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DAC8512 Datasheet(PDF) 9 Page - Analog Devices |
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DAC8512 Datasheet(HTML) 9 Page - Analog Devices |
9 / 20 page DAC8512 –9– REV. A APPLICATIONS SECTION Power Supplies, Bypassing, and Grounding All precision converter products require careful application of good grounding practices to maintain full rated performance. Because the DAC8512 has been designed for +5 V applications, it is ideal for those applications under microprocessor or micro- computer control. In these applications, digital noise is preva- lent; therefore, special care must be taken to assure that its inherent precision is maintained. This means that particularly good engineering judgment should be exercised when address- ing the power supply, grounding, and bypassing issues using the DAC8512. The power supply used for the DAC8512 should be well filtered and regulated. The device has been completely characterized for a +5 V supply with a tolerance of ±5%. Since a +5 V logic sup- ply is almost universally available, it is not recommended to connect the DAC directly to an unfiltered logic supply without careful filtering. Because it is convenient, a designer might be inclined to tap a logic circuit’s supply for the DAC’s supply. Unfortunately, this is not wise because fast logic with nanosec- ond transition edges induce high current pulses. The high tran- sient current pulses can generate glitches hundreds of millivolts in amplitude due to wiring resistances and inductances. This high frequency noise will corrupt the analog circuits internal to the DAC and cause errors. Even though their spike noise is lower in amplitude, directly tapping the output of a +5 V system supply can cause errors because these supplies are of the switch- ing regulator type that can and do generate a great deal of high frequency noise. Therefore, the DAC and any associated analog circuitry should be powered directly from the system power sup- ply outputs using appropriate filtering. Figure 23 illustrates how a clean, analog-grade supply can be generated from a +5 V logic supply using a differential LC filter with separate power supply and return lines. With the values shown, this filter can easily handle 100 mA of load current without saturating the ferrite cores. Higher current capacity can be achieved with larger ferrite cores. For lowest noise, all electrolytic capacitors should be low ESR (Equivalent Series Resistance) type. 100 µF ELECT . 10-22 µF TANT. 0.1 µF CER. TTL/CMOS LOGIC CIRCUITS +5V POWER SUPPLY +5V +5V RETURN FERRITE BEADS: 2 TURNS, FAIR-RITE #2677006301 Figure 23. Properly Filtering a +5 V Logic Supply Can Yield a High Quality Analog Supply In order to fit the DAC8512 in an 8-pin package, it was neces- sary to use only one ground connection to the device. The ground connection of the DAC serves as the return path for supply currents as well as the reference point for the digital in- put thresholds. The ground connection also serves as the supply rail for the internal voltage reference and the output amplifier. Therefore, to minimize any errors, it is recommended that the ground connection of the DAC8512 be connected to a high quality analog ground, such as the one described above. Gener- ous bypassing of the DAC’s supply goes a long way in reducing supply line-induced errors. Local supply bypassing consisting of a 10 µF tantalum electrolytic in parallel with a 0.1 µF ceramic is recommended. The decoupling capacitors should be connected between the DAC’s supply pin (Pin 1) and the analog ground (Pin 7). Figure 24 shows how the ground and bypass connec- tions should be made to the DAC8512. 6 2 GND V DD 8 DAC8512 10 µF 0.1 µF V OUT 1 +5V TO ANALOG GROUND CS CLR 5 3 4 LD SCLK SDI V OUT 7 Figure 24. Recommended Grounding and Bypassing Scheme for the DAC8512 Unipolar Output Operation This is the basic mode of operation for the DAC8512. As shown in Figure 24, the DAC8512 has been designed to drive loads as low as 2 k Ω in parallel with 500 pF. The code table for this op- eration is shown in Table II. 10 µF 0.1 µF 0V ≤ V OUT ≤ 4.095V +5V 2k Ω 500pF 6 2 V DD 8 DAC8512 1 CS CLR 5 3 4 LD SCLK SDI 7 GND V OUT Figure 25. Unipolar Output Operation Table II. Unipolar Code Table Hexadecimal Number Decimal Number Analog Output in DAC Register in DAC Register Voltage (V) FFF 4095 +4.095 801 2049 +2.049 800 2048 +2.048 7FF 2047 +2.047 000 0 0 Typical Performance Characteristics— |
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