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Data Sheet

AD7981

Rev. B | Page 21 of 26

CS MODE, 4-WIRE WITH A BUSY INDICATOR

The 4-wire CS mode with a busy indicator is typically used

when a single AD7981 is connected to an SPI-compatible digital

host that has an interrupt input, and 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 requirement is

particularly important in applications where low jitter on CNV

is desired.

The connection diagram is shown in Figure 48, and the

corresponding timing is given in Figure 49.

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

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 conversion time to

guarantee the generation of the busy signal indicator. When the

conversion is complete, SDO goes from high impedance to low.

With a pull-up resistor on the SDO line, use this transition as an

interrupt signal to initiate the data readback controlled by the

digital host. The AD7981 then enters the acquisition phase and

powers down. The data bits are clocked out, MSB first, by

subsequent SCK falling edges. The data is valid on both SCK

edges. Although the rising edge captures the data, a digital host

using the SCK falling edge allows a faster reading rate provided

it has an acceptable hold time. After the optional 17th SCK

falling edge or SDI going high, whichever is earlier, the SDO

returns to high impedance.

Figure 48. 4-Wire CS Mode with Busy Indicator Connection Diagram

Figure 49. 4-Wire CS Mode with Busy Indicator Serial Interface Timing

AD7981 SDO

SDI

DATA INPUT

IRQ

DIGITAL HOST

CONVERT

CS1

CLK

VIO

47kΩ

CNV

SCK

tCONV

tCYC

ACQUISITION

tSSDICNV

ACQUISITION

tACQ

tSCK

tSCKH

tSCKL

CONVERSION

SDI

tHSDICNV

SCK

CNV

SDO

tEN

D15

D14

tHSDO

tDSDO

tDIS

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