LTC2654 Linear Technology, LTC2654 Datasheet - Page 16

• Manufacturer Part Number: LTC2654
• Description: Quad 16-/12-Bit Rail-to-Rail DACs
• Manufacturer: Linear Technology

OPERATION
LTC2654
The LTC2654 is a family of quad voltage output DACs in
20-lead 4mm × 4mm QFN and in 16-lead narrow SSOP
packages. Each DAC can operate rail-to-rail in external
reference mode, or with its full-scale voltage set by an
integrated reference. Four combinations of accuracy (16-
bit and 12-bit), and full-scale voltage (2.5V or 4.096V)
are available. The LTC2654 is controlled using a 4-wire
SPI/MICROWIRE compatible interface.
Power-On Reset
The LTC2654-L/LTC2654-H clear the output to zero-scale
if PORSEL pin is tied to GND, when power is fi rst applied,
making system initialization consistent and repeatable.
For some applications, downstream circuits are active
during DAC power-up, and may be sensitive to nonzero
outputs from the DAC during this time. The LTC2654 con-
tains circuitry to reduce the power-on glitch. The analog
outputs typically rise less than 10mV above zero-scale
during power-on if the power supply is ramped to 5V in
1ms or more. In general, the glitch amplitude decreases
as the power supply ramp time is increased. See Power-
On-Reset Glitch in the Typical Performance Characteristics
section.
Alternatively, if PORSEL pin is tied to V
The LTC2654-L/LTC2654-H set the output to mid-scale
when power is fi rst applied.
Power Supply Sequencing and Start-Up
For LTC2654 family of parts, the internal reference is
powered-up at start-up by default. If an external reference
is to be used, the REFCOMP pin (Pin 2/Pin 3) must be
hardwired to GND. Having REFCOMP hardwired to GND
at power up, will cause the REFIN/OUT pin to become
high-impedance and will allow for the use of an external
reference at start-up. However in this confi guration, internal
reference will still be ON, even though it is disconnected
from the REFIN/OUT pin and it will draw supply current. In
order to use external reference after power-up, the com-
mand Select External Reference (0111b) should be used
to turn the internal reference off (See Table 1).
16
CC
(Pin 18/Pin 15),
The voltage at REFIN/OUT (Pin 4/Pin 5) should be kept
within the range –0.3V ≤ REFIN/OUT ≤ V
Absolute Maximum Ratings). Particular care should be
taken to observe these limits during power supply turn-on
and turn-off sequences, when the voltage at V
Pin 15) is in transition.
Transfer Function
The digital-to-analog transfer function is
where k is the decimal equivalent of the binary DAC input
code, N is the resolution of the DAC, and V
age at the REFIN/OUT Pin. The resulting DAC output span
is 0V to 2•V
V
LTC2654-H, in Internal Reference Mode.
Table 1. Command and Address Codes
COMMAND*
ADDRESS ( n )*
*Command and address codes not shown are reserved and should not
be used.
A3
C3
0
0
0
0
1
REF
0
0
0
0
0
0
0
0
1
V
OUT(IDEAL)
C2 C1 C0
A2 A1 A0
is nominally 1.25V for LTC2654-L and 2.048V for
0
0
0
0
1
1
1
1
1
0
0
0
0
1
0
0
1
1
0
0
1
1
1
0
0
1
1
1
REF
0
1
0
1
0
1
0
1
1
0
1
0
1
1
=
, as it is necessary to tie REFLO to GND.
⎛
⎝ ⎜
Write to Input Register n
Update (Power-Up) DAC Register n
Write to Input Register n , Update (Power-Up) All
Write to and Update (Power-Up) n
Power-Down n
Power-Down Chip (All DAC’s and Reference)
Select Internal Reference (Power-Up Reference)
Select External Reference (Power-Down Reference)
No Operation
DAC A
DAC B
DAC C
DAC D
All DACs
2
k
N
⎞
⎠ ⎟
• 2 • V
⎡ ⎣
REF
− V
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REFLO
CC
REF
⎤ ⎦ + V
+ 0.3V (see
CC
is the volt-
REFLO
(Pin 18/
2654f