AD7985BCPZ datasheet(19/29 Pages) AD

19 / 29 page

AD7985

Data Sheet

Rev. C | Page 18 of 28

DATA READING OPTIONS

There are three different data reading options for the AD7985.

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

across acquisition and conversion (see Figure 26 and Figure 27),

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

SCK frequency largely determines which 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 must 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

CNVH

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 = 16; tDATA = 190 ns; tCNVH = 10 ns; tEN = 5 ns

fSCK = 16/(190 ns – 10 ns – 5 ns) = 91.5 MHz

For normal mode (2.0 MSPS),

Number_SCK_Edges = 16; tDATA = 290 ns; tCNVH = 10 ns; tEN = 5 ns

fSCK = 16/(290 ns − 10 ns − 5 ns) = 58.2 MHz

The time between tDATA and tCONV is an input/output quiet time

during which digital activity must 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 must 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 = 75 MHz; tDATA = 190 ns; tCNVH = 10 ns; tEN = 5 ns

Number_SCK_Edges = 75 MHz × (190 ns − 10 ns − 5 ns) = 13.1

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

read during acquisition (n).

For normal mode (2.0 MSPS),

fSCK = 50 MHz; tDATA = 290 ns; tCNVH = 10 ns; tEN = 5 ns

Number_SCK_Edges = 50 MHz × (290 ns – 10 ns – 5 ns) = 13.75

Thirteen 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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