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AD5671R Datasheet(PDF) 22 Page - Analog Devices

Part No. AD5671R
Description  Base station power amplifiers
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD5671R Datasheet(HTML) 22 Page - Analog Devices

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AD5671R/AD5675R
Data Sheet
Rev. B | Page 22 of 32
THEORY OF OPERATION
DIGITAL-TO-ANALOG CONVERTER (DAC)
The AD5671R/AD5675R are octal, 12-/16-bit, serial input, voltage
output DACs with an internal reference. The devices operate from
supply voltages of 2.7 V to 5.5 V. Data is written to the AD5671R/
AD5675R in a 24-bit word format via a 2-wire serial interface.
The AD5671R/AD5675R incorporate a power-on reset circuit
to ensure that the DAC output powers up to a known output
state. The devices also have a software power-down mode that
reduces the typical current consumption to 1 µA.
TRANSFER FUNCTION
The internal reference is on by default.
Gain is the gain of the output amplifier and is set to 1 by default.
This can be set to ×1 or ×2 using the gain select pin (GAIN). When
this pin is tied to GND, all eight DAC outputs have a span from
0 V to VREF. If this pin is tied to VLOGIC, all eight DACs output a
span of 0 V to 2 × VREF.
DAC ARCHITECTURE
The AD5671R/AD5675R implement segmented string DAC
architecture with an internal output buffer. Figure 56 shows the
internal block diagram.
Figure 56. Single DAC Channel Architecture Block Diagram
The resistor string structure is shown in Figure 57. The code
loaded to the DAC register determines the node on the string
where the voltage is tapped off and fed into the output amplifier.
The voltage is tapped off by closing one of the switches and
connecting the string to the amplifier. Because each resistance
in the string has same value, R, the string DAC is guaranteed
monotonic.
Figure 57. Resistor String Structure
Internal Reference
The AD5671R/AD5675R on-chip reference is enabled at power-up,
but can be disabled via a write to the control register. See the
Internal Reference and Amplifier Gain Selection section for
details.
The AD5671R/AD5675R have a 2.5 V, 2 ppm/°C reference, giving
a full-scale output of 2.5 V or 5 V, depending on the state of the
GAIN pin. The internal reference associated with the device is
available at the VREFOUT pin. This buffered reference is capable of
driving external loads of up to 15 mA.
Output Amplifiers
The output buffer amplifier generates rail-to-rail voltages on its
output, which gives an output range of 0 V to VDD. The actual
range depends on the value of VREF, the GAIN pin, the offset
error, and the gain error. The GAIN pin selects the gain of the
output. If the GAIN pin is tied to GND, all eight outputs have a
gain of 1, and the output range is 0 V to VREF. If the GAIN pin is
tied to VLOGIC, all eight outputs have a gain of 2, and the output
range is 0 V to 2 × VREF.
These amplifiers are capable of driving a load of 1 kΩ in parallel
with 10 nF to GND. The slew rate is 0.8 V/µs with a typical ¼ to
¾ scale settling time of 5 µs.
INPUT
REGISTER
2.5V
REF
DAC
REGISTER
RESISTOR
STRING
REF (+)
REF (–)
VREF
VOUTX
GND
GAIN
(GAIN = 1 OR 2)
R
R
R
R
R
TO OUTPUT
AMPLIFIER
VREF


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