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DAC712U Arkusz danych(PDF) 9 Page - Burr-Brown (TI) |
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DAC712U Arkusz danych(HTML) 9 Page - Burr-Brown (TI) |
9 / 12 page 9 ® DAC712 DAC712 CALIBRATION VALUES 1 LEAST SIGNIFICANT BIT = 305 µV DIGITAL INPUT CODE ANALOG BINARY TWO’S OUTPUT COMPLEMENT, BTC (V) DESCRIPTION 7FFFH +9.999695 + Full Scale –1LSB | 4000H +5.000000 3/4 Scale | 0001H +0.000305 BPZ + 1LSB 0000H 0.000000 Bipolar Zero (BPZ) FFFFH –0.000305 BPZ – 1LSB | C000H –5.000000 1/4 Scale | 8000H –10.00000 Minus Full Scale TABLE I. Digital Input and Analog Output Voltage Calibra- tion Values. Gain Adjustment Apply the digital input that gives the maximum positive voltage output. Adjust the gain potentiometer or the gain adjust D/A converter for this positive full scale voltage. INSTALLATION GENERAL CONSIDERATIONS Due to the high-accuracy of these D/A converters, system design problems such as grounding and contact resistance become very important. A 16-bit converter with a 20V full- scale range has a 1LSB value of 305 µV. With a load current of 5mA, series wiring and connector resistance of only 60m Ω will cause a voltage drop of 300µV. To understand what this means in terms of a system layout, the resistivity of a typical 1 ounce copper-clad printed circuit board is 1/2 m Ω per square. For a 5mA load, a 10 milli-inch wide printed circuit conductor 60 milli-inches long will result in a voltage drop of 150 µV. The analog output of DAC712 has an LSB size of 305 µV (–96dB). The noise floor of the D/A must remain below this level in the frequency range of interest. The DAC712’s noise spectral density (which includes the noise contributed by the internal reference,) is shown in the Typical Performance Curves section. Wiring to high-resolution D/A converters should be routed to provide optimum isolation from sources of RFI and EMI. The key to elimination of RF radiation or pickup is small loop area. Signal leads and their return conductors should be kept close together such that they present a small capture cross-section for any external field. Wire-wrap construction is not recommended. POWER SUPPLY AND REFERENCE CONNECTIONS Power supply decoupling capacitors should be added as shown in Figure 4. Best performance occurs using a 1 to 10 µF tantalum capacitor at –V CC. Applications with less 1 2 3 4 5 6 7 8 9 10 11 12 13 14 28 27 26 25 24 23 22 21 20 19 18 17 16 15 + 0.01µF DCOM ACOM V OUT V REF OUT +V CC –V CC 0.01µF + +12V to +15V –12V to –15V FIGURE 4. Power Supply Connections. critical settling time may be able to use 0.01 µF at –V CC as well as at +VCC. The capacitors should be located close to the package. DAC712 has separate ANALOG COMMON and DIGITAL COMMON pins. The current through DCOM is mostly switching transients and are up to 1mA peak in amplitude. The current through ACOM is typically 5 µA for all codes. Use separate analog and digital ground planes with a single interconnection point to minimize ground loops. The analog pins are located adjacent to each other to help isolate analog from digital signals. Analog signals should be routed as far as possible from digital signals and should cross them at right angles. A solid analog ground plane around the D/A package, as well as under it in the vicinity of the analog and power supply pins, will isolate the D/A from switching currents. It is recommended that DCOM and ACOM be connected directly to the ground planes under the package. If several DAC712s are used or if DAC712 shares supplies with other components, connecting the ACOM and DCOM lines to together once at the power supplies rather than at each chip may give better results. LOAD CONNECTIONS Since the reference point for VOUT and VREF OUT is the ACOM pin, it is important to connect the D/A converter load directly to the ACOM pin. Refer to Figure 5. Lead and contact resistances are represented by R1 through R3. As long as the load resistance RL is constant, R1 simply introduces a gain error and can be removed by gain adjust- ment of the D/A or system-wide gain calibration. R2 is part of RL if the output voltage is sensed at ACOM. In some applications it is impractical to return the load to the ACOM pin of the D/A converter. Sensing the output voltage at the SYSTEM GROUND point is reasonable, because |
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