Datasheet

13
®
DAC7641
ANALOG OUTPUTS
When V
SS
= –5V (dual supply operation), the output ampli-
fier can swing to within 2.25V of the supply rails, guaran-
teed over the –40°C to +85°C temperature range. With V
SS
= 0V (single-supply operation), and with R
LOAD
also con-
nected to ground, the output can swing to ground. Care must
be taken when measuring the zero-scale error with V
SS
= 0V.
Since the output voltage cannot swing below ground, the
output voltage may not change for the first few digital input
codes (0000
H
, 0001
H
, 0002
H
, etc.) if the output amplifier has
a negative offset. At the negative limit of –2mV, the first
specified output starts at code 0040
H
.
Due to the high accuracy of these D/A converters, system
design problems such as grounding and contact resistance
become very important. A 16-bit converter with a 2.5V full-
scale range has a 1LSB value of 38µV. With a load current
of 1mA, series wiring and connector resistance (see Figure
4) of only 40mΩ (R
W2
) will cause a voltage drop of 40µV.
To understand what this means in terms of a system layout,
the resistivity of a typical 1 ounce copper-clad printed circuit
board is 1/2 mΩ per square. For a 1mA load, a 10 milli-inch
wide printed circuit conductor 600 milli-inches long will
result in a voltage drop of 30µV.
The DAC7641 offers a force and sense output configuration
for the high open-loop gain output amplifier. This feature
allows the loop around the output amplifier to be closed at the
load (see Figure 4), thus ensuring an accurate output voltage.
REFERENCE INPUTS
The reference inputs, V
REF
L and V
REF
H, can be any voltage
between V
SS
+ 2.5V and V
CC
– 2.5V provided that V
REF
H is
at least 1.25V greater than V
REF
L. The minimum output of
each DAC is equal to V
REF
L plus a small offset voltage
(essentially, the offset of the output op amp). The maximum
output is equal to V
REF
H plus a similar offset voltage. Note
that V
SS
(the negative power supply) must either be
FIGURE 4. Analog Output Closed-Loop Configuration. R
W
represents wiring resistances.
connected to ground or must be in the range of –4.75V to
–5.25V. The voltage on V
SS
sets several bias points within
the converter. If V
SS
is not in one of these two configura-
tions, the bias values may be in error and proper operation
of the device is not guaranteed.
The current into the V
REF
H input and out of V
REF
L depends
on the DAC output voltages and can vary from a few
microamps to approximately 0.5mA. The reference input
appears as a varying load to the reference. If the reference
can sink or source the required current, a reference buffer is
not required. The DAC7641 features a reference drive and
sense connection such that the internal errors caused by the
changing reference current and the circuit impedances can
be minimized. Figures 5 through 13 show different reference
configurations and the effect on the linearity and differential
linearity.
FIGURE 5. Dual Supply Configuration-Buffered References, used for Dual Supply Performance Curves.
V
REFL
V
REFL
Sense
V
REFH
V
REFH
Sense
V
CC
AGND
30 29 28 27 26 25
DAC7641
R
W1
R
W2
V
OUT
Sense
V
OUT
V
SS
24
23
22
+2.5V
+V
V
OUT
500pF
OPA2234
500pF
+2.5V
+V
–2.5V
–V
V
REFL
V
REFL
Sense
V
REFH
V
REFH
Sense
V
CC
AGND
30 29 28 27 26 25
DAC7641
V
OUT
Sense
V
OUT
V
SS
24
23
22
V
OUT