Preliminary Technical Data

ADIS16260/ADIS16265

Rev. PrB | Page 9 of 19

THEORY OF OPERATION

OVERVIEW

The core angular rate sensor integrated inside the ADIS16260/

ADIS16265 is based on the Analog Devices iMEMS technology.

This sensor operates on the principle of a resonator gyroscope.

Two polysilicon sensing structures each contain a dither frame

electrostatically driven to resonance. This provides the necessary

velocity element to produce a Coriolis force during rotation. At

two of the outer extremes of each frame, orthogonal to the

dither motion, are movable fingers placed between fixed fingers to

form a capacitive pickoff structure that senses Coriolis motion.

The resulting signal is fed to a series of gain and demodulation

stages that produce the electrical rate signal output.

The base sensor output signal is sampled using an ADC, and then

the digital data is fed into a proprietary digital calibration circuit.

This circuit contains calibration coefficients from the factory

calibration, along with user-defined calibration registers that can

be used to calibrate system-level errors.

The calibrated gyroscope data (GYRO_OUT) is made available

through output data registers along with temperature, power

supply, auxiliary ADC, and relative angle output calculations.

RELATIVE ANGLE ESTIMATE

The ANGL_OUT register offers the integration of the

GYRO_OUT data. In order for this information to be useful,

the reference angle must be known. This can be accomplished

by reading the register contents at the initial time, before starting

the monitoring, or by setting its contents to zero. This number

is reset to zero when the NULL command is used, after a RESET

command is used, and during power-up. This function can be

used to estimate change in angle over a period. The user is

cautioned to fully understand the stability requirements and

the time period over which to use this estimated relative angle

position.

FACTORY CALIBRATION

The ADIS16260/ADIS16265 provide a factory calibration that

includes correction for initial tolerance and power supply

variation. In addition, the ADIS16265 provides correction for

temperature variation. This calibration includes individual

sensor characterization and custom correction coefficient

calculation.

AUXILIARY ADC FUNCTION

The auxiliary ADC function integrates a standard 12-bit ADC

into the ADIS16260/ADIS16265 to digitize other system-level

analog signals. The output of the ADC can be monitored

through the AUX_ADC control register, as defined in Table 6.

The ADC is a 12-bit successive approximation converter. The

output data is presented in straight binary format with the full-

scale range extending from 0 V to 2.5 V. The 2.5 V upper limit

is derived from the on-chip precision internal reference.

Figure 8 shows the equivalent circuit of the analog input

structure of the ADC. The input capacitor (C1) is typically 4 pF

and can be attributed to parasitic package capacitance. The two

diodes provide ESD protection for the analog input. Care must

be taken to ensure that the analog input signals never exceed

the range of −0.3 V to +3.5 V. This causes the diodes to become

forward-biased and to start conducting. The diodes can handle

10 mA without causing irreversible damage. The resistor is a

lumped component that represents the on resistance of the

switches. The value of this resistance is typically 100 Ω.

Capacitor C2 represents the ADC sampling capacitor and is

typically 16 pF.

C2

C1

R1

VDD

D

D

Figure 8. Equivalent Analog Input Circuit

Conversion Phase: Switch Open

Track Phase: Switch Closed

For ac applications, it is recommended to remove high frequency

components from the analog input signal by using a low-pass

filter on the analog input pin.

In applications where harmonic distortion and signal-to-noise

ratio are critical, the analog input must be driven from a low

impedance source. Large source impedances significantly affect

the ac performance of the ADC. This can necessitate the use of

an input buffer amplifier. When no input amplifier is used to drive

the analog input, the source impedance should be limited to

values lower than 1 kΩ.