ltc2640-lz8 Linear Technology Corporation, ltc2640-lz8 Datasheet - Page 17

• Manufacturer Part Number: ltc2640-lz8
• Description: Single 12-/10-/8-bit Spi Vout Dacs With Bidirectional Reference
• Manufacturer: Linear Technology Corporation

OPERATION
The LTC2640 is a family of single voltage output DACs in
8-lead ThinSOT packages. Each DAC can operate rail-to-rail
using an external reference, or with its full-scale voltage
set by an integrated reference. Twelve combinations of
accuracy (12-, 10-, and 8-bit), power-on reset value (zero
or midscale), and full-scale voltage (2.5V or 4.096V) are
available. The LTC2640 is controlled using a 3-wire SPI/
MICROWIRE compatible interface.
Power-On Reset
The LTC2640-HZ/LTC2640-LZ clear the output to zero scale
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 LTC2640
contains circuitry to reduce the power-on glitch: the
analog output typically rises less than 5mV 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.
INPUT WORD (LTC2640-12)
INPUT WORD (LTC2640-10)
INPUT WORD (LTC2640-8)
C3
C3
C3
COMMAND
COMMAND
COMMAND
C2
C2
C2
C1
C1
C1
C0
C0
C0
X
X
X
4 DON'T-CARE BITS
4 DON'T-CARE BITS
4 DON'T-CARE BITS
X
X
X
X
X
X
Figure 2. Command and Data Input Format
X
X
X
MSB
MSB
MSB
D11 D10
D9
D7
D8
D6
D9
D7
D5
D8
D6
D4
The LTC2640-HM/LTC2640-LM provide an alternative
reset, setting the output to midscale when power is fi rst
applied.
Default reference mode selection is described in the Refer-
ence Modes section.
Power Supply Sequencing
The voltage at REF (Pin 6) should be kept within the range
–0.3V ≤ V
ings). Particular care should be taken to observe these
limits during power supply turn-on and turn-off sequences,
when the voltage at V
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, and V
LM/LTC2640-LZ) or 4.096V (LTC2640-HM/LTC2640-HZ)
when in Internal Reference mode, and the voltage at REF
(Pin 6) when in External Reference mode.
D7
D5
D3
V
DATA (12 BITS + 4 DON'T-CARE BITS)
DATA (10 BITS + 6 DON'T-CARE BITS)
DATA (8 BITS + 8 DON'T-CARE BITS)
OUT(IDEAL)
D6
D4
D2
D5
D3
D1
REF
LSB
D4
D2
D0
≤ V
=
D3
D1
X
CC
2
k
LSB
N
D2
D0
+ 0.3V (see Absolute Maximum Rat-
X
CC
V
D1
X
X
(Pin 5) is in transition.
REF
LSB
D0
X
X
REF
X
X
X
is either 2.5V (LTC2640-
X
X
X
LTC2640
X
X
X
2640 F02
X
X
X
17
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