ADIS16228SLASHPCBZ datasheet(16/28 Pages) AD

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ADIS16228

Data Sheet

Rev. B | Page 16 of 28

FFT

The FFT process converts each 512-sample time record into

a 256-point spectral record that provides magnitude vs.

frequency data.

FFT Averaging

The FFT averaging function combines multiple FFT records to

reduce the variation of the FFT noise floor, which enables detection

of lower vibration levels. Each SRx option in the REC_CTRL1

number of FFT records to average into the final FFT record.

To enable this function, write the number of averages for each

SRx option that is enabled in the REC_CTRL1 register to the

FFT_AVGx registers. For example, set FFT_AVG2[8:0] = 0x4A

(DIN = 0x9E4A) to set the number of FFT averages to 16 for the

SR2 sample rate option and 1024 for the SR3 sample rate option.

Table 19. FFT_AVG1 (Base Address = 0x0C), Read/Write

Bits

Description (Default = 0x0108)

[15:8]

FFT averages for a single record, SR1 sample rate,

NF in Figure 14; range = 1 to 255, binary

[7:0]

FFT averages for a single record, SR0 sample rate,

NF in Figure 14; range = 1 to 255, binary

Table 20. FFT_AVG2 (Base Address = 0x0E), Read/Write

Bits

Description (Default = 0x0101)

[15:8]

FFT averages for a single record, SR3 sample rate,

NF in Figure 14; range = 1 to 255, binary

[7:0]

FFT averages for a single record, SR2 sample rate,

NF in Figure 14; range = 1 to 255, binary

RECORDING TIMES

When using automatic FFT mode, the automatic recording period

(REC_PRD) must be greater than the total recording time. Use

the following equations to calculate the recording time:

Manual time mode

tR = tS + tPT + tST + tAST

tS = (512/20480) × 2AVG_CNT

Note that the AVG_CNT variable in this relationship refers to

the decimal equivalent of the applicable nibble in the

AVG_CNT register (See Table 11).

FFT modes

tR = NF × (tS + tPT + tFFT) + tST + tAST

Table 21 provides a list of the processing times and settings that

are used in these equations.

Table 21. Typical Processing Times

Function

Time (ms)

Processing Time, tPT

18.7

FFT Time, tFFT

32.7

Number of FFT Averages, NF

Per FFT_AVG1, FFT_AVG2

Storage Time, tST

120.0

Alarm Scan Time, tAST

2.21

The storage time (tST) applies only when a storage method is

selected in REC_CTRL1[3:2] (see Table 9 for more details about

the record storage settings). The alarm scan time (tAST) applies

only when the alarms are enabled in ALM_CTRL[4:0] (see

Table 26 for more information). Understanding the recording

time helps predict when data is available, for systems that cannot

use DIO1 to monitor the status of these operations. Note that

when using automatic FFT mode, the automatic recording period

(REC_PRD) must be greater than the total recording time.

DATA RECORDS

After the ADIS16228 finishes processing FFT data, it stores the

data into the data buffer, where it is available for external access

using the SPI and x_BUF registers (see Table 49 to Table 51).

REC_CTRL1[3:2] (see Table 9) provides programmable conditions

for writing buffer data into the FFT records, which are in

nonvolatile flash memory locations. Set REC_CTRL1[3:2] = 01

to store data buffer data into the flash memory records only

when an alarm condition is met. Set REC_CTRL1[3:2] = 10 to

store every set of FFT data into the flash memory locations. The

flash memory record provides space for a total of 14 records.

Each record stored in flash memory contains a header and

frequency domain (FFT) data from all three axes (x, y, and z).

When all 14 records are full, new records do not load into the

flash memory. The REC_CNTR register (see Table 22) provides

a running count for the number of records that are stored. Set

GLOB_CMD[8] = 1 (DIN = 0xBF01) to clear all of the records in

flash memory.

Table 22. REC_CNTR (Base Address = 0x78), Read Only

Bits

Description (Default = 0x0000)

[15:5]

Not used

[4:0]

Total number of records taken; range = 0 to 14, binary

When used in conjunction with automatic trigger mode and record

storage, FFT analysis for each sample rate option requires no addi-

tional inputs. Depending on the number of FFT averages, the time

between each sample rate selection may be quite large. Note that

selecting multiple sample rates reduces the number of records

available for each sample rate setting, as shown in Table 23.

Table 23. Available Records per Sample Rate Selected

Number of Sample Rates Selected

Available Records

FFT RECORD FLASH ENDURANCE

The REC_FLSH_CNT register (see Table 24) increments when

all 14 records contain FFT data.

Table 24. REC_FLSH_CNT (Base Address = 0x5E), Read Only

Bits

Description

[15:0]

Flash write cycle count; record data only, binary

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