ADXRS613

Rev. 0 | Page 9 of 12

THEORY OF OPERATION

The ADXRS613 operates on the principle of a resonator

gyroscope. Two polysilicon sensing structures each contain a

dither frame that is electrostatically driven to resonance,

producing the necessary velocity element to produce a Coriolis

force while rotating. At two of the outer extremes of each frame,

orthogonal to the dither motion, are movable fingers that are

placed between fixed pickoff 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 dual-sensor design rejects

external g forces and vibration. Fabricating the sensor with the

signal conditioning electronics preserves signal integrity in

noisy environments.

The electrostatic resonator requires 18 V to 20 V for operation.

Because only 5 V are typically available in most applications, a

charge pump is included on-chip. If an external 18 V to 20 V

supply is available, the two capacitors on CP1 through CP4 can

be omitted, and this supply can be connected to the CP5 pin

(6D, 7D). Note that CP5 should not be grounded when power is

applied to the ADXRS613. Although no damage occurs, under

certain conditions, the charge pump may fail to start up after

the ground is removed without first removing power from the

ADXRS613.

SETTING BANDWIDTH

bandwidth of the ADXRS613 rate response. The –3 dB

()

OUT

OUT

OUT

C

R

f

×

×

×

=

π

2

1

during manufacturing to be 180 kΩ ± 1%. Any external resistor

applied between the RATEOUT pin (1B, 2A) and SUMJ pin

(1C, 2C) results in:

(

)

()

EXT

EXT

OUT

R

R

R

+

×

=

kΩ

180

kΩ

180

In general, an additional hardware or software filter is added to

attenuate high frequency noise arising from demodulation

spikes at the gyroscope’s 14 kHz resonant frequency (the noise

spikes at 14 kHz can be clearly seen in the power spectral

density curve shown in Figure 21). Typically, this additional

filter’s corner frequency is set to greater than 5× the required

bandwidth to preserve good phase response.

Figure 22 shows the effect of adding a 250 Hz filter to the

output of an ADXRS613 set to 40 Hz bandwidth (as shown

in Figure 21). High frequency demodulation artifacts are

attenuated by approximately 18 dB.

0.1

0.01

0.000001

0.00001

0.0001

0.001

10

100k

1k

100

(Hz)

10k

Figure 22. Noise Spectral Density with Additional 250 Hz Filter

TEMPERATURE OUTPUT AND CALIBRATION

It is common practice to temperature-calibrate gyroscopes to

improve their overall accuracy. The ADXRS613 has a tempera-

ture proportional voltage output that provides input to such a

calibration method. The temperature sensor structure is shown

in Figure 23. The temperature output is characteristically

nonlinear, and any load resistance connected to the TEMP

output results in decreasing the TEMP output and temperature

coefficient. Therefore, buffering the output is recommended.

The voltage at the TEMP pin (3F, 3G) is nominally 2.5 V at

at 25°C. Although the TEMP output is highly repeatable, it has

only modest absolute accuracy.

Figure 23. ADXRS613 Temperature Sensor Structure

CALIBRATED PERFORMANCE

Using a 3-point calibration technique, it is possible to calibrate

the null and sensitivity drift of the ADXRS613 to an overall

accuracy of nearly 200°/hour. An overall accuracy of 40°/hour

or better is possible using more points.

Limiting the bandwidth of the device reduces the flat-band

noise during the calibration process, improving the measure-

ment accuracy at each calibration point.