Datasheet

18
®
DAC7744
REFERENCE INPUTS
The reference inputs, V
REF
L and V
REF
H, can be any voltage
between V
SS
+ 4V and V
CC
– 4V, 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 –14.25V 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 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 2.0mA. 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 DAC7744 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.
The analog supplies (or the analog supplies and the reference
power supplies) have to come up first. If the power supplies
for the reference come up first, then the V
CC
and V
SS
supplies will be “powered from the reference via the ESD
protection diode”, see page 4.
FIGURE 5. Dual Supply Configuration-Buffered References, used for Dual Supply Performance Curves (1/2 DAC7744).
FIGURE 6. Single-Supply Buffered Reference with a Reference Low of 50mV Used for Single-Supply Performance Curves
(1/2 DAC7744).
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
DAC7744
+10V
+V
–10V
–V
V
OUT
V
OUT
1000pF
2200pF
+V
OPA2234
100Ω
1000pF
2200pF
100Ω
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
DAC7744
99kΩ
2kΩ
+0.050V
+10V
+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.
1000pF
2200pF
+V
OPA350
OPA227
100Ω
1000pF
2200pF
100Ω