Datasheet

DAC7734
18
SBAS138A
www.ti.com
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 ensured.
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. The DAC7734
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
9 show different reference configurations, and the effect on
the linearity and differential linearity.
The analog supplies must come up first. If V
CC
and V
SS
do not
come up together, then V
SS
should 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 the ESD protection circuits on page 4.
FIGURE 5. Dual-Supply Configuration-Buffered References, used for Dual-Supply Performance (1/2 DAC7734).
FIGURE 6. Single-Supply Buffered Reference with a Reference Low of 50mV used for Single-Supply Performance Curves
(1/2 DAC7734).
+10V
+V
–10V
–V
V
OUT
–15V
V
OUT
V
OUT
A Sense
V
OUT
A
AGND
V
SS
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
DAC7734
2200pF
100Ω
1000pF
1000pF
2200pF
+V
OPA2234
–V
100Ω
V
OUT
V
OUT
A Sense
V
OUT
A
AGND
V
SS
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
DAC7734
+10V
+V
OPA350
OPA227
99.5kΩ
+0.050V
500Ω
50Ω
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.
2200pF
100Ω
1000pF
1000pF
2200pF
+V
100Ω