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AD5678 Datasheet(PDF) 20 Page - Analog Devices |
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AD5678 Datasheet(HTML) 20 Page - Analog Devices |
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20 / 28 page 
AD5678 Rev. C | Page 20 of 28 THEORY OF OPERATION D/A SECTION The AD5678 DAC is fabricated on a CMOS process. The archi- tecture consists of a string of DACs followed by an output buffer amplifier. The parts include an internal 1.25 V/2.5 V, 5 ppm/°C reference with an internal gain of 2. Figure 45 shows a block diagram of the DAC architecture. DAC REGISTER REF (+) VDD VOUT GND REF (–) RESISTOR STRING OUTPUT AMPLIFIER (GAIN = +2) Figure 45. DAC Architecture Because the input coding to the DAC is straight binary, the ideal output voltage when using an external reference is given by ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × = N REFIN OUT D V V 2 the ideal output voltage when using an internal reference is given by ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × × = N REFOUT OUT D V V 2 2 where: D = decimal equivalent of the binary code that is loaded to the DAC register. 0 to 4,095 for AD5678 DAC C, D, E, F (12 bits). 0 to 65,535 for AD5678 DAC A, B, G, H (16 bits). N = the DAC resolution. RESISTOR STRING The resistor string section is shown in Figure 46. It is simply a string of resistors, each of value R. The code loaded into the DAC register determines at which node on the string the voltage is tapped off to be fed into the output amplifier. The voltage is tapped off by closing one of the switches connecting the string to the amplifier. Because it is a string of resistors, it is guaranteed monotonic. TO OUTPUT AMPLIFIER R R R R R Figure 46. Resistor String INTERNAL REFERENCE The AD5678 has an on-chip reference with an internal gain of 2. The AD5678-1 has a 1.25 V 5 ppm/°C reference, giving a full- scale output of 2.5 V. The AD5678-2 has a 2.5 V 5 ppm/°C reference, giving a full-scale output of 5 V. The on-board reference is off at power-up, allowing the use of an external reference. The internal reference is enabled via a write to a control register. See Table 7. The internal reference associated with each part is available at the VREFOUT pin. A buffer is required if the reference output is used to drive external loads. When using the internal reference, it is recommended that a 100 nF capacitor be placed between the reference output and GND for reference stability. Individual channel power-down is not supported while using the internal reference. |
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