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AD7982 Datasheet(PDF) 23 Page - Analog Devices

Part No. AD7982
Description  18-Bit, 1 MSPS PulSAR 7 mW ADC in MSOP/LFCSP
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
Logo AD - Analog Devices

AD7982 Datasheet(HTML) 23 Page - Analog Devices

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AD7982
Data Sheet
Rev. D | Page 22 of 25
CHAIN MODE WITHOUT BUSY INDICATOR
Chain mode without busy indicator can be used to daisy-chain
multiple AD7982 devices on a 3-wire serial interface. The chain
mode without busy indicator feature reduces 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.
Figure 37 shows a connection diagram example using two AD7982
devices, and Figure 38 shows the corresponding timing.
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 completes, the MSB is output onto SDO
and the AD7982 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 capture the data, a digital host using the SCK falling edge
allows a faster reading rate and consequently more AD7982
devices in the chain, provided the digital host has an acceptable
hold time. The maximum conversion rate can be reduced due to
the total readback time.
CONVERT
DATA IN
CLK
DIGITAL HOST
AD7982
SDI
SDO
CNV
B
SCK
AD7982
SDI
SDO
CNV
A
SCK
Figure 37. Chain Mode Without Busy Indicator Connection Diagram
SDOA = SDIB
DA17
DA16
DA15
SCK
1
2
3
34
35
36
tSSDISCK
tHSDISCK
tEN
CONVERSION
ACQUISITION
tCONV
tCYC
tACQ
ACQUISITION
CNV
DA1
16
17
tSCK
tSCKL
tSCKH
DA0
19
20
18
SDIA = 0
SDOB
DB17
DB16
DB15
DA1
DB1DB0DA17
DA16
tHSDO
tDSDO
tSSCKCNV
tHSCKCNV
DA0
Figure 38. Chain Mode Without Busy Indicator Serial Interface Timing


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