AD8021 datasheet(22/28 Pages) AD

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AD7986

Rev. B | Page 22 of 28

CS MODE, 4-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7986 is connected

to an SPI-compatible digital host with an interrupt input and

when it is desired to keep CNV, which is used to sample the

analog input, independent of the signal used to select the data

reading. This independence is particularly important in

applications where low jitter on CNV is desired. This mode is

only available in normal conversion mode (TURBO = low).

The connection diagram is shown in Figure 31, and the

corresponding timing is given in Figure 32.

With SDI high, a rising edge on CNV initiates a conversion,

selects the CS mode, and forces SDO to high impedance. In this

mode, CNV must be held high during the conversion phase and

the subsequent data readback. (If SDI and CNV are low, SDO is

driven low.) Prior to the minimum conversion time, SDI can be

used to select other SPI devices, such as analog multiplexers,

but SDI must be returned low before the minimum conversion

time elapses and then held low for the maximum possible

conversion time to guarantee the generation of the busy signal

indicator. When the conversion is complete, SDO goes from

high impedance to low impedance. With a pull-up on the SDO

line, this transition can be used as an interrupt signal to initiate

the data readback controlled by the digital host. The AD7986

then enters the acquisition phase and powers down. The data

bits are then clocked out, MSB first, by subsequent SCK falling

edges. The data is valid on both SCK edges. Although the rising

edge can be used to capture the data, a digital host using the

SCK falling edge allows a faster reading rate, provided that it

has an acceptable hold time. After the optional 19th SCK falling

edge or SDI going high (whichever occurs first), SDO returns to

high impedance.

AD7986

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

IRQ

VIO

47k

Ω

CS1

TURBO

Figure 31. CS Mode, 4-Wire with Busy Indicator Connection Diagram

(I/O QUIET

TIME)

SDO

D17

D16

tDIS

tQUIET

SCK

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

SDI

CNV

tSSDICNV

tHSDICNV

TURBO = 0

Figure 32. CS Mode, 4-Wire with Busy Indicator Serial Interface Timing

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