LTC2751-14 Linear Technology Corporation, LTC2751-14 Datasheet - Page 17

• Manufacturer Part Number: LTC2751-14
• Description: Ltc2751-14 - Current Output 14-bit Softspan Dacs With Parallel I/o
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Op amp offset will contribute mostly to output offset and
gain error and has minimal effect on INL and DNL. For
the LTC2751-16, a 250µV op amp offset will cause about
0.8LSB INL degradation and 0.2LSB DNL degradation
with a 5V reference. For the LTC2751 programmed in 5V
unipolar mode, the same 250µV op amp offset will cause
a 3.3LSB zero-scale error and a 3.3LSB gain error.
While not directly addressed by the simple equations in
Tables 3 and 4, temperature effects can be handled just as
easily for unipolar and bipolar applications. First, consult
an op amp’s data sheet to fi nd the worst-case V
over temperature. Then, plug these numbers in the V
and I
ture-induced effects.
For applications where fast settling time is important,
Application Note 74, “Component and Measurement
Advances Ensure 16-Bit DAC Settling Time,” offers a
thorough discussion 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 LTC2751 is critical to the performance
of the system, selecting a precision voltage reference
also requires due diligence. The output voltage of the
LTC2751 is directly affected by the voltage reference;
thus, any voltage reference error will appear as a DAC
output voltage 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 affects
not only the full-scale error, but can also affect the circuit’s
INL and DNL performance. If a reference is chosen with
a loose output voltage temperature coeffi cient, then the
B
equations from Table 4 and calculate the tempera-
OS
and I
OS
B
DAC output voltage along its transfer characteristic will
be very dependent on ambient conditions. Minimizing
the error due to reference temperature coeffi cient can be
achieved by choosing a precision reference with a low
output voltage temperature coeffi cient and/or tightly con-
trolling 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 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 6. Partial List of LTC Precision References Recommended
for Use with the LTC2751 with Relevant Specifi cations
REFERENCE
LT1019A-5,
LT1019A-10
LT1236A-5,
LT1236A-10
LT1460A-5,
LT1460A-10
LT1790A-2.5
Grounding
As with any high resolution converter, clean grounding is
important. A low impedance analog ground plane and star
grounding techniques should be used. I
to the star ground with as low a resistance as possible.
When it is not possible to locate star ground close to
I
pin to star ground. This minimizes the voltage drop from
this pin to ground caused by the code dependent current
fl owing to ground. When the resistance of this circuit
board trace becomes greater than 1Ω, a force/sense am-
plifi ed confi guration should be used to drive this pin (see
Figure 2). This preserves the excellent accuracy (1LSB
INL and DNL) of the LTC2751-16.
OUT2
, a low resistance trace should be used to route this
TOLERANCE
±0.075%
±0.05%
±0.05%
±0.05%
INITIAL
TEMPERATURE
10ppm/°C
10ppm/°C
5ppm/°C
5ppm/°C
DRIFT
LTC2751
OUT2
must be tied
0.1Hz to 10Hz
12µV
12µV
20µV
3µV
NOISE
17
P-P
P-P
P-P
P-P
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