Datasheet

7
LTC1590
TIMING DIAGRAMS
 WUW
D11 A
MSB
D10 A D9 A D1 B
t
1
t
6
D0 B
LSB
t
2
t
4
t
3
t
9
CLK
D
IN
D
OUT
CS/LD
t
5
1590 TD02
D11 A
PREVIOUS WORD
D10 A
PREVIOUS WORD
D0 B
PREVIOUS WORD
D11 A
CURRENT WORD
D9 A
PREVIOUS WORD
t
8
t
7
12
3
23 24
TIMING DIAGRAM
APPLICATIONS INFORMATION
WUU
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Description
The LTC1590 is a dual 12-bit multiplying DAC that has
serial inputs and current outputs. It uses precision R/2R
resistor ladder technology to provide exceptional linearity
and stability. The device operates from a single 5V supply
and provides a ±10V reference input and voltage output
range when used with an external op amp.
Serial I/O
The LTC1590 has a 3-wire SPI/MICROWIRE
TM
compatible
serial port that accepts 24-bit serial words. Data is loaded
MSB first with the first 12 bits controlling DAC A and the
second 12 bits controlling DAC B. Data is shifted into the
D
IN
input on the rising edge of CLK. The CS/LD input must
be taken low before transferring data to enable the CLK
input. After transferring data, CS/LD is pulled high to load
data from the shift register to the DAC registers which
updates both DACs.
The buffered output of the 24-bit shift register is available
on the D
OUT
pin. Multiple DACs can be daisy-chained on
one 3-wire interface by connecting the D
OUT
pin to the D
IN
pin of the next DAC (see the Timing Diagrams section).
MICROWIRE is a trademark of National Semiconductor Corporation.
R
R
R
O
V
REF A
V
REF B
R
FB A
R
FB B
OUT1 A
OUT1 B
OUT2 A
OUT2 B
AGND
1590 F01
I
LKG
C
OUT
CODE
4096
V
REF

R
()()
Equivalent Circuit
Figure 1 shows an equivalent analog circuit for the LTC1590
DACs. R is the reference input, R
REF
, which is nominally
11k. The DAC output is represented by the Thevinin
equivalent current source with a value of:
(Code/4096)(V
REF
/R)
The current source I
LKG
models the junction leakage of the
DAC output switches. I
LKG
is typically less than 5nA at
85°C and decreases by roughly two times for every 10°C
reduction in temperature. C
OUT
is the output capacitance,
and it also comes from the DAC output switches and varies
from 30pF at zero scale to 60pF at full scale. R
O
is the
equivalent output resistance, which varies with digital
input code (see Op Amp Selection section).
Figure 1. Equivalent Circuit