13

®

DAC7641

ANALOG OUTPUTS

When VSS = –5V (dual supply operation), the output ampli-

fier can swing to within 2.25V of the supply rails, guaran-

teed over the –40

°C to +85°C temperature range. With V

SS

= 0V (single-supply operation), and with RLOAD also con-

nected to ground, the output can swing to ground. Care must

be taken when measuring the zero-scale error with VSS = 0V.

Since the output voltage cannot swing below ground, the

output voltage may not change for the first few digital input

codes (0000H, 0001H, 0002H, etc.) if the output amplifier has

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

specified output starts at code 0040H.

Due to the high accuracy of these D/A converters, 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, series wiring and connector resistance (see Figure

4) of only 40m

Ω (R

W2) will cause a voltage drop of 40µV.

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 DAC7641 offers a force and sense output configuration

for the high open-loop gain output amplifier. This feature

allows the loop around the output amplifier to be closed at the

load (see Figure 4), thus ensuring an accurate output voltage.

REFERENCE INPUTS

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

FIGURE 4. Analog Output Closed-Loop Configuration. RW

represents wiring resistances.

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.

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 DAC7641 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 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 Performance Curves.

DAC7641

R

W1

R

W2

V

OUT Sense

V

OUT

V

SS

24

23

22

+2.5V

+V

V

OUT

500pF

OPA2234

500pF

+2.5V

+V

–2.5V

–V

DAC7641

V

OUT Sense

V

OUT

V

SS

24

23

22

V

OUT