Datasheet
17
®
DAC7644
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
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 DAC7644 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 12 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 (1/2 DAC7644).
FIGURE 6. Single-Supply Buffered Reference with a Reference Low of 50mV (1/2 DAC7644).
NC
NC
NC
NC
V
OUT
A Sense
V
OUT
A
V
REF
L AB Sense
V
REF
L AB
V
REF
H AB
V
REF
H AB Sense
V
OUT
B Sense
V
OUT
B
48
47
46
45
44
43
42
41
40
39
38
37
DAC7644
500pF
+V
OPA2234
500pF
+2.5V
–V
+V
–2.5V
–V
V
OUT
V
OUT
NC
NC
NC
NC
V
OUT
A Sense
V
OUT
A
V
REF
L AB Sense
V
REF
L AB
V
REF
H AB
V
REF
H AB Sense
V
OUT
B Sense
V
OUT
B
48
47
46
45
44
43
42
41
40
39
38
37
DAC7644
1000pF
2200pF
+V
OPA2350
100Ω
1000pF
2200pF
98kΩ
0.050V
2kΩ
+2.5V
100Ω
+V
V
OUT
V
OUT
NOTE: V
REF
L has been chosen to be 50mV to allow for current sinking voltage
drops across the 100Ω resistor and the output stage of the buffer op amp.