Datasheet

LTC2656
18
2656fa
OPERATION
The LTC2656 is a family of octal voltage output DACs in
20-lead 4mm × 5mm QFN and in 20-lead thermally en-
hanced TSSOP 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 ac-
curacy (16-bit and 12-bit), and full-scale voltage (2.5V or
4.096V) are available. The LTC2656 is controlled using a
4-wire SPI/MICROWIRE compatible interface.
Power-On Reset
The LTC2656-L/ LTC2656-H clear the output to zero-scale if
the PORSEL pin is tied to GND, when power is first 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 LTC2656 contains
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.
Alternatively, if the PORSEL pin is tied to V
CC
, the
LTC2656-L/ LTC2656-H sets the output to mid-scale when
power is first applied.
Power Supply Sequencing and Start-Up
For the LTC2656 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 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 configuration, the internal reference
will still be on even though it is disconnected from the
REFIN/OUT pin and will draw supply current. In order
to use external reference after power-up, the command
Select External Reference (0111b) should be used to turn
the internal reference off (see Table 1.)
The voltage at REFIN/OUT should be kept within the range
– 0.3V ≤ REFIN/OUT ≤ V
CC
+ 0.3V if the external reference
is to be used (see 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
CC
is in transition.
Transfer Function
The digital-to-analog transfer function is:
V
OUT(IDEAL)
=
k
2
N






• 2 • V
REF
– V
REFLO
( )
+ V
REFLO
where k is the decimal equivalent of the binary DAC input
code, N is the resolution of the DAC, and V
REF
is the volt-
age at the REFIN/OUT pin. The resulting DAC output span
is0Vto2•V
REF
, as it is necessary to tie REFLO to GND.
V
REF
is nominally 1.25V for LTC2656-L and 2.048V for
LTC2656-H, in internal reference mode.
Table 1. Command and Address Codes
COMMAND*
C3 C2 C1 C0
0 0 0 0 Write to Input Register n
0 0 0 1 Update (Power Up) DAC Register n
0 0 1 0 Write to Input Register n, Update (Power Up) All
0 0 1 1 Write to and Update (Power Up) n
0 1 0 0 Power Down n
0 1 0 1 Power Down Chip (All DACs and Reference)
0 1 1 0 Select Internal Reference (Power-Up Reference)
0 1 1 1 Select External Reference (Power-Down
Reference)
1 1 1 1 No Operation
ADDRESS (n)*
A3 A2 A1 A0
0 0 0 0 DAC A
0 0 0 1 DAC B
0 0 1 0 DAC C
0 0 1 1 DAC D
0 1 0 0 DAC E
0 1 0 1 DAC F
0 1 1 0 DAC G
0 1 1 1 DAC H
1 1 1 1 All DACs
*Command and address codes not shown are reserved and should not
be used.
Serial Interface
The CS/LD input is level triggered. When this input is taken
low, it acts as a chip-select signal, powering on the SDI
and SCK buffers and enabling the input shift register. Data
(SDI input) is transferred at the next 24 rising SCK edges.