Datasheet

Data Sheet AD5697R
Rev. 0 | Page 17 of 28
THEORY OF OPERATION
DIGITAL-TO-ANALOG CONVERTER
The AD5697R is a dual, 12-bit, serial input, voltage output DAC
with an internal reference. The part operates from supply voltages
of 2.7 V to 5.5 V. Data is written to the AD5697R in a 24-bit word
format via a 2-wire serial interface. The AD5697R incorporates a
power-on reset circuit to ensure that the DAC output powers up to
a known output state. The device also has a software power-down
mode that reduces the typical current consumption to 4 µA.
TRANSFER FUNCTION
The internal reference is on by default. To use an external reference,
only a nonreference option is available. Because the input coding
to the DAC is straight binary, the ideal output voltage when using
an external reference is given by






×=
N
REF
OUT
D
GainVV
2
where:
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. When this
pin is tied to GND, both DAC outputs have a span from 0 V to
V
REF
. If this pin is tied to V
LOGIC
, both DAC output a span of 0 V to
2 × V
REF
.
D is the decimal equivalent of the binary code that is loaded to the
DAC register as 0 to 4,095 for the 12-bit device.
N is the DAC resolution.
DAC ARCHITECTURE
The DAC architecture consists of a string DAC followed by an
output amplifier. Figure 41 shows a block diagram of the DAC
architecture.
Figure 41. Single DAC Channel Architecture Block Diagram
The resistor string structure is shown in Figure 42. It is a string
of resistors, each of Value R. The code loaded to the DAC register
determines the node on the string where the voltage is to be
tapped off and 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.
Figure 42. Resistor String Structure
Internal Reference
The AD5697R on-chip reference is on at power-up but can
be disabled via a write to a control register. See the Internal
Reference Setup section for details.
The AD5697R has 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 V
REF
pin. This buffered reference is capable of driving external
loads of up to 10 mA.
Output Amplifiers
The output buffer amplifier can generate rail-to-rail voltages on
its output, which gives an output range of 0 V to V
DD
. The actual
range depends on the value of V
REF
, the GAIN pin, the offset error,
and the gain error. The GAIN pin selects the gain of the output.
• If GAIN is tied to GND, both outputs have a gain of 1, and
the output range is 0 V to V
REF
.
• If GAIN is tied to V
LOGIC
, both outputs have a gain of 2, and
the output range is 0 V to 2 × V
REF
.
These amplifiers are capable of driving a load of 1 kΩ in parallel
with 2 nF to GND. The slew rate is 0.8 V/µs with a ¼ to ¾ scale
settling time of 5 µs.
INPUT
REGISTER
2.5V
REF
DAC
REGISTER
RESISTOR
STRING
REF (+)
V
REF
GND
REF (–)
V
OUT
X
GAIN
(GAIN = 1 OR 2)
11253-042
R
R
R
R
R
TO OUTPUT
AMPLIFIER
V
REF
11253-043