Datasheet

LTC2656
21
2656fa
OPERATION
are powered down, then the main bias generation circuit
block has been automatically shut down in addition to the
individual DAC amplifiers and integrated reference. In this
case, the power-up delay time is 14µs. The power up of
the integrated reference depends on the command that
powered it down. If the reference is powered down using
the select external reference command (0111b), then it can
only be powered back up using select internal reference
command (0110b). However if the reference was powered
down using power-down chip command (0101b), then in
addition to select internal reference command (0110b),
any command that powers up the DACs will also power
up the integrated reference.
Asynchronous DAC Update Using LDAC
In addition to the update commands shown in Table 1, the
LDAC pin asynchronously updates all the DAC registers
with the contents of the input registers.
If CS/LD is high, a low on the LDAC pin causes all the
DAC registers to be updated with the contents of the
input registers.
If CS/LD is low, a low going pulse on the LDAC pin before
the rising edge of CS/LD powers up all the DAC outputs but
does not cause the output to be updated. If LDAC remains
low after the rising edge of CS/LD, then LDAC is recognized,
the command specified in the 24-bit word just transferred
is executed and the DAC outputs are updated.
The DAC outputs are powered up when LDAC is taken
low, independent of the state of CS/LD. The integrated
reference is also powered up if it was powered down us-
ing power-down chip (0101b) command. The integrated
reference will not power up when LDAC is taken low,
if it was powered down using select external reference
(0111b) command.
If LDAC is low at the time CS/LD goes high, it inhibits any
software power-down command (power down n, power-
down chip, select external reference) that was specified
in the input word.
Reference Modes
For applications where an accurate external reference is
not available, the LTC2656 has a user-selectable, integrated
reference. The LTC2656-L has a 1.25V reference that pro-
vides a full-scale DAC output of 2.5V. The LTC2656-H has
a 2.048V reference that provides a full-scale DAC output
of 4.096V. Both references exhibit a typical temperature
drift of 2ppm/°C. Internal reference mode can be selected
by using command 0110b, and is the power-on default. A
buffer is needed if the internal reference is required to drive
external circuitry. For reference stability and low noise, it
is recommended that a 0.1µF capacitor be tied between
REFCOMP and GND. In this configuration, the internal
reference can drive up to 0.1µF capacitive load without any
stability problems. In order to ensure stable operation, the
capacitive load on the REFIN/OUT pin should not exceed
the capacitive load on the REFCOMP pin.
The DAC can also operate in external reference mode us-
ing command 0111b. In this mode, the REFIN/OUT pin
acts as an input that sets the DAC’s reference voltage. The
input is high impedance and does not load the external
reference source. The acceptable voltage range at this
pin is 0.5V ≤ REFIN/OUT ≤ V
CC
/2. The resulting full-scale
outputvoltageis2•V
REFIN/OUT
. For using external refer-
ence at start-up, see the Power Supply Sequencing and
Start-Up section.
Integrated Reference Buffers
Each of the eight DACs in LTC2656 has its own integrated
high performance reference buffer. The buffers have very
high input impedance and do not load the reference voltage
source. These buffers shield the reference voltage from
glitches caused by DAC switching and thus minimize DAC-
to-DAC dynamic crosstalk. Typically DAC-to-DAC crosstalk
is less than 3nV•s. By tying 0.1µF capacitors between
REFCOMP and GND, and also between REFIN/OUT and
GND,thisnumbercanbereducedtolessthan1nV•s.See
the curve DAC-to-DAC Dynamic Crosstalk in the Typical
Performance Characteristics section.
Voltage Outputs
Each of the LTC2656’s eight rail-to-rail output amplifiers con-
tained in these parts has a guaranteed load regulation when
sourcing or sinking up to 15mA at 5V (7.5mA at 3V).
Load regulation is a measure of the amplifier’s ability to
maintain the rated voltage accuracy over a wide range of