Datasheet
AD5666
Rev. D | Page 20 of 28
THEORY OF OPERATION
D/A SECTION
The AD5666 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 40 shows a block
diagram of the DAC architecture.
05298-023
DAC REGISTER
REF (+)
V
DD
V
OUT
GND
REF (–)
RESISTOR
STRING
OUTPUT
AMPLIFIER
(GAIN = +2)
Figure 40. 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
VV
2
The ideal output voltage when using and internal reference is
given by
⎟
⎠
⎞
⎜
⎝
⎛
××=
N
REFOUTOUT
D
VV
2
2
where:
D = decimal equivalent of the binary code that is loaded to the
DAC register.
0 to 65,535 for AD5666 (16 bits).
N = the DAC resolution.
RESISTOR STRING
The resistor string section is shown in Figure 41. 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.
05298-024
TO OUTPUT
AMPLIFIER
R
R
R
R
R
Figure 41. Resistor String
INTERNAL REFERENCE
The AD5666 has an on-chip reference with an internal gain of 2.
The AD5666-1 has a 1.25 V 5 ppm/°C reference, giving a full-scale
output of 2.5 V. The AD5666-2 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.
The internal reference associated with each part is available at
the V
REFOUT
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.