12
LTC1821
Precision Voltage Reference Considerations
Because of the extremely high accuracy of the 16-bit
LTC1821, careful thought should be given to the selection
of a precision voltage reference. As shown in the section
describing the basic operation of the LTC1821, the output
voltage of the DAC circuit 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 applica-
tions: output voltage initial tolerance, output voltage tem-
perature coefficient (TC), 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 due to 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 INL and DNL performance. If a reference 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 condi-
tions. Minimizing the error due to reference temperature
coefficient can be achieved by choosing a precision refer-
ence 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
Table 2. Partial List of LTC Precision References Recommended
for Use with the LTC1821, with Relevant Specifications
INITIAL TEMPERATURE 0.1Hz to 10Hz
REFERENCE TOLERANCE DRIFT NOISE
LT1019A-5, ±0.05% 5ppm/°C12µV
P-P
LT1019A-10
LT1236A-5, ±0.05% 5ppm/°C3µV
P-P
LT1236A-10
LT1460A-5, ±0.075% 10ppm/°C20µV
P-P
LT1460A-10
LT1790A-2.5 ±0.05% 10ppm/°C12µV
P-P
APPLICATIONS INFORMATION
WUU
U
reference with as low an output noise voltage as practical
for the system resolution desired. Precision voltage refer-
ences, 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 band-
widths increase, filtering the output of the reference may
be required to minimize output noise.
Grounding
As with any high resolution converter, clean grounding is
important. A low impedance analog ground plane and star
grounding should be used. AGNDF and AGNDS must be
tied to the star ground with as low a resistance as possible.
When it is not possible to locate star ground close to
AGNDF and AGNDS, separate traces should be used to
route these pins to the star ground. This minimizes the
voltage drop from these pins to ground due to the code
dependent current flowing into the ground plane. If the
resistance of these separate circuit board traces exceeds
1Ω, the circuit of Figure␣ 3 eliminates this code dependent
voltage drop error for high resistance traces.
To calculate PC track resistance in squares, divide the
length of the PC track by the width and multiply this result
by the sheet resistance of copper foil. For 1 oz copper
(≈1.4 mils thick), the sheet resistance is 0.045Ω per
square.
