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AD9713B Datasheet(PDF) 7 Page - Analog Devices |
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AD9713B Datasheet(HTML) 7 Page - Analog Devices |
7 / 12 page AD9712B/AD9713B REV. B –7– D (LSB) 12 D6 D9 D8 D7 D11 D10 D 5 D 3 D 4 D2 ANALOG RETURN DIGITAL –VS D (MSB) 1 LATCH ENABLE R SET REFERENCE GROUND REFERENCE OUT CONTROL AMP IN REFERENCE IN CONTROL AMP OUT ANALOG –VS AD9712B AD9713B 0.1µF 0.01µF 20 Ω RL R L SYSTEM GROUND DIGITAL GROUND –5.2V 13 22 27 3 4 26 11 5 6 7 8 9 10 14 I OUT 12,21 15,25 20 16 24 19 18 17 28 1 2 ECL DRIVE LOGIC 0.1µF 0.01µF 0.1µF VOUT = IFS x RL I OUT Figure 4. Typical Resistive Load Connection An operational amplifier can also be used to perform the I to V conversion of the DAC output. Figure 5 shows an example of a circuit which uses the AD9617, a high speed, current feedback amplifier. CONTROL AMP IN AD9712B AD9713B 19 20 14 16 10k 10k 400 RFB R 1 200 I FS I OS REF OUT I OUT 25 R L I OUT – + 1/2 AD708 R FF 25 V ±2.048V OUT 100 R2 – + 1/2 AD708 – + AD9617 12.5 Figure 5. I/VConversion Using Current Feedback DAC current across feedback resistor RFB determines the AD9617 output swing. A current divider formed by RL and RFF limits the current used in the I-to-V conversion, and provides an output voltage swing within the specifications of the AD9617. Current through R2 provides dc offset at the output of the AD9617. Adjusting the value of R1 adjusts the value of offset current. This offset current is based on the reference of the AD9712B/AD9713B, to avoid coupling noise into the output signal. The resistor values in Figure 5 provide a 4.096 V swing, cen- tered at ground, at the output of the AD9617 amplifier. Power and Grounding Maintaining low noise on power supplies and ground is critical for obtaining optimum results with the AD9712B or AD9713B. DACs are most often used in circuits which are predominantly digital. To preserve 12-bit performance, especially at conversion speeds up to 100 MSPS, special precautions are necessary for power supplies and grounding. Ideally, the DAC should have a separate analog ground plane. All ground pins of the DAC, as well as reference and analog output components, should be tied directly to this analog ground plane. The DAC’s ground plane should be connected to the system ground plane at a single point. Ferrite beads such as the Stackpole 57-1392 or Amidon FB-43B-101, along with high frequency, low-inductance decou- pling capacitors, should be used for the supply connections to isolate digital switching currents from the DAC supply pins. Separate isolation networks for the digital and analog supply connections will further reduce supply noise coupling to the output. Molded socket assemblies should be avoided even when prototyping circuits with the AD9712B or AD9713B. When the DAC cannot be directly soldered into the board, individual pin sockets such as AMP #6-330808-0 (knock-out end), or #60330808-3 (open end) should be used. These have much less effect on inter-lead capacitance than do molded assemblies. DDS Applications Numerically controlled oscillators (NCOs) are digital devices which generate samples of a sine wave. When the NCO is com- bined with a high performance D/A converter (DAC), the com- bination system is referred to as a Direct Digital Synthesizer (DDS). The digital samples generated by the NCO are reconstructed by the DAC and the resulting sine wave is usable in any system which requires a stable, spectrally pure, frequency-agile refer- ence. The DAC is often the limiting factor in DDS applications, since it is the only analog function in the circuit. The AD9712B/ AD9713B D/A converters offer the highest level of performance available for DDS applications. DC linearity errors of a DAC are the dominant effect in low- frequency applications and can affect both noise and harmonic content in the output waveform. Differential Nonlinearity (DNL) errors determine the quantization error between adja- cent codes, while Integral Nonlinearity (INL) is a measure of how closely the overall transfer function of the DAC compares with an ideal device. Together, these errors establish the limits of phase and amplitude accuracy in the output waveform. |
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