REFERENCE INPUTS
The current into the VREFH input and out of VREFL 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.
The reference inputs, VREFL and VREFH, can be any voltage
between VSS + 2.5V and VCC – 2.5V provided that VREFH is
at least 1.25V greater than VREFL. The minimum output of
each DAC is equal to VREFL plus a small offset voltage
(essentially, the offset of the output op amp). The maximum
output is equal to VREFH plus a similar offset voltage. Note
that VSS (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 VSS sets several bias points within
the converter. If VSS 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.
NC 48
NC 47
NC 46
NC 45
+V
V
OUTA Sense 44
OUTA 43
REFL AB Sense 42
REFL AB 41
REFH AB 40
REFH AB Sense 39
OUTB Sense 38
OUTB 37
OPA2234
VOUT
V
DAC7644
V
–2.5V
V
500pF
V
–V
+V
V
500pF
V
V
VOUT
+2.5V
–V
FIGURE 5. Dual Supply Configuration-Buffered References, used for Dual Supply Performance Curves (1/2 DAC7644).
NC 48
NC 47
NC 46
+V
NC 45
VOUTA Sense 44
OPA2350
VOUT
V
OUTA 43
REFL AB Sense 42
REFL AB 41
REFH AB 40
REFH AB Sense 39
OUTB Sense 38
OUTB 37
DAC7644
V
2kΩ
100Ω
100Ω
2200pF
2200pF
V
1000pF
1000pF
0.050V
V
+V
98kΩ
V
V
+2.5V
V
VOUT
NOTE: VREFL 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.
FIGURE 6. Single-Supply Buffered Reference with a Reference Low of 50mV (1/2 DAC7644).
®
17
DAC7644
BB [ BURR-BROWN CORPORATION ]
