Datasheet
AD7985
Rev. A | Page 21 of 28
CS
MODE, 4-WIRE WITHOUT BUSY INDICATOR
This mode is usually used when multiple AD7985 devices are
connected to an SPI-compatible digital host. A connection dia-
gram example using two AD7985 devices is shown in Figure 30,
and the corresponding timing is given in Figure 31.
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 multi-
plexers, but SDI must be returned high before the minimum
conversion time elapses and then held high for the maximum
possible conversion time to avoid the generation of the busy
signal indicator.
When the conversion is complete, the AD7985 enters the
acquisition phase and powers down. Each ADC result can be
read by bringing its SDI input low, which consequently outputs
the MSB 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 16
th
SCK falling edge, SDO returns to high impedance and another
AD7985 can be read.
AD7985
SDI SDO
CNV
SCK
CONVERT
DATA IN
CLK
DIGITAL HOST
CS1
CS2
AD7985
SDI SDO
CNV
SCK
07947-013
Figure 30.
CS
Mode, 4-Wire Without Busy Indicator Connection Diagram
ACQUISITION (n)
ACQUISITION
(n – 1)
ACQUISITION
(n + 1)
1
2
BEGIN DATA (n – 1)
CONVERSION (n)
END DATA (n – 1)
SCK
CNV
SDO
14 15
CONVERSION (n – 1)
END DATA (n – 2)
t
CONV
t
DATA
0
(I/O QUIET
TIME)
(I/O QUIET
TIME)
16 14 15 16
1 15 14 132 012
SDI
(I/O QUIET
TIME)
t
CYC
t
ACQ
t
QUIET
t
SCK
t
DIS
t
DIS
t
HSDO
t
HSDICNV
t
SSDICNV
t
EN
t
EN
t
DSDO
t
HSDO
t
DATA
t
CONV
07947-014
Figure 31.
CS
Mode, 4-Wire Without Busy Indicator Serial Interface Timing