EVAL-SDP-CB1Z View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'EVAL-SDP-CB1Z' PDF : 28 Pages View PDF
AD7685
CHAIN MODE WITH BUSY INDICATOR
This mode can also be used to daisy chain multiple AD7685s on
a 3-wire serial interface while providing a BUSY indicator. 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 three AD7685s is shown
in Figure 44, and the corresponding timing is given in Figure 45.
When SDI and CNV are low, SDO is driven low. With SCK
high, a rising edge on CNV initiates a conversion, selects the
chain mode, and enables the BUSY indicator feature. In this
mode, CNV is held high during the conversion phase and the
subsequent data readback. When all ADCs in the chain have
completed their conversions, the near-end ADC (ADC C in
Figure 44) SDO is driven high. This transition on SDO can be
used as a BUSY indicator to trigger the data readback controlled
by the digital host. The AD7685 then enters the acquisition
phase and powers down. The data bits stored in the internal
shift register are then clocked out, MSB first, 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 + 1
clocks are required to readback the N ADCs. Although the
rising edge can be used to capture the data, a digital host also
using the SCK falling edge allows a faster reading rate and,
consequently, more AD7685s in the chain, provided the digital
host has an acceptable hold time. For instance, with a 5 ns
digital host setup time and 3 V interface, up to eight AD7685s
running at a conversion rate of 220 kSPS can be daisy-chained
to a single 3-wire port.
CNV
SDI AD7685 SDO
A
SCK
CNV
SDI AD7685 SDO
B
SCK
CNV
SDI AD7685 SDO
C
SCK
Figure 44. Chain Mode with BUSY Indicator Connection Diagram
CONVERT
DIGITAL HOST
DATA IN
IRQ
CLK
CNV = SDIA
ACQUISITION
SCK
tHSCKCNV
SDOA = SDIB
SDOB = SDIC
SDOC
tCYC
tCONV
CONVERSION
tSSCKCNV
tEN
tDSDOSDI
tDSDOSDI
tACQ
ACQUISITION
tSCKH
tSCK
1
2
3
4
15 16 17 18 19
31 32 33 34 35
tSSDISCK
tHSDISCK
tSCKL
DA15 DA14 DA13
tHSDO
tDSDO
DA1 DA0
DB15 DB14 DB13
DB1 DB0 DA15 DA14
DA1 DA0
DC15 DC14 DC13
DC1 DC0 DB15 DB14
DB1 DB0 DA15 DA14
47 48 49
tDSDOSDI
tDSDOSDI
tDSDOSDI
DA1 DA0
Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing
Rev. C | Page 23 of 28
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