Datasheet

Data Sheet AD5628/AD5648/AD5668
Rev. G | Page 21 of 32
THEORY OF OPERATION
D/A SECTION
The AD5628/AD5648/AD5668 DACs are fabricated on a
CMOS process. The architecture consists of a string of DACs
followed by an output buffer amplifier. Each part includes an
internal 1.25 V/2.5 V, 5 ppm/°C reference with an internal gain
of 2. Figure 55 shows a block diagram of the DAC architecture.
05302-153
DAC
REGISTER
V
REFIN
RESISTOR
STRING
REF
GND
V
DD
OUTPUT
AMPLIFIER
(GAIN = ×2)
V
OUT
Figure 55. 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 the 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 4095 for AD5628 (12 bits).
0 to 16,383 for AD5648 (14 bits).
0 to 65,535 for AD5668 (16 bits).
N = the DAC resolution.
RESISTOR STRING
The resistor string section is shown in Figure 56. 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.
05302-053
TO OUTPUT
AMPLIFIER
R
R
R
R
R
Figure 56. Resistor String
INTERNAL REFERENCE
The AD5628/AD5648/AD5668 have an on-chip reference with
an internal gain of 2. The AD5628/AD5648/AD5668-1 have a
1.25 V, 5 ppm/°C reference, giving a full-scale output of 2.5 V;
the AD5628/AD5648/AD5668-2, -3 have 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 the
control register (see Table 8).
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.