DAC7642, DAC7643

14

SBAS233

www.ti.com

nected to ground or must be in the range of –4.75V to

the bias values may be in error and proper operation of the

device may be affected.

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 references

applied can sink or source the required current, a reference

buffer is not required. The DAC7642 and DAC7643 feature

reference drive and sense connections such that the internal

errors caused by the changing reference current and the

circuit impedances can be minimized. Figures 5 through 13

show different reference configurations and the effect on the

linearity and differential linearity.

FIGURE 5. Dual Supply Configuration-Buffered References, Used for Dual-Supply Characteristic Curves.

V

V

V

V

V

V

V

V

32

31

30

29

28

27

26

25

DAC7642

DAC7643

1000pF

V

OUT

V

OUT

100

Ω

100

Ω

+2.5V

+V

–2.5V

–V

–V

+V

OPA2234

2200pF

1000pF

2200pF

ANALOG OUTPUTS

can swing to within 2.25V of the supply rails over the –40

°C

to +85

°C temperature range. When V

output can swing to ground. Care must also be taken when

DAC output cannot swing below ground, the output voltage

offset. At the negative limit of –2mV, the first specified output

Due to the high accuracy of these DACs, system design

problems such as grounding and contact resistance become

very important. A 16-bit converter with a 2.5V full-scale range

has a 1LSB value of 38

µV. With a load current of 1mA, a series

wiring and connector resistance of only 40m

Ω (R

a voltage drop of 40

µV, as shown in Figure 4. To understand

what this means in terms of a system layout, the resistivity of

a typical 1 ounce copper-clad printed circuit board is 1/2 m

Ω

per square. For a 1mA load, a 10 milli-inch wide printed circuit

conductor 600 milli-inches long will result in a voltage drop of

30

µV.

The DAC7642 and DAC7643 offer a force and sense output

configuration for the high open-loop gain output amplifiers.

This feature allows the loop around the output amplifier to be

closed at the load (shown in Figure 4), thus ensuring an

accurate output voltage.

REFERENCE INPUTS

(essentially, the offset of the output op amp). The maximum

represents wiring resistances.

V

V

V

V

V

V

V

V

32

31

30

29

28

27

26

25

DAC7642

DAC7643

R

W2

R

W1

+2.5V

+V

V

OUT

R

W1

R

W2

V

OUT