (ms)

560g, 0.1ms, SIMULATED SHOCK

70g, FILTERED RESPONSE

Figure 19. ADIS16204 Shock Response