Datasheet

AD7988-1/AD7988-5 Data Sheet
Rev. D | Page 20 of 24
CHAIN MODE
This mode can be used to daisy-chain multiple AD7988-x
devices on a 3-wire serial interface. 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 AD7988-x devices is
shown in Figure 38, and the corresponding timing is given in
Figure 39.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion and selects the chain
mode. 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 AD7988-x
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 16 × 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
AD7988-x 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.
DIGITAL HOST
CONVERT
CLK
DATA IN
AD7988-1/
AD7988-5
SDO
CNV
A
SCK
AD7988-1/
AD7988-5
SDO
CNV
B
SCK
10231-039
SDISDI
Figure 38. Chain Mode Connection Diagram
t
CONV
t
CYC
t
SSDISCK
t
SCKL
t
SCK
t
HSDISCK
t
ACQ
ACQUISITION
t
SSCKCNV
ACQUISITION
t
SCKH
CONVERSION
t
HSCKCNV
SCK
CNV
SDO
B
t
EN
D
A
15 D
A
14 D
A
13
D
B
15 D
B
14 D
B
13 D
B
1 D
B
0 D
A
15 D
A
14 D
A
0D
A
1
D
A
1 D
A
0
t
HSDO
1 2 3 15 16 1714 18 30 31 32
t
DSDO
10231-040
SDI
A
= 0
SDO
A
= SDI
B
Figure 39. Chain Mode Serial Interface Timing