AD7944BCPZ datasheet(18/28 Pages) AD

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AD7944

Rev. 0 | Page 18 of 28

DATA READING OPTIONS

There are three different data reading options for the AD7944.

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

across acquisition and conversion (see Figure 28 and Figure 29),

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

SCK frequency largely determines the reading option to use.

Reading During Conversion, Fast Host (Turbo or

Normal Mode)

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

the previous (n − 1) conversion. Reading should occur only 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 minimum SCK frequency, follow these

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

For turbo mode (2.5 MSPS)

Number_SCK_Edges = 14; tDATA = 190 ns

fSCK = 14/190 ns = 73.7 MHz

For normal mode (2.0 MSPS)

Number_SCK_Edges = 14; tDATA = 290 ns

fSCK = 14/290 ns = 48.3 MHz

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

which digital activity should not occur, or sensitive bit decisions

may be corrupted.

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

To allow for a 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

occur only 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 (2.5 MSPS)

fSCK = 50 MHz; tDATA = 190 ns

Number_SCK_Edges = 50 MHz × 190 ns = 9.5

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

during acquisition (n).

For normal mode (2.0 MSPS)

fSCK = 40 MHz; tDATA = 290 ns

Number_SCK_Edges = 40 MHz × 290 ns = 11.6

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

read during acquisition (n).

For slow throughputs, the time restriction is dictated by the

throughput required by the user; the host is free to run at any

speed. Similar to reading during acquisition, data access for

slow hosts must take place during the acquisition phase with

additional time into the conversion.

Note that data access spanning conversion requires the CNV

pin 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 Host (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 (2.0 MSPS); however, in turbo

mode, 2.5 MSPS throughput is not achievable.

For the maximum throughput, the only time restriction is that

reading take place during the tACQ (minimum) time. For slow

throughputs, the time restriction is dictated by the throughput

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

for slow hosts, data access must take place during the acquisi-

tion phase.

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