ADIS16228

Data Sheet

Rev. B | Page 12 of 28

Real-Time Mode

Set REC_CTRL1[1:0] = 11 to place the device into real-time mode.

In this mode, the device samples only one axis, at a rate of

20.48 kSPS, and provides data on its output register at the SR0

sample rate setting in AVG_CNT[3:0] (see Table 11). Select the

axis of measurement in this mode by reading its assigned register.

For example, select the x-axis by reading X_BUF, using DIN =

0x1400. See Table 49, Table 50, or Table 51 for more information

on the x_BUF registers. Use DIO1 (Pin 15) to help manage external

access to real-time data. For example, this signal is suitable for

driving an interrupt line to initiate a service routine in an

external processor.

SPECTRAL RECORD PRODUCTION

The ADIS16228 produces a spectral record by taking a time

record of data on all three axes, then scaling, windowing, and

performing an FFT process on each time record. This process

repeats for a programmable number of FFT averages, with the FFT

result of each cycle accumulating in the data buffer. After com-

pleting the selected number of cycles, the FFT averaging process

completes by scaling the data buffer contents. Then the data

buffer contents are available to the SPI and output data registers.

SAMPLE RATE/FILTERING

The sample rate for each axis is 20.48 kSPS. The internal ADC

samples all three axes in a time-interleaving pattern (x1, y1, z1,

x2, y2…) that provides even distribution of data across the data

record. The averaging/decimating filter provides a control for

the final sample rate in the time record. By averaging and

decimating the time domain data, this filter provides the ability

to focus the spectral record on lower bandwidths, which produces

finer frequency resolution in each FFT frequency bin. AVG_CNT

(see Table 11) provides the setting for the four dif-ferent sample

rate options in REC_CTRL1[11:8] (SRx, see Table 9). All four

options are available when using the manual FFT, automatic

FFT, and manual time capture modes. When more than one

sample rate option is enabled while the device is in one of the

manual modes, the device produces a spectral record for one

SRx at a time, starting with the lowest number. After completing

the spectral record for one SRx option, the device waits for

another start command before producing a spectral record for

the next SRx option that is enabled in REC_CTRL1[11:8]. When

more than one sample rate option is enabled while the device is in

the automatic FFT mode, the device produces a spectral record

for one SRx option, and then waits for the next automatic trigger,

which occurs based on the time setting in the REC_PRD register

(see Table 10). See Figure 15 for more details on how multiple

SRx options influence data collection and spectral record

production. When in real-time mode, the output data rate

reflects the SR0 setting.

Table 12 provides a list of SRx settings available in the AVG_CNT

register (see Table 11), along with the resulting sample rates, FFT

bin widths, bandwidth, and estimated total noise. Note that each

SRx setting also has associated range settings in the REC_CTRL2

register (see Table 14) and the FFT averaging settings that are

shown in the FFT_AVG1 and FFT_AVG2 registers (see Table 19

and Table 20, respectively).

Table 11. AVG_CNT (Base Address = 0x3A), Read/Write

Bits

Description (Default = 0x9630)

[15:12]

Sample Rate Option 3, binary (0 to 10),

[11:8]

Sample Rate Option 2, binary (0 to 10),

[7:4]

Sample Rate Option 1, binary (0 to 10),

[3:0]

Sample Rate Option 0, binary (0 to 10),

Table 12. Sample Rate Settings and Filter Performance

SRx

Option

Sample

(SPS)

Bin

Width

(Hz)

Bandwidth

(Hz)

Peak Noise

per Bin

(mg)

0

20,480

40

10,240

5.18

1

10,240

20

5120

3.66

2

5120

10

2560

2.59

3

2560

5

1280

1.83

4

1280

2.5

640

1.29

5

640

1.250

320

0.91

6

320

0.625

160

0.65

7

160

0.313

80

0.46

8

80

0.156

40

0.32

9

40

0.078

20

0.23

10

20

0.039

10

0.16

K = 1

Na

1

TRIAXIS

MEMS

ACCEL

20.48kSPS

FFT

FFT

AVERAGE

FREQUENCY

RESPONSE

CORRECTION

SAMPLE RATE SETTING

REC_CTRL1[11:8]

WINDOW SETTING

REC_CTRL1[13:12]

RANGE-SCALE SETTING

WINDOW

FFT

RECORD

1

FFT

RECORD

m

FFT

RECORD

13

FFT

RECORD

0

FFT RECORDS—NONVOLATILE FLASH MEMORY

m = REC_CNTR

REC_CTRL2[3:2]

DATA

BUFFER

SPI

REGISTER

ACCESS

SENSITIVITY ADJUSTMENT

X_SENS, Y_SENS, Z_SENS

CAL_ENABLE[4]

Figure 14. Signal Flow Diagram, REC_CTRL1[1:0] = 00 or 01, FFT Analysis Modes