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AD5339 Datasheet(PDF) 19 Page - Analog Devices |
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AD5339 Datasheet(HTML) 19 Page - Analog Devices |
19 / 24 page AD5337/AD5338/AD5339 Rev. A | Page 19 of 24 DOUBLE-BUFFERED INTERFACE The AD5337/AD5338/AD5339 DACs all have a double-buffered interface consisting of two banks of registers—an input register and a DAC register per channel. The input register is directly connected to the input shift register, and the digital code is transferred to the relevant input register upon completion of a valid write sequence. The DAC register contains the digital code used by the resistor string. Access to the DAC register is controlled by the LDAC bit. When the LDAC bit is set high, the DAC register is latched and therefore the input register may change state without affecting the DAC register. This is useful if the user requires simultaneous updating of all DAC outputs. The user may write to three of the input registers individually; by setting the LDAC bit low when writing to the remaining DAC input register, all outputs will update simultaneously. These parts contain an extra feature whereby the DAC register is only updated if its input register has been updated since the last time that LDAC was brought low, thereby removing unnecessary digital crosstalk. POWER-DOWN MODES The AD5337/AD5338/AD5339 have very low power consumption, typically dissipating 0.75 mW with a 3 V supply and 1.5 mW with a 5 V supply. Power consumption can be further reduced when the DACs are not in use by putting them into one of three power-down modes, which are selected by Bits 15 and 14 (PD1 and PD0) of the data byte. Table 8 shows how the state of the bits corresponds to the mode of operation of the DAC. Table 8. PD1/PD0 Operating Modes PD1 PD0 Operating Mode 0 0 Normal Operation 0 1 Power-Down (1 kΩ Load to GND) 1 0 Power-Down (100 kΩ Load to GND) 1 1 Power-Down (3-State Output) When both bits are 0, the DAC works with its normal power consumption of 300 µA at 5 V. However, for the three power- down modes, the supply current falls to 200 nA at 5 V (80 nA at 3 V). Not only does the supply current drop, but the output stage is also internally switched from the output of the amplifier to a resistor network of known values. This is advantageous in that the output impedance of the part is known while the part is in power-down mode, which provides a defined input condition for whatever is connected to the output of the DAC amplifier. There are three options. The output may be connected internally to GND through a 1 kΩ resistor, a 100 kΩ resistor, or may be left open-circuited (3-state). Resistor tolerance = ±20%. The output stage is illustrated in Figure 35. AMPLIFIER POWER-DOWN CIRCUITRY RESISTOR NETWORK RESISTOR STRING DAC VOUT Figure 35. Output Stage during Power-Down The bias generator, the output amplifiers, the resistor string, and all other associated linear circuitry are shut down when power- down mode is activated. However, the contents of the DAC registers remain unchanged when power-down mode is activated. The time to exit power-down is typically 2.5 µs for VDD = 5 V and 5 µs when VDD = 3 V. This is the time from the rising edge of the eighth SCL pulse to the time when the output voltage deviates from its power-down voltage. See Figure 23 for a plot. |
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