LTC2752
21
2752f
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 LTC2752 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 0.75LSB 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 find 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 amplifier
for use with the LTC2752 is critical to the performance of
the system, selecting a precision voltage reference also
requires due diligence. The output voltage of the LTC2752
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
applications: output voltage initial tolerance, output voltage
temperature coefficient 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 coefficient 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
coefficient, then the DAC output voltage along its transfer
characteristic will be very dependent on ambient conditions.
Minimizing the error due to reference temperature coef-
ficient can be achieved by choosing a precision reference
with a low output voltage temperature coefficient 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 contrib-
ute a dominant share of the system’s noise floor. This in
turn can degrade system dynamic range and signal-to-noise
ratio. Care should be exercised in selecting a voltage refer-
ence with as low an output noise voltage as practical for the
system resolution desired. Precision voltage references,
like the LT1236 and LTC6655, produce low output noise in
the 0.1Hz to 10Hz region, well below the 16-bit LSB level
Table 7. Partial List of LTC Precision References Recommended
for Use with the LTC2752 with Relevant Specifications
REFERENCE
INITIAL
TOLERANCE
TEMPERATURE
DRIFT
0.1Hz to 10Hz
NOISE
LT1019A-5,
LT1019A-10
±0.05% Max 5ppm/°C Max 12µV
P-P
LT1236A-5,
LT1236A-10
±0.05% Max 5ppm/°C Max 3µV
P-P
LT1460A-5,
LT1460A-10
±0.075% Max 10ppm/°C Max 20µV
P-P
LT1790A-2.5 ±0.05% Max 10ppm/°C Max 12µV
P-P
LTC6652A-2.048 ±0.05% Max 5ppm/°C Max 2.1ppm
P-P
LTC6652A-2.5 2.1ppm
P-P
LTC6652A-3 2.1ppm
P-P
LTC6652A-3.3 2.2ppm
P-P
LTC6652A-4.096 2.3ppm
P-P
LTC6652A-5 2.8ppm
P-P
LT6655A-25,
LT6655A-5
±0.025% Max 2ppm/°C Max 0.25ppm
P-P
