ADIS16250

Rev. 0 | Page 10 of 20

THEORY OF OPERATION

OVERVIEW

The core angular rate sensor integrated inside the ADIS16250

is based on the Analog Devices iMEMS technology. This sensor

operates on the principle of a resonator gyro. 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 gyro 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.

TEMPERATURE SENSOR

An internal temperature sensor monitors the sensor’s junction

temperature. The TEMP_OUT data register provides a digital

representation of this measurement. This sensor provides

a convenient temperature measurement for system-level

characterization and calibration feedback.

AUXILIARY ADC FUNCTION

The auxiliary ADC function integrates a standard 12-bit ADC

into the ADIS16250 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

calibrated 2.5 V reference is also provided.

Figure 19 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 supply rails by more than 300 mV. 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 19. Equivalent Analog Input Circuit

Conversion Phase: Switch Open

Track Phase: Switch Closed

For ac applications, removing high frequency components from

the analog input signal is recommended by the use of 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.