ADIS16448SLASHPCBZ datasheet(17/24 Pages) AD

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Data Sheet

ADIS16448

Rev. B | Page 17 of 24

DIGITAL PROCESSING CONFIGURATION

GYROSCOPES/ACCELEROMETERS

Figure 20 provides a diagram that describes 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 register in Table 34 provides two functional controls

that affect sampling and register update rates. SMPL_PRD[12:8]

provides 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[15:8] =

0x04 (DIN = 0xB704) to set the decimation factor to 16. This

reduces the update rate to 51.2 SPS and the bandwidth to

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

for the TEMP_OUT register (see Table 23) as well.

Table 34. SMPL_PRD (Base Address = 0x36), Read/Write

Bits

Description (Default = 0x0001)

[15:13]

Not used

[12:8]

D, decimation rate setting, binomial, see Figure 20

[7:1]

Not used

Clock

1 = internal sampling clock, 819.2 SPS

0 = external sampling clock

INPUT CLOCK CONFIGURATION

SMPL_PRD[0] (see Table 34) provides a control for synchro-

nizing the internal sampling to an external clock source. Set

SMPL_PRD[0] = 0 (DIN = 0xB600) and GPIO_CTRL[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 35 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 20). For example, set SENS_AVG[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[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 19. Bartlett Window, FIR Filter Frequency Response

(Phase Delay = N Samples)

Dynamic Range

The SENS_AVG[10:8] bits 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[10:8] =

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

Because this setting can influence the filter settings, program

SENS_AVG[10:8] before programming SENS_AVG[2:0] if more

filtering is required.

Table 35. 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 19 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[0] = 0

GYROSCOPES

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

ACCELEROMETERS

LOW-PASS, SINGLE-POLE (330Hz)

B = SENS_AVG[2:0]

NB = 2B

NB = NUMBER OF TAPS

(PER STAGE)

D = SMPL_PRD[12:8]

ND = 2D

ND = NUMBER OF TAPS

÷ND

x(n)

n = 1

x(n)

n = 1

x(n)

n = 1

Figure 20. Sampling and Frequency Response Block Diagram

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