Datasheet

AD7946
Rev. A | Page 21 of 24
CHAIN MODE, NO BUSY INDICATOR
This mode can be used to daisy-chain multiple AD7946s 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 AD7946s is shown in
Figure 41, and the corresponding timing is given in Figure 42.
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 is complete, the MSB is output
onto SDO, and the AD7946 enters the acquisition phase and
powers down. The remaining data bits stored in the internal
shift register are then 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 14 × N clocks are required to
readback 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 AD7946s in the chain, provided the
digital host has an acceptable hold time. The maximum conver-
sion rate may be reduced due to the total readback time. For
instance, with a 3 ns digital host setup time and 3 V interface,
up to four AD7946s running at a conversion rate of 360 kSPS
can be daisy-chained on a 3-wire port.
CLK
CONVERT
DATA IN
DIGITAL HOST
04656-040
CNV
SCK
SDOSDI
AD7946
B
CNV
SCK
SDOSDI
AD7946
A
Figure 41. Chain Mode, No BUSY Indicator Connection Diagram
SDO
A
= SDI
B
D
A
13 D
A
12 D
A
11
SCK
1 2 3 262728
t
SSDISCK
t
HSDISCK
t
EN
CONVERSIONACQUISITION
t
CONV
t
CYC
t
ACQ
ACQUISITION
CNV
D
A
1
12 13
t
SCK
t
SCKL
t
SCKH
D
A
0
15 1614
SDI
A
= 0
SDO
B
D
B
13 D
B
12 D
B
11 D
A
1D
B
1D
B
0D
A
13 D
A
12
t
HSDO
t
DSDO
t
SSCKCNV
t
HSCKCNV
D
A
0
04656-041
Figure 42. Chain Mode, No BUSY Indicator Serial Interface Timing