AD8400 Analog Devices, AD8400 Datasheet - Page 15

• Manufacturer Part Number: AD8400
• Description: Single-channel Digital Potentiometer
• Manufacturer: Analog Devices

CLK CS
L
P
X
X
X
X
X
NOTE
P = positive edge, X = don’t care, SR = shift register.
The serial data-output (SDO) pin contains an open drain n-channel
FET. This output requires a pull-up resistor in order to transfer data
to the next package’s SDI pin. The pull-up resistor termination
voltage may be larger than the V
V
could operate at V
next device could be set at 5 V. This allows for daisy-chaining
several RDACs from a single processor serial data line. The
clock period needs to be increased when using a pull-up resistor
to the SDI pin of the following device in the series. Capacitive
loading at the daisy-chain node SDO–SDI between devices must
be accounted for to successfully transfer data. When daisy chaining
is used, the CS should be kept low until all the bits of every package
are clocked into their respective serial registers ensuring that the
address bits and data bits are in the proper decoding location.
This would require 20 bits of address and data complying to the
word format provided in Table I if two AD8403 four-channel
RDACs are daisy-chained. Note, only the AD8403 has a SDO
pin. During shutdown SHDN the SDO output pin is forced to
the off (logic high) state to disable power dissipation in the pull-up
resistor. See Figure 6 for equivalent SDO output circuit schematic.
The data setup and data hold times in the specification table
determine the data valid time requirements. The last 10 bits of
the data word entered into the serial register are held when CS
returns high. At the same time CS goes high it gates the address
decoder, which enables one of the two (AD8402) or four (AD8403)
positive edge triggered RDAC latches. See Figure 5 detail and
Table III Address Decode Table.
A1
0
0
1
1
DD
of 8 V) of the AD8403 SDO output device, e.g., the AD8403
L
L
P
H
X
H
H
Table II. Input Logic Control Truth Table
RS
H
H
H
H
L
P
H
Table III. Address Decode Table
A0
0
1
0
1
DD
SHDN Register Activity
H
H
H
H
H
H
L
= 3.3 V and the pull-up for interface to the
No SR effect, enables SDO pin.
Shift one bit in from the SDI pin.
The tenth previously entered bit is
shifted out of the SDO pin.
Load SR data into RDAC latch
based on A1, A0 decode (Table III).
No Operation
Sets all RDAC latches to midscale,
wiper centered, and SDO latch
cleared.
Latches all RDAC latches to 80
Open circuits all resistor
A-terminals, connects W to B,
turns off SDO output transistor.
DD
Latch Decoded
RDAC#1
RDAC#2
RDAC#3 AD8403 Only
RDAC#4 AD8403 Only
supply (but less than max
H
.
The target RDAC latch is loaded with the last eight bits of the
serial data word completing one DAC update. In the case of the
AD8403 four separate 10-bit data words must be clocked in to
change all four VR settings.
All digital pins are protected with a series input resistor and
parallel Zener ESD structure shown in Figure 7a. This structure
applies to digital pins CS, SDI, SDO, RS, SHDN, CLK. The
digital input ESD protection allows for mixed power supply
applications where 5 V CMOS logic can be used to drive an
AD8400, AD8402, or AD8403 operating from a 3 V power sup-
ply. The analog pins A, B, and W are protected with a 20 Ω
series resistor and parallel Zener (see Figure 7b).
C
A
= 90.4pF
SHDN
CLK
SDI
CS
RS
CLK
SDI
CS
(DW / 256) + 30pF
A
REGISTER
DIGITAL
SERIAL
Ω
C
A
AD8400/AD8402/AD8403
PINS
AD8403
A, B, W
RDAC
10k
D
CK RS
1k
W
20
Q
120pF
C
C
B
DECODE
REGISTER
W
ADDR
= 90.4pF
SERIAL
LOGIC
C
B
[1 – (DW / 256)] + 30pF
RDAC 1
RDAC 2
RDAC 4
B
SDO