Using DAC to achieve high-precision, bipolar voltage output digital-to-analog conversion

Circuit Description

The AD5763 is a high performance digital-to-analog converter that features guaranteed monotonicity, ±1 LSB INL (C-grade), low noise, and 10 μs settling time. Performance is guaranteed over the following supply voltage ranges: AVDD supply voltage range of +4.75 V to +5.25 V, and AVSS supply voltage range of -4.75 V to -5.25 V. With a 2.048 V reference input, the nominal full-scale output range is ±4.096 V.

To achieve the best performance of this DAC over the entire operating temperature range, a precision voltage reference must be used. The AD5763 has internal reference buffers, eliminating the need for external positive and negative references and associated buffers, further saving cost and board space. Because the voltages applied to the reference inputs (REFA, REFB) are used to generate the internally buffered positive and negative reference voltages used by the DAC core, any errors in the external reference voltages will be reflected through the output of the device.

When selecting a voltage reference for a high-precision application, there are four possible sources of error to consider: initial accuracy, temperature coefficient of the output voltage, long-term drift, and output voltage noise. Table 1 lists other 2.048 V precision voltage reference candidates from Analog Devices and their characteristics.

Table 1: 2.048 V Precision Voltage References

Circuit Function and Advantages

This circuit uses the AD5763 dual-channel, 16-bit, serial input, bipolar voltage output DAC to provide high-precision, bipolar data conversion. It uses the ADR420 precision voltage reference to achieve optimal DAC performance over the entire operating temperature range. The only external components required for this 16-bit precision DAC are the reference voltage source, decoupling capacitors on the power pins and reference input, and an optional short-circuit current setting resistor, so this implementation saves cost and board space. This circuit is ideal for closed-loop servo control and open-loop control applications.

Figure 1: AD5763 DAC high-precision, bipolar configuration using a precision voltage reference

In any circuit where accuracy is important, careful consideration of the power supply and ground return layout helps ensure that the rated performance is achieved. The PCB used for the AD5763 must be designed with the analog and digital sections separated and confined to certain areas of the board. If the AD5763 is in a system where multiple devices require an AGND to DGND connection, the connection should be made at only one point. The star ground point should be as close to the device as possible. The AD5763 must use ample 10 μF power supply bypassing capacitors in parallel with 0.1 μF capacitors on each supply as close to the package as possible, preferably right up against the device. The 10 μF capacitors should be tantalum bead type. The 0.1 μF capacitors must have low effective series resistance (ESR) and low effective series inductance (ESL), such as the common ceramic types that provide a low impedance path to ground at high frequencies to handle transient currents caused by internal logic switching.

The power supply traces for the AD5763 must be as wide as possible to provide a low impedance path and reduce the effects of glitches on the power supply lines. Fast switching signals such as clocks must be shielded with digital ground to prevent radiating noise to other devices on the board and should never be placed close to the reference input. A ground trace between the SDIN line and the SCLK line helps reduce crosstalk between them (not necessary on a multilayer board because it has a separate ground plane; however, a ground trace helps separate the different lines). Noise on the reference input must be minimized because this noise can be coupled to the DAC output. Overlapping of digital and analog signals should be avoided. Traces on opposite sides of the board must be perpendicular to each other, which helps reduce feedthrough effects on the board. Microstrip techniques are recommended, but may not always be possible with double-sided boards. With this technique, the component side of the board is dedicated to the ground plane, and signal traces are placed on the solder side. A minimum of four layers are required for optimal layout and performance: one ground plane, one power plane, and two signal layers.