AD7691
CHAIN MODE WITHOUT BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7691 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 AD7691 devices is
shown in Figure 43, and the corresponding timing is given in
Figure 44.
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
Data Sheet
readback. When the conversion is complete, the MSB is output
onto SDO and the AD7691 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 can allow a faster reading
rate and, consequently, more AD7691 devices in the chain,
provided the digital host has an acceptable hold time. The
maximum conversion rate may be reduced due to the total
readback time.
CONVERT
CNV
SDI AD7691 SDO
A
SCK
CNV
SDI AD7691 SDO
B
SCK
DIGITAL HOST
DATA IN
CLK
Figure 43. Chain Mode Without Busy Indicator Connection Diagram
SDIA = 0
CNV
ACQUISITION
SCK
tHSCKCNV
SDOA = SDIB
SDOB
tCYC
tCONV
tACQ
CONVERSION
ACQUISITION
tSSCKCNV
tSCKL
tSCK
1
2
3
16
17
18
19
20
34
tSSDISCK
tSCKH
tEN
tHSDISCK
DA17 DA16 DA15
DA1
DA0
tHSDO
tDSDO
DB17 DB16 DB15
DB1
DB0 DA17 DA16
Figure 44. Chain Mode Without Busy Indicator Serial Interface Timing
35
36
DA1
DA0
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