EVAL-AD7985FMCZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-AD7985FMCZ' PDF : 28 Pages View PDF
Data Sheet
AD7985
CHAIN MODE WITHOUT BUSY INDICATOR
This mode can daisy-chain multiple AD7985 devices on a 3-wire
serial interface. It is available only in normal conversion mode
(TURBO is low). This feature is useful for reducing component
count and wiring connections, for example, in isolated multi-
converter applications or for systems with a limited interfacing
capacity. Data readback is analogous to clocking a shift register.
A connection diagram example using two AD7985 devices is
shown in Figure 34, and the corresponding timing is given in
Figure 35.
When SDI and CNV are low, SDO is driven low. With SCK low,
a rising edge on CNV initiates a conversion, selects 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 AD7985 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 the 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
capture the data, a digital host using the SCK falling edge allows a
faster reading rate and consequently more AD7985 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.
TURBO = 0
SDIA = 0
CNV
ACQUISITION
SCK
tHSCKCNV
SDOA = SDIB
SDOB
CONVERT
CNV
SDI AD7985 SDO
A TURBO
SCK
CNV
SDI AD7985 SDO
B TURBO
SCK
DIGITAL HOST
DATA IN
CLK
Figure 34. Chain Mode Without Busy Indicator Connection Diagram
tCYC
tCONV
tACQ
CONVERSION
ACQUISITION
tQUIET
tSCKL
tSCK
1
2
3
14
15
16
17
18
30
tSSDISCK
tSCKH
tEN
tHSDISCK
DA15 DA14 DA13
DA1 DA0
tHSDO
tDSDO
DB15 DB14 DB13
DB1 DB0 DA15 DA14
Figure 35. Chain Mode Without Busy Indicator Serial Interface Timing
31
32
DA1 DA0
Rev. C | Page 23 of 28
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