ADIS16488

Data Sheet

Rev. B | Page 20 of 36

DIGITAL SIGNAL PROCESSING

GYROSCOPES/ACCELEROMETERS

Figure 20 provides a signal flow diagram for all of the compo-

nents and settings that influence the frequency response for

the accelerometers and gyroscopes. The sample rate for each

accelerometer and gyroscope is 9.84 kHz. Each sensor has

its own averaging/decimation filter stage, which reduces the

update rate to 2.46 kSPS. When using the external clock option

(FNCTIO_CTRL[7:4], see Table 117), the input clock drives a

4-sample burst at a sample rate of 9.84kSPS, which feeds into

the 4x averaging/decimation filter. This results in a data rate

that is equal to the input clock frequency.

AVERAGING/DECIMATION FILTER

The DEC_RATE register (see Table 55) provides user control for

the final filter stage (see Figure 20), which averages and decimates

the accelerometers, gyroscopes, delta angle, and delta velocity data.

The output sample rate is equal to 2460/(DEC_RATE + 1). When

using the external clock option (FNCTIO_CTRL[7:4], see Table

117), replace the “2460” number in this relationship, with the input

clock frequency. For example, turn to Page 3 (DIN = 0x8003),

and set DEC_RATE = 0x18 (DIN = 0x8C18, then DIN = 0x8D00)

to reduce the output sample rate to 98.4 SPS (2460 ÷ 25).

Table 55. DEC_RATE (Page 3, Base Address = 0x0C)

Bits

Description (Default = 0x0000)

[15:11]

Don’t care

[10:0]

Decimation rate, binary format, maximum = 2047

See Figure 20 for impact on sample rate

MAGNETOMETER/BAROMETER

When using the internal sampling clock, the magnetometer

output registers (xMAGN_OUT) update at a rate of 102.5 SPS

and the barometer output registers (BARO_xxx) update at a rate

of 51.25 SPS. When using the external clock, the magnetome-

ters update at a rate of 1/24th of the input clock frequency and

the barometers update at a rate that is 1/48th of the input clock

frequency. The update rates for the magnetometer and barome-

ters do not change with the DEC_RATE register settings.

SYS_E_FLAG[9:8] (see Table 47) offers new data bits for these

registers and the SEQ_CNT register provides a counter function

to help determine when there is new data in the magnetometer

and barometer registers. When SEQ_CNT = 0x0001, there is

new data in the magnetometer and barometer output registers.

The SEQ_CNT register can be useful during initialization to

help synchronize read loops for new data in both magnetometer

and barometer outputs. When beginning a continuous read loop,

read SEQ_CNT, then subtract this value from the maximum value

shown (range) in Table 56 to calculate the number of internal

sample cycles until both magnetometer and barometer data is

new.

Table 56. SEQ_CNT (Page 0, Base Address = 0x06)

Bits

Description

[15:11]

Don’t care

[6:0]

Binary counter: range = 1 to 48/(DEC_RATE + 1)

MEMS

SENSOR

330Hz

÷4

GYROSCOPE

2-POLE: 404Hz, 757Hz

ACCELEROMETER

1-POLE: 330Hz

4×

AVERAGE

DECIMATION

FILTER

SELECTABLE

FIR FILTER BANK

FILTR_BNK_0

FILTR_BNK_1

AVERAGE/DECIMATION FILTER

D = DEC_RATE[10:0] + 1

1

4

4

÷D

1

D

D

FIR

FILTER

BANK

INTERNAL

CLOCK

9.84kHz

DIOx

OPTIONAL INPUT CLOCK

FNCTIO_CTRL[7] = 1

NOTES

1. WHEN FNCTIO_CTRL[7] = 1, EACH CLOCK PULSE ON THE DESIGNATED DIOx LINE (FNCTIO_CTRL[5:4]) STARTS A 4-SAMPLE BURST,

AT A SAMPLE RATE OF 9.84kHz. THESE FOUR SAMPLES FEED INTO THE 4x AVERAGE/DECIMATION FILTER, WHICH PRODUCES A

DATA RATE THAT IS EQUAL TO THE INPUT CLOCK FREQUENCY.

Figure 20. Sampling and Frequency Response Block Diagram