Data Sheet

AD7980

Rev. C | Page 17 of 28

CS MODE, 3-WIRE, WITHOUT BUSY INDICATOR

This mode is usually used when a single AD7980 is connected

to an SPI-compatible digital host. The connection diagram is

shown in Figure 31, and the corresponding timing is given in

Figure 32.

With SDI tied to VIO, a rising edge on CNV initiates a

conversion, selects the CS mode, and forces SDO to high

impedance. Once a conversion is initiated, it continues until

completion irrespective of the state of CNV. This can be useful,

for instance, to bring CNV low to select other SPI devices, such

as analog multiplexers; however, CNV must be returned high

before the minimum conversion time elapses and then held

high for the maximum conversion time to avoid the generation

of the busy signal indicator. When the conversion is complete,

the AD7980 enters the acquisition phase and powers down.

When CNV goes low, the MSB is output onto SDO. The

remaining data bits are then clocked 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 16th SCK falling edge or

when CNV goes high, whichever is earlier, SDO returns to high

impedance.

SDI

DATA IN

DIGITAL HOST

CONVERT

CLK

VIO

CNV

SCK

Figure 31. 3-Wire CS Mode Without Busy Indicator

Connection Diagram (SDI High)

SDI=1

CNV

AQUISITION

AQUISITION

CONVERSION

SCK

SDO

D15

D14

D13

D1

D0

1

2

3

14

15

16

Figure 32. 3-Wire CS Mode Without Busy Indicator Serial Interface Timing (SDI High)