18

®

DAC7744

REFERENCE INPUTS

The reference inputs, VREFL and VREFH, can be any voltage

between VSS + 4V and VCC – 4V, 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 –14.25V to

–15.75V. 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.

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 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 VCC and VSS

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

OUTA Sense

V

OUTA

V

REFL AB Sense

V

REFL AB

V

REFH AB

V

REFH AB Sense

V

OUTB Sense

V

OUTB

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

100

Ω

1000pF

2200pF

100

Ω

NC

NC

NC

NC

V

OUTA Sense

V

OUTA

V

REFL AB Sense

V

REFL AB

V

REFH AB

V

REFH AB Sense

V

OUTB Sense

V

OUTB

48

47

46

45

44

43

42

41

40

39

38

37

DAC7744

99k

Ω

2k

Ω

+0.050V

+10V

+V

V

OUT

V

OUT

NOTE: V

REFL has been chosen to be 50mV to allow for current sinking voltage

drops across the 100

Ω resistor and the output stage of the buffer op amp.

1000pF

2200pF

+V

OPA350

OPA227

100

Ω

1000pF

2200pF

100

Ω