High Accuracy, Bipolar Voltage Output Digital-to-Analog Conversion Using the AD5765 DAC
Figure 1: High Accuracy, Bipolar Configuration of the AD5765 DAC Using a Precision Reference
A precision voltage reference must be used in order for the DAC to achieve the optimum performance over its full operating temperature range. The AD5765 incorporates reference buffers, which eliminate the need for both a positive and negative external reference and associated buffers. This leads to further savings in both cost and board space. Because the voltages applied to the reference inputs (REFAB, REFCD) are used to generate the buffered positive and negative internal references for the DAC cores, any error in the external 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. Table 1 lists other 2.048 V precision reference candidates from Analog Devices, Inc., and their respective attributes.
In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps to ensure the rated performance. The PCB on which the AD5765 is mounted must be designed so that the analog and digital sections are physically separated and confined to certain areas of the board. If the AD5765 is in a system where multiple devices require an AGND-to-DGND connection, the connec-tion is to be made at one point only. The star ground point is established as close as possible to the device. The AD5765 must have ample supply bypassing of 10 μF in parallel with 0.1 μF on each supply, located as close to the package as possible, ideally right up against the device. The 10 μF capacitors are the tantalum bead type. The 0.1 μF capacitor must have low effective series resistance (ESR) and low effective series inductance (ESL), such as the common ceramic types, which provide a low impedance path to ground at high frequencies to handle transient currents due to internal logic switching.
Table 1. Precision 2.048 V References
||Initial Accuracy Max (mV)
||Long-Term Drift Typ (ppm)
||Temp Drift Max (ppm/°C)||0.1 Hz to 10 Hz Noise Typ (μV p-p)
The power supply traces of the AD5765 must be as wide as possible to provide low impedance paths and reduce the effects of glitches on the power supply line. Fast switching signals, such as clocks, must be shielded with digital ground to avoid radiating noise to other parts of the board and must never be run near the reference inputs. A ground line routed between the SDIN and SCLK lines helps reduce crosstalk between them (not required on a multilayer board, which has a separate ground plane; however, it is helpful to separate the lines). 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 reduces the effects of feedthrough on the board. A microstrip technique is recommended but 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. Best layout and performance are achieved with at least a 4-layer multilayer board, where there are a ground plane layer, a power supply layer, and two signal layers.