Datasheet

9
LTC1661
Voltage Outputs
Each of the rail-to-rail output amplifiers contained in the
LTC1661 can typically source or sink up to 5mA
(V
CC
= 5V). The outputs swing to within a few millivolts
of either supply when unloaded and have an equivalent
output resistance of 85Ω (typical) when driving a load to
the rails. The output amplifiers are stable driving capaci-
tive loads up to 1000pF.
A small resistor placed in series with the output can be
used to achieve stability for any load capacitance. A 1µF
load can be successfully driven by inserting a 20Ω resistor
in series with the V
OUT
pin. A 2.2µF load needs only a 10Ω
resistor, and a 10µF electrolytic capacitor can be used
without any resistor (the equivalent series resistance of
the capacitor itself provides the required small resis-
tance). In any of these cases, larger values of resistance,
capacitance or both may be substituted for the values
given.
OPERATIO
U
Rail-to-Rail Output Considerations
In any rail-to-rail DAC, the output swing is limited to
voltages within the supply range.
If the DAC offset is negative, the output for the lowest
codes limits at 0V as shown in Figure 2b.
Similarly, limiting can occur near full scale when the REF
pin is tied to V
CC
. If V
REF
= V
CC
and the DAC full-scale error
(FSE) is positive, the output for the highest codes limits at
V
CC
as shown in Figure 2c. No full-scale limiting can occur
if V
REF
is less than V
CC
– FSE.
Offset and linearity are defined and tested over the region
of the DAC transfer function where no output limiting can
occur.
Figure 2. Effects of Rail-to-Rail Operation On a DAC Transfer Curve. (a) Overall Transfer Function (b) Effect of Negative
Offset for Codes Near Zero Scale (c) Effect of Positive Full-Scale Error for Input Codes Near Full Scale When V
REF
= V
CC
1661 F02
INPUT CODE
(b)
OUTPUT
VOLTAGE
NEGATIVE
OFFSET
0V
5120 1023
INPUT CODE
OUTPUT
VOLTAGE
(a)
V
REF
= V
CC
V
REF
= V
CC
(c)
INPUT CODE
OUTPUT
VOLTAGE
POSITIVE
FSE