ADIS1644XSLASHFLEX datasheet(17/23 Pages) AD

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Data Sheet

ADIS16446

Rev. 0 | Page 17 of 23

DIGITAL PROCESSING CONFIGURATION

GYROSCOPES AND ACCELEROMETERS

Figure 19 details the all signal processing components for the

gyroscopes and accelerometers. The internal sampling system

produces new data in the xGYRO_OUT and xACCL_OUT

output data registers at a rate of 819.2 SPS. The SMPL_PRD

sampling and register update rates. SMPL_PRD, Bits[12:8]

provide a control for reducing the update rate, using an averaging

filter with a decimated output. These bits provide a binomial

control that divides the data rate by a factor of 2 every time this

number increases by 1. For example, set SMPL_PRD, Bits[15:8] =

0x04 (DIN = 0xB704) to set the decimation factor to 16, which

reduces the update rate to 51.2 SPS and the bandwidth to

~25 Hz. The SMPL_PRD, Bits[12:8], setting affects the update

rate for the TEMP_OUT register (see Table 17) as well.

Table 28. SMPL_PRD (Base Address = 0x36), Read/Write

Bits

Description (Default = 0x0001)

[15:13]

Not used

[12:8]

D, decimation rate setting, binomial, see Figure 19

[7:1]

Not used

Clock

1 = internal sampling clock, 819.2 SPS

0 = external sampling clock

INPUT CLOCK CONFIGURATION

SMPL_PRD, Bit 0 (see Table 28) provides a control for

synchronizing the internal sampling to an external clock source.

Set SMPL_PRD, Bit 0 = 0 (DIN = 0xB600) and GPIO_CTRL,

Bit 3 = 0 (DIN = 0xB200) to enable the external clock. See Table 2

and Figure 4 for timing information.

Digital Filtering

The SENS_AVG register in Table 29 provides user controls for

the low-pass filter. This filter contains two cascaded averaging

filters that provide a Bartlett window, FIR filter response (see

Figure 18). For example, set SENS_AVG, Bits[2:0] = 100 (DIN =

0xB804) to set each stage to 16 taps. When used with the

default sample rate of 819.2 SPS and zero decimation

(SMPL_PRD, Bits[15:8] = 0x00), this value reduces the sensor

bandwidth to approximately 16 Hz.

–20

–40

–60

–80

–100

–120

–140

0.001

0.01

0.1

FREQUENCY (

f/fS)

N = 2

N = 4

N = 16

N = 64

Figure 18. Bartlett Window, FIR Filter Frequency Response

(Phase Delay = N Samples)

Dynamic Range

The SENS_AVG, Bits[10:8] provide three dynamic range

settings for the gyroscopes. The lower dynamic range settings

(±250°/sec and ±500°/sec) limit the minimum filter tap sizes to

maintain resolution. For example, set SENS_AVG, Bits[10:8] =

010 (DIN = 0xB902) for a measurement range of ±500°/sec.

Because this setting can influence the filter settings, program

SENS_AVG, Bits[10:8] before programming SENS_AVG, Bits[2:0]

if more filtering is required.

Table 29. SENS_AVG (Base Address = 0x38), Read/Write

Bits

Description (Default = 0x0402)

[15:11]

Not used

[10:8]

Measurement range (sensitivity) selection

100 = ±1000°/sec (default condition)

010 = ±500°/sec, filter taps ≥ 4 (Bits[2:0] ≥ 0x02)

001 = ±250°/sec, filter taps ≥ 16 (Bits[2:0] ≥ 0x04)

[7:3]

Not used

[2:0]

Filter Size Variable B

Number of taps in each stage; NB = 2B

See Figure 18 for filter response

MEMS

SENSOR

LOW-PASS

FILTER

330Hz

CLOCK

819.2SPS

ADC

BARTLETT WINDOW

FIR FILTER

AVERAGE/

DECIMATION

FILTER

EXTERNAL CLOCK ENABLED

BY SMPL_PRD, BIT 0

GYROSCOPES

LOW-PASS, TWO-POLE (404Hz, 757Hz)

ACCELEROMETERS

LOW-PASS, SINGLE-POLE (330Hz)

B = SENS_AVG, BITS[2:0]

NB = 2B

NB = NUMBER OF TAPS

(PER STAGE)

D = SMPL_PRD, BITS[12:8]

ND = 2D

ND = NUMBER OF TAPS

÷ND

x(n)

n = 1

x(n)

n = 1

x(n)

n = 1

Figure 19. Sampling and Frequency Response Block Diagram

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