AD8842
CLOCK
LD
CLK
DATA
SDI
ADDRESSS
DECODE
AD8842
#1
CODED
PACKAGE
ADDRESS
LD
•
•
•
EN
CLK
SDI
AD8842
#2
•
•
•
Figure 32 shows a three-wire interface for a single AD8842 that
easily cascades for multiple packages. This circuit topology often
called daisy chaining requires preformating all the serial data for
each package in the chain. In the case of the 3 packages shown a
36 bit data word must be completely clocked into all the
AD8842 serial data input registers then the LD strobe would
transfer the data bits into the DAC registers updating one DAC
in each package.
µC
PA0
PA1
DATA
CLOCK
PA2
LD
WR
LD
CLK
SDI
ANALOG CONNECTIONS OMITTED FOR CLARITY
AD8842
#N
SDI
CLK
DAC A
•
V
OU
•
•
V
O
AD8842 #1
Figure 31. Addressing Multiple AD8842 Packages
LD
SDO
DAC H
There is some digital feedthrough from the digital input pins.
Operating the clock only when the DAC registers require updat-
ing minimizes the effect of the digital feedthrough on the analog
signal channels. Measurements of DAC switch feedthrough
shown in the electrical characteristics table were accomplished
by grounding the V
IN
x inputs and cycling the data codes be-
tween all zeros and all ones. Under this condition 5 nV-s of
feedthrough was measured on the output of the switched DAC
channel. An adjacent channel measured less than 1 nV-s of digi-
tal crosstalk. The digital feedthrough and crosstalk photographs
shown in the typical performance characteristics section display
these characteristics (Figures 15 and 17).
SDI
CLK
DAC A
AD8842 #2
•
LD
SDO
DAC H
SDI
CLK
DAC A
AD8842 #3
•
LD
SDO
DAC H
Figure 32. Three-Wire Interface Updates Multiple
AD8842s
REV. 0
–11–
AD [ ANALOG DEVICES ]
