Datasheet

DAC7642, DAC7643
14
SBAS233
www.ti.com
that V
SS
(the negative power supply) must either be con-
nected 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 configurations,
the bias values may be in error and proper operation of the
device may be affected.
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 references
applied can sink or source the required current, a reference
buffer is not required. The DAC7642 and DAC7643 feature
reference drive and sense connections 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 Characteristic Curves.
V
OUT
A
V
OUT
A Sense
V
REF
L Sense
V
REF
L
V
REF
H
V
REF
H Sense
V
OUT
B Sense
V
OUT
B
32
31
30
29
28
27
26
25
DAC7642
DAC7643
1000pF
V
OUT
V
OUT
100Ω
100Ω
+2.5V
+V
–2.5V
–V
–V
+V
OPA2234
2200pF
1000pF
2200pF
ANALOG OUTPUTS
When V
SS
= –5V (dual-supply operation), the output amplifier
can swing to within 2.25V of the supply rails over the –40°C
to +85°C temperature range. When V
SS
= 0V (single-supply
operation), and with R
LOAD
also connected to ground, the
output can swing to ground. Care must also be taken when
measuring the zero-scale error when V
SS
= 0V. Since the
DAC output 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 DACs, 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, a series
wiring and connector resistance of only 40mΩ (R
W2
) will cause
a voltage drop of 40µV, as shown in Figure 4. 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 DAC7642 and DAC7643 offer a force and sense output
configuration for the high open-loop gain output amplifiers.
This feature allows the loop around the output amplifier to be
closed at the load (shown in 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
FIGURE 4. Analog Output Closed-Loop Configuration. R
W
represents wiring resistances.
V
OUT
A
V
OUT
A Sense
V
REF
L Sense
V
REF
L
V
REF
H
V
REF
H Sense
V
OUT
B Sense
V
OUT
B
32
31
30
29
28
27
26
25
DAC7642
DAC7643
R
W2
R
W1
+2.5V
+V
V
OUT
R
W1
R
W2
V
OUT