DAC8734_091 Datasheet, PDF (32/41 Pages) Texas Instruments – Quad, 16-Bit, High-Accuracy, ±16V Output, Serial Input DIGITAL-TO-ANALOG CONVERTER

DAC8734
SBAS465A – MAY 2009 – REVISED SEPTEMBER 2009 .................................................................................................................................................. www.ti.com
LAYOUT AND GROUNDING
Precision analog circuits require careful layout, adequate bypass capacitors, and a clean, well-regulated power
supply to obtain the best possible dc and ac performance. A careful consideration of the power-supply and
ground-return layout helps to ensure the rated performance.
The PCB must be designed so that the analog and digital sections are separated and confined to certain areas of
the board. Fast switching signals, such as clocks, must be shielded with the digital ground to avoid radiating
noise to other sections of the board, and 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 may not always
be 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.
DGND is the return path for digital currents and AGND is the analog 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. If multiple devices require an AGND-to-DGND connection, the connection must be made at one point
only. The star ground point must be established as close as possible to the device. Each DAC has a ground pin
(SGND-x) that must be connected directly to the corresponding reference ground in low-impedance paths to
achieve the best performance. SGND-0 and SGND-1 must be connected with REFGND-A, and SGND-2 and
SGND-3 must be connected with REFGND-B. It is critical that this trace resistance be extremely small in order to
prevent the voltage drops across the path from affecting device linearity and gain performance. The reference
ground pins, REFGND-A and REFGND-B, must be connected to analog ground AGND.
POWER-SUPPLY NOISE
The DAC8734 should 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, placed next to the device. The 1µF to 10µF capacitors must
be a 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 that provide a low-impedance path to ground at high
frequencies to handle transient currents because of internal logic switching. The power-supply lines must use
traces as wide 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 being fed to the DAC in order to obtain the best noise performance possible.
PRECISION VOLTAGE REFERENCE SELECTION
To achieve the optimum performance from the DAC8734 over its full operating temperature range, a precision
voltage reference must be used. Consideration should be given to the selection of a precision voltage reference.
The DAC8734 has two reference inputs, REF-A and REF-B. The voltages applied to the reference inputs are
used to provide a buffered positive and negative 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 a low
initial accuracy error specification is preferred. Long-term drift is a measurement 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 INL, DNL, gain error, and zero error. 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 that
have a relatively low noise budget, reference output voltage noise must be considered. Choosing a reference
with as low an output noise voltage as practical for the system resolution required is important. Precision voltage
references such as the TI REF50xx (2V to 5V) and REF32xx (1.25V to 4V), provide low-drift and high-accuracy
reference voltage.
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