AD8021 datasheet(18/28 Pages) AD

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AD7986

Rev. B | Page 18 of 28

DATA READING OPTIONS

There are three different data reading options for the AD7986.

There is the option to read during conversion, to split the read

across acquisition and conversion (see Figure 27 and Figure 28),

and in normal mode, to read during acquisition. The desired

SCK frequency largely determines which reading option to

pursue.

Reading During Conversion, Fast Hosts (Turbo or

Normal Mode)

When reading during conversion (n), conversion results are for

the previous (n − 1) conversion. Reading should only occur up

to tDATA and, because this time is limited, the host must use a

fast SCK.

The required SCK frequency is calculated by

DATA

SCK

Edges

SCK

Number

≥

To determine the SCK frequency, follow these examples to read

data from conversion (n − 1).

Turbo mode (2 MSPS):

Number_SCK_Edges = 18; tDATA = 200 ns

fSCK = 18/200 ns = 90 MHz

Normal mode (1.5 MSPS):

Number_SCK_Edges = 18; tDATA = 300 ns

fSCK = 18/300 ns = 60 MHz

The time between tDATA and tCONV is an I/O quiet time where

digital activity should not occur, or sensitive bit decisions may

be corrupt.

Split-Reading, Any Speed Host (Turbo or Normal Mode)

To allow for slower SCK, there is the option of a split read where

data access starts at the current acquisition (n) and spans into the

conversion (n). Conversion results are for the previous (n − 1)

conversion.

Similar to reading during conversion, split-reading should only

occur up to tDATA. For the maximum throughput, the only time

restriction is that split-reading take place during the tACQ

(minimum) + tDATA − tQUIET time. The time between the falling

edge of SCK and CNV rising is an acquisition quiet time, tQUIET.

To determine how to split the read for a particular SCK frequency,

follow these examples to read data from conversion (n − 1).

For turbo mode (2 MSPS),

fSCK = 65 MHz; tDATA = 200 ns

Number_SCK_Edges = 65 MHz × 200 ns = 13

Thirteen bits are read during conversion (n), and five bits are

read during acquisition (n).

For normal mode (1.5 MSPS),

fSCK = 50 MHz; tDATA = 300 ns

Number_SCK_Edges = 50 MHz × 300 ns = 15

Fifteen bits are read during conversion (n), and three bits are

read during acquisition (n).

For slow throughputs, the time restriction is dictated by the

user’s required throughput, and the host is free to run at any

speed. Similar to the reading during acquisition, for slow hosts,

the data access must take place during the acquisition phase

with additional time into the conversion.

Note that data access spanning conversion requires the CNV to

be driven high to initiate a new conversion, and data access is

not allowed when CNV is high. Thus, the host must perform

two bursts of data access when using this method.

Reading During Acquisition, Any Speed Hosts (Turbo or

Normal Mode)

When reading during acquisition (n), conversion results are

for the previous (n − 1) conversion. Maximum throughput is

achievable in normal mode (1.5 MSPS); however, in turbo

mode, 2 MSPS throughput is not achievable.

For the maximum throughput, the only time restriction is that

the reading takes place during the tACQ (minimum) time. For

slow throughputs, the time restriction is dictated by throughput

required by the user, and the host is free to run at any speed.

Thus for slow hosts, data access must take place during the

acquisition phase.

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