DAC8718 Datasheet, PDF (54/60 Pages) Texas Instruments – Octal, 16-Bit, Low-Power, High-Voltage Output, Serial Input DIGITAL-TO-ANALOG CONVERTER

DAC8718
SBAS467A – MAY 2009 – REVISED DECEMBER 2009
www.ti.com
PRECISION VOLTAGE REFERENCE SELECTION
To achieve the optimum performance from the DAC8718 over the full operating temperature range, a precision
voltage reference must be used. Careful consideration should be given to the selection of a precision voltage
reference. The DAC8718 has two reference inputs, REF-A and REF-B. The voltages applied to the reference
inputs are used to provide a buffered positive reference for the DAC cores. Therefore, any error in the voltage
reference is reflected in the outputs of the device. There are four possible sources of error to consider when
choosing a voltage reference for high-accuracy applications: initial accuracy, temperature coefficient of the output
voltage, long-term drift, and output voltage noise. Initial accuracy error on the output voltage of an external
reference can lead to a full-scale error in the DAC. Therefore, to minimize these errors, a reference with low
initial accuracy error specification is preferred. Long-term drift is a measure of how much the reference output
voltage drifts over time. A reference with a tight, long-term drift specification ensures that the overall solution
remains relatively stable over its entire lifetime. The temperature coefficient of a reference output voltage affects
the output drift when the temperature changes. Choose a reference with a tight temperature coefficient
specification to reduce the dependence of the DAC output voltage on ambient conditions. In high-accuracy
applications, which have a relatively low noise budget, the reference output voltage noise also must be
considered. Choosing a reference with as low an output noise voltage as practical for the required system
resolution is important. Precision voltage references such as TI's REF50xx (2V to 5V) and REF32xx (1.25V to
4V) provide a low-drift, high-accuracy reference voltage.
POWER-SUPPLY NOISE
The DAC8718 must have ample supply bypassing of 1μF to 10μF in parallel with 0.1μF on each supply, located
as close to the package as possible; ideally, immediately next to the device. The 1μF to 10μF capacitors must be
the tantalum-bead type. The 0.1μF capacitor must have low effective series resistance (ESR) and low effective
series inductance (ESI), such as common ceramic types, which provide a low-impedance path to ground at high
frequencies to handle transient currents because of internal logic switching. The power-supply lines must be as
large a trace as possible to provide low-impedance paths and reduce the effects of glitches on the power-supply
line. Apart from these considerations, the wideband noise on the AV_DD, AV_SS, DV_DD and IOV_DD supplies should
be filtered before feeding to the DAC to obtain the best possible noise performance.
LAYOUT
Precision analog circuits require careful layout, adequate bypassing, and a clean, well-regulated power supply to
obtain the best possible dc and ac performance. Careful consideration of the power-supply and ground-return
layout helps to meet the rated performance. DGND is the return path for digital currents and AGND is the power
ground for the DAC. For the best ac performance, care should be taken to connect DGND and AGND with very
low resistance back to the supply ground. The PCB must be designed so that the analog and digital sections are
separated and confined to certain areas of the board. If multiple devices require an AGND-to-DGND connection,
the connection is to be made at one point only. The star ground point is established as close as possible to the
device.
The power-supply traces must be as large as possible to provide low impedance paths and reduce the effects of
glitches on the power-supply line. Fast switching signals must never be run near the reference inputs. It is
essential to minimize noise on the reference inputs because it couples through to the DAC output. Avoid
crossover of digital and analog signals. Traces on opposite sides of the board must run at right angles to each
other. This configuration reduces the effects of feedthrough on the board. A microstrip technique may be
considered, but is not always possible with a double-sided board. In this technique, the component side of the
board is dedicated to the ground plane, and signal traces are placed on the solder-side.
Each DAC group has a ground pin, AGND-x, which is the ground of the output from the DACs in the group. It
must be connected directly to the corresponding reference ground in low-impedance paths to get the best
performance. AGND-A must be connected with REFGND-A and AGND-B must be connected with REFGND-B.
AGND-A and AGND-B must be tied together and connected to the analog power ground and DGND.
During single-supply operation, the OFFSET-x pins must be connected to AGND-x with a low-impedance path
because these pins carry DAC-code-dependent current. Any resistance from OFFSET-x to AGND-x causes a
voltage drop by this code-dependent current. Therefore, it is very important to minimize routing resistance to
AGND-x or to any ground plane that AGND-x is connected to.
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