ADPD4200 datasheet(18/93 Pages) AD

18 / 93 page

Data Sheet

ADPD4200

THEORY OF OPERATION

analog.com

Rev. 0 | 18 of 93

Decimation

The DECIMATE_FACTOR_x bits determine the number of time slot

values used to create a 32-bit final sample value at a rate of

Sample Rate= 1/TIMESLOT_PERIOD_x/

DECIMATE_FACTOR_x+1

If DECIMATE_FACTOR_x is 0, the output sample rate equals the

time slot rate. The final value is the sum of the decimated samples.

There is no divide by (DECIMATE_FACTOR_x + 1) operation

performed on the decimated data, but final data values can be

bit shifted to the right before being written to the FIFO, creating

a direct average when the number of samples is a power of 2.

DECIMATE_TYPE_x selects the method of decimation used. A

setting of 0 selects a simple block sum with other settings allowing

higher order CIC filters up to fourth order. If using higher order

CIC filters for the signal data, the dark data still uses the simple

block sum at the same decimation rate. Each time slot maintains

its own block sum or CIC filter state. The entire decimation path

uses a 32-bit datapath. When using the CIC filter, the number of

bits required for the result is dependent on the number of pulses,

the decimation rate, and the order of the CIC filter according to the

following equation:

NBITS=14+log2Number of Pulses +

log2Decimation Rate CIC Order

It is up to the user to ensure that there is no undesired overflow.

Final data results can be read from data registers or a 512-byte

data FIFO. Data written to the FIFO is configurable to allow the

different data registers, formats, and data sizes as required. Each

time slot can use its own decimation rate. Data from each time slot

is written to the FIFO at its respective ODR.

Subsampling

The ADPD4200 supports a subsampling mode that allows selected

time slots to run at slower sampling rates than the programmed

sampling rate. For example, in a multiparameter application where

most of the measurements must be taken at a sampling rate of

300 Hz but one of the measurements must only be taken at

25 Hz, the subsampling mode can be used on the time slot that

only needs to operate at 25 Hz. To enable subsampling mode

for a specific time slot, set the SUBSAMPLE_x bit to 1 and set

the DECIMATE_FACTOR_x bits to the desired subsampling rate.

The subsampled time slot then samples only once every (DECI-

MATE_FACTOR_x + 1) cycle, instead of operating every time slot

sequence. If other time slots are decimating at the same rate, the

subsampled cycles occur at the same time the decimated data is

presented to the FIFO. For example, if Time Slot A is operating

at 300 Hz but decimating to 25 Hz, and Time Slot B is set to

subsample by 12, both time slots write the FIFO during the same

time slot sequence and at the same rate.

More complicated patterns can be made if the decimate and sub-

sample rates for the enabled time slots are different. The user

must manage the varying packet sizes by reading the data in

multiples of the repeating packet size. For example, if Time Slot A

is not decimating or subsampling, Time Slot B is subsampling every

second cycle, and Time Slot C is subsampling every fourth cycle,

the data pattern written to the FIFO is A, AB, A, ABC, and so on, as

the repeating packet.

Decimation and subsampling have the same effect on the output

data rate. The only difference is that the decimated time slots

operate every input cycle but produce data at the slower rate using

the on-chip decimating filter. The subsampling time slots only occur

at the slower rate.

Status bytes are written to the FIFO every wake-up period, regard-

less of which time slots execute. Using the same example as the

different decimate and subsample rates scenario, but with a status

byte enabled, the pattern is AS, ABS, AS, ABCS, and so on, where

S is a status byte.

FIFO

Data is written to the FIFO at the end of each sampling period.

This packet can include 0, 8-, 16-, 24-, or 32-bit data for each

of the dark data and signal data values. The bit alignment of

the data written to the FIFO is selectable with a shift of 0 bits

to 31 bits, with saturation provided. Lower bits are ignored. The

DARK_SHIFT_x and SIGNAL_SHIFT_x bit fields select the number

of bits to shift the output data to the right before writing to the

FIFO. The DARK_SIZE_x and SIGNAL_SIZE_x bit fields select the

number of bytes of each field to be written from 0 bytes to 4 bytes.

When set to 0, no data is written for that data type. If there are any

nonzero bits at more significant bit positions than those selected,

the data written to the FIFO is saturated. If both channels are

enabled, all selected Channel 1 data values are written to the FIFO

first, followed by the Channel 2 data.

For example, in modes that utilize dark data, the eight upper bits

of the dark data can be stored with 24 appropriately selected bits

from the signal data for each time slot to allow detection of whether

the ambient light is becoming large, while limiting the size of the

amount of data transferred.

Data is written to the FIFO at the end of the sampling period only

if there is enough FIFO space left to write data for each active

timeslot. For example, if one active timeslot is running at an ODR

of 100 Hz and a second timeslot is decimating by 4 or subsampling

at 1/4th the rate of the first timeslot for an ODR of 25 Hz, data is

only going to be written to the FIFO at the end of the sampling

period if there is enough room for both active timeslots to write

data, regardless of whether or not the timeslot that is decimating

or subsampling is supposed to write data during that sampling

period. It is up to the user to manage the data appropriately at the

microprocessor end when using timeslots with different decimation

and/or subsampling rates.

ADPD4200 датащи(PDF) 18 Page - Analog Devices

ADPD4200 датащи(HTML) 18 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93

датащи скачать

ссылки URL