Datasheet

®
12
DAC7714
THEORY OF OPERATION
The DAC7714 is a quad, serial input, 12-bit, voltage output
DAC. The architecture is a classic R-2R ladder configura-
tion followed by an operational amplifier that serves as a
buffer. Each DAC has its own R-2R ladder network and
output op amp, but all share the reference voltage inputs, as
shown in Figure 1. The minimum voltage output (“zero-
scale”) and maximum voltage output (“full-scale”) are set by
external voltage references (V
REFL
and V
REFH
, respectively).
The digital input is a 16-bit serial word that contains the
12-bit DAC code and a 2-bit address code that selects one of
the four DACs (the two remaining bits are unused). The
converter can be powered from a single +15V supply or a
dual ±15V supply. Each device offers a reset function which
immediately sets all DAC output voltages and internal
registers to either zero-scale (code 000
H
) or mid-scale (code
800
H
). The reset code is selected by the state of the
RESETSEL pin (LOW = 000
H
, HIGH = 800
H
). Figures 2
and 3 show the basic operation of the DAC7714.
ANALOG OUTPUTS
When V
SS
= –15V (dual supply operation), the output
amplifier can swing to within 4V of the supply rails, over the
–40°C to +85°C temperature range. With V
SS
= 0V (single-
supply operation), the output can swing to ground. Note that
the settling time of the output op amp will be longer with
voltages very near ground. Care must also be taken when
measuring the zero-scale error when V
SS
= 0V. If the output
amplifier has a negative offset, the output voltage may not
change for the first few digital input codes (000
H
, 001
H
,
002
H
, etc.) since the output voltage cannot swing below
ground.
FIGURE 1. DAC7714 Architecture.
R
2R
2R2R 2R 2R 2R 2R 2R 2R
V
REF
H
V
OUT
RRRRRR
V
REF
L
R
F
At the negative offset limit of –4LSB (–9.76mV), for the
single-supply case, the first specified output starts at code
004
H
.
REFERENCE INPUTS
The reference inputs, V
REFL
and V
REFH
, can be any voltage
between V
SS
+ 4V and V
CC
– 4V provided that V
REFH
is at
least 1.25V greater than V
REFL
. The minimum output of
each D/A is equal to V
REFL
– 1LSB plus a small offset
voltage (essentially, the offset of the output op amp). The
maximum output is equal to V
REFH
plus a similar offset
voltage. Note that V
SS
(the negative power supply) must
either be connected to ground or must be in the range of
–14.75V to –15.75V. The voltage on V
SS
sets several bias
points within the converter. If V
SS
is not in one of these two
configurations, the bias values may be in error and proper
operation of the device is not guaranteed.
The current into the reference inputs depends on the DAC
output voltages and can vary from a few microamps to
approximately 3mA. The reference input appears as a vary-
ing load to the reference. If the reference can sink or source
the required current, a reference buffer is not required. See
“Reference Current vs Code” in the Typical Performance
Curves.
The analog supplies must come up before the reference
power supplies, if they are separate. If the power supplies for
the references come up first, then the V
CC
and V
SS
supplies
will be powered from the reference via the ESD protection
diodes (see page 4).