Datasheet

AD7986
Rev. B | Page 23 of 28
CHAIN MODE WITHOUT BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7986 devices
on a 3-wire serial interface. It is only available in normal conver-
sion mode (TURBO = low). This feature is useful for reducing
component count and wiring connections, for example, in
isolated multiconverter applications or for systems with a
limited interfacing capacity. Data readback is analogous to
clocking a shift register.
A connection diagram example using two AD7986 devices is
shown in Figure 33, and the corresponding timing is given in
Figure 34.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion, selects the chain
mode, and disables the busy indicator. In this mode, CNV is
held high during the conversion phase and the subsequent data
readback. When the conversion is complete, the MSB is output
onto SDO, and the AD7986 enters the acquisition phase and
powers down. The remaining data bits stored in the internal
shift register are clocked by subsequent SCK falling edges. For
each ADC, SDI feeds the input of the internal shift register and
is clocked by the SCK falling edge. Each ADC in the chain
outputs its data MSB first, and 18 × N clocks are required to
read back the N ADCs. 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 and consequently more AD7986 devices in the chain,
provided that the digital host has an acceptable hold time. The
maximum conversion rate may be reduced due to the total
readback time.
CONVERT
DATA IN
CLK
DIGITAL HOST
AD7986
SDI SDO
CNV
B
SCK
AD7986
SDI SDO
CNV
A
SCK
TURBO TURBO
07956-026
Figure 33. Chain Mode Without Busy Indicator Connection Diagram
SDO
A
= SDI
B
D
A
17 D
A
16 D
A
15
SCK
1 2 3 39 53 54
t
EN
CONVERSION
ACQUISITION
t
CONV
t
CYC
t
ACQ
ACQUISITION
CNV = SDI
A
D
A
1
4 17
t
SCK
t
SCKH
t
SCKL
D
A
0
19 3818
SDO
B
= SDI
C
D
B
17 D
B
16 D
B
15 D
A
1D
B
1 D
B
0 D
A
17 D
A
16
55
t
SSDISCK
t
HSDISCK
t
HSDO
t
DSDO
SDO
C
D
C
17 D
C
16 D
C
15 D
A
1 D
A
0D
C
1 D
C
0 D
A
16
21 35 3620
37
D
B
1 D
B
0 D
A
17D
B
17 D
B
16
t
DSDOSDI
t
HSCKCNV
D
A
0
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
t
DSDOSDI
07956-027
TURBO= 0
t
SSCKCNV
Figure 34. Chain Mode Without Busy Indicator Serial Interface Timing