LTC2604 LINER [Linear Technology], LTC2604 Datasheet - Page 12

• Manufacturer Part Number: LTC2604
• Description: Quad 16-Bit Rail-to-Rail DACs in 16-Lead SSOP
• Manufacturer: LINER [Linear Technology]

LTC2604/LTC2614/LTC2624
OPERATIO
other hand, all four DACs are powered down, then the main
bias generation circuit block has been automatically shut
down in addition to the individual DAC amplifiers and
reference inputs. In this case, the power up delay time is
12 s (for V
Voltage Outputs
Each of the four rail-to-rail amplifiers contained in these
parts has 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
load conditions. The measured change in output voltage
per milliampere of forced load current change is ex-
pressed in LSB/mA.
DC output impedance is equivalent to load regulation, and
may be derived from it by simply calculating a change in
units from LSB/mA to Ohms. The amplifiers’ DC output
impedance is 0.025 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
30 typical channel resistance of the output devices; e.g.,
when sinking 1mA, the minimum output voltage = 30 •
1mA = 25mV. See the graph Headroom at Rails vs Output
Current in the Typical Performance Characteristics
section.
The amplifiers are stable driving capacitive loads of up to
1000pF.
Board Layout
The excellent load regulation and DC crosstalk perfor-
mance of these devices is achieved in part by keeping
“signal” and “power” grounds separate.
12
CC
= 5V) or 30 s (for V
U
CC
= 3V).
The PC board should have separate areas for the analog
and digital sections of the circuit. This keeps digital signals
away from sensitive analog signals and facilitates the use
of separate digital and analog ground planes which have
minimal capacitive and resistive interaction with each
other.
Digital and analog ground planes should be joined at only
one point, establishing a system star ground as close to
the device’s ground pin as possible. Ideally, the analog
ground plane should be located on the component side of
the board, and should be allowed to run under the part to
shield it from noise. Analog ground should be a continu-
ous and uninterrupted plane, except for necessary lead
pads and vias, with signal traces on another layer.
The GND pin functions as a return path for power supply
currents in the device and should be connected to analog
ground. Resistance from the GND pin to system star
ground should be as low as possible. When a zero scale
DAC output voltage of zero is desired, the REFLO pin
(pin 2) should be connected to system star ground.
Rail-to-Rail Output Considerations
In any rail-to-rail voltage output device, the output is
limited to voltages within the supply range.
Since the analog outputs of the device cannot go below
ground, they may limit for the lowest codes as shown in
Figure 3b. Similarly, limiting can occur near full scale
when the REF pins are tied to V
DAC full-scale error (FSE) is positive, the output for the
highest codes limits at V
scale limiting can occur if REF x is less than V
Offset and linearity are defined and tested over the region
of the DAC transfer function where no output limiting can
occur.
CC
as shown in Figure 3c. No full-
CC
. If REF x = V
CC
CC
and the
– FSE.
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