LTC2755
19
2755f
APPLICATIONS INFORMATION
Op amp offset will contribute mostly to output offset and
gain error, and has minimal effect on INL and DNL. For
example, for the LTC2755-16 with a 5V reference in 5V
unipolar mode, a 250µV op amp offset will cause a 3.3LSB
zero-scale error and a 3.3LSB gain error; but only 0.8LSB
of INL degradation and 0.2LSB of DNL degradation.
While not directly addressed by the simple equations in
Tables 4 and 5, temperature effects can be handled just
as easily for unipolar and bipolar applications. First, con-
sult an op amp’s data sheet to fi nd the worst-case V
OS
and I
B
over temperature. Then, plug these numbers into
the V
OS
and I
B
equations from Table 5 and calculate the
temperature-induced effects.
For applications where fast settling time is important, Ap-
plication Note 74, Component and Measurement Advances
Ensure 16-Bit DAC Settling Time, offers a thorough discus-
sion of 16-bit DAC settling time and op amp selection.
Precision Voltage Reference Considerations
Much in the same way selecting an operational amplifi er
for use with the LTC2755 is critical to the performance of
the system, selecting a precision voltage reference also
requires due diligence. The output voltage of the LTC2755
is directly affected by the voltage reference; thus, any
voltage reference error will appear as a DAC output volt-
age error.
There are three primary error sources to consider when
selecting a precision voltage reference for 16-bit appli-
cations: output voltage initial tolerance, output voltage
temperature coeffi cient and output voltage noise.
Initial reference output voltage tolerance, if uncorrected,
generates a full-scale error term. Choosing a reference
with low output voltage initial tolerance, like the LT1236
(±0.05%), minimizes the gain error caused by the reference;
however, a calibration sequence that corrects for system
zero- and full-scale error is always recommended.
A reference’s output voltage temperature coeffi cient af-
fects not only the full-scale error, but can also affect the
circuit’s apparent INL and DNL performance. If a refer-
ence is chosen with a loose output voltage temperature
coeffi cient, then the DAC output voltage along its transfer
characteristic will be very dependent on ambient conditions.
Minimizing the error due to reference temperature coef-
fi cient can be achieved by choosing a precision reference
with a low output voltage temperature coeffi cient and/or
tightly controlling the ambient temperature of the circuit
to minimize temperature gradients.
As precision DAC applications move to 16-bit and higher
performance, reference output voltage noise may con-
tribute a dominant share of the system’s noise fl oor. This
in turn can degrade system dynamic range and signal-to-
noise ratio. Care should be exercised in selecting a voltage
reference with as low an output noise voltage as practi-
cal for the system resolution desired. Precision voltage
references, like the LT1236, produce low output noise in
the 0.1Hz to 10Hz region, well below the 16-bit LSB level
in 5V or 10V full-scale systems. However, as the circuit
bandwidths increase, fi ltering the output of the reference
may be required to minimize output noise.
Table 7. Partial List of LTC Precision References Recommended
for Use with the LTC2755 with Relevant Specifi cations
REFERENCE
INITIAL
TOLERANCE
TEMPERATURE
DRIFT
0.1Hz to 10Hz
NOISE
LT1019A-5,
LT1019A-10
±0.05% 5ppm/°C 12µV
P-P
LT1236A-5,
LT1236A-10
±0.05% 5ppm/°C 3µV
P-P
LT1460A-5,
LT1460A-10
±0.075% 10ppm/°C 20µV
P-P
LT1790A-2.5 ±0.05% 10ppm/°C 12µV
P-P
