LTC2668CUJ-16TRPBF View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC2668CUJ-16TRPBF
Linear Technology
'LTC2668CUJ-16TRPBF' PDF : 28 Pages View PDF
LTC2668
Operation
The DAC outputs are powered up when LDAC is taken low,
independent of the state of CS/LD.
If LDAC is low at the time CS/LD goes high, any software
power-down command (power down n, power-down chip,
config/select external reference) that was specified in the
input word is inhibited.
Reference Modes
The LTC2668 has two reference modes (internal and ex-
ternal) with which the reference source can be selected.
In either mode, the voltage at the REF pin and the output
range settings determine the full-scale voltage of each of
the channels.
The device has a precision 2.5V integrated reference with
a typical temperature drift of 2ppm/°C. To use the internal
reference, the REFCOMP pin should be left floating (no
DC path to ground). In addition, the RD bit in the config
register must have a value of 0. This value is reset to 0 at
power-up, or it can be reset using the Config command,
0111b. Figure 9 shows the command syntax.
A buffer is needed if the internal reference is to drive exter-
nal circuitry. For reference stability and low noise, a 0.1μF
capacitor should be tied between REFCOMP and GND. In
this configuration, the internal reference can drive up to
0.1μF with excellent stability. In order to ensure stable
operation, the capacitive load on the REF pin should not
exceed that on the REFCOMP pin.
To use an external reference, tie the REFCOMP pin to
ground. This disables the output of the internal reference
at start-up, so that the REF pin becomes a high impedance
input. Apply the desired reference voltage at the REF pin
after powering up, and set the RD bit to 1 using the Config
command (0111b). This reduces AVP supply current by
approximately 200µA.
The acceptable external reference voltage range is:
0.5V ≤ VREF ≤ AVP – 1.75V.
Integrated Reference Buffers
Each channel has its own integrated high performance
reference buffer. The buffers have very high input imped-
ance 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
6nV • s (0V to 10V range). See the DAC-to-DAC Crosstalk
graph in the Typical Performance Characteristics section.
Voltage Outputs
An amplifier’s ability to maintain its rated voltage accuracy
over a wide range of load conditions is characterized in its
load regulation specification. The change in output voltage
is measured per milliampere of forced load current change.
Each of the LTC2668's high voltage, rail-to-rail output
amplifiers has guaranteed load regulation when sourcing
or sinking up to 10mA with supply headroom as low as
1.4V. Additionally, the amplifiers can drive up to ±14mA
if available headroom is increased to 2.2V or more.
DC output impedance is equivalent to load regulation, and
may be derived from it by simply calculating a change in
units from µV/mA to ohms. The amplifier’s DC output
impedance is typically 0.08Ω when driving a load well
away from the rails.
When drawing a load current from either rail, the output
voltage headroom with respect to that rail is limited by the
60Ω typical channel resistance of the output devices—
e.g., when sinking 1mA, the minimum output voltage
(above V–) is 60Ω • 1mA = 60mV. See the Headroom at
Rails vs Output Current graphs in the Typical Performance
Characteristics section.
CONFIG COMMAND
DON’T CARE
CONFIG
BITS
0 1 1 1 X X X X X X X X X X X X X X X X X X TS RD
2668 F09
Figure 9. Config Command Syntax—Thermal Shutdown (TS) and Reference Disable (RD)
For more information www.linear.com/LTC2668
2668fa
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