ADIS16COM1SLASHPCBZ datasheet(21/37 Pages) AD

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Preliminary Technical Data

ADIS16000/ADIS16229

Rev. PrA | Page 21 of 37

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 = 0x904A) to set the number of FFT averages to 16 for the

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

Table 34. FFT_AVG1

Page 0, Low Byte Address = 0x0E, Read/Write

Bits

Description (Default = 0x0108)

[15:8]

FFT averages for a single record, SR1 sample rate,

NF in Figure 16; range = 1 to 255, binary

[7:0]

FFT averages for a single record, SR0 sample rate,

NF in Figure 16; range = 1 to 255, binary

Table 35. FFT_AVG2

Page 0, Low Byte Address = 0x10, Read/Write

Bits

Description (Default = 0x0101)

[15:8]

FFT averages for a single record, SR3 sample rate,

NF in Figure 16; range = 1 to 255, binary

[7:0]

FFT averages for a single record, SR2 sample rate,

NF in Figure 16; 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

FFT modes

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

Table 36 provides a list of the processing times and settings that

are used in these equations.

Table 36. Typical Processing Times

Function

Time (ms)

Sample Time, tS

1 ÷ fS, per AVG_CNT

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 27 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 41 for more information). Understanding the

recording time helps predict when data is available, for systems

that cannot use DO1 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 ADIS16229 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 59 and Table 60).

REC_CTRL1[3:2] (see Table 27) 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

axes (x, and y). When all 14 records are full, new records do not

load into the flash memory. The REC_CNTR register (see Table

37) 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 37. REC_CNTR

Page 0, Low Byte Address = 0x50), 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 38.

Table 38. Available Records per Sample Rate Selected

Number of Sample Rates Selected

Available Records

FFT RECORD FLASH ENDURANCE

The REC_FLSH_CNT register (see Table 39) increments when

all 14 records contain FFT data.

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