9

®

DAC707/708/709

In Figures 7 and 8, lead and contact resistances are repre-

sented by R

is constant, R

removed with gain calibration. R

voltage is sensed at ANALOG COMMON.

Figures 8 and 9 show two methods of connecting the current

output model with an external precision output op amp. By

sensing the output voltage at the load resistor (connecting R

F

to the output of the amplifier at R

is greatly reduced. R

dent of the value of R

calibration adjustments. The effect of R

cause it is inside the feedback loop of the output op amp and

is therefore greatly reduced by the loop gain.

In many applications it is impractical to sense the output

voltage at ANALOG COMMON. Sensing the output volt-

age at the system ground point is permissible because these

converters have separate analog and digital common lines

and the analog return current is a near-constant 2mA and

varies by only 10

µA to 20µA over the entire input code

range. R

4

can be as large as 3

Ω without adversely affecting

the linearity of the D/A converter. The voltage drop across

R

4

is constant and appears as a zero error that can be nulled

with the zero calibration adjustment.

Another approach senses the output at the load as shown in

Figure 9. In this circuit the output voltage is sensed at the

load common and not at the D/A converter common as in the

previous circuits. The value of R

for maximum common-mode rejection across R

of R

The D/A converter and the wiring to its connectors should be

located to provide optimum isolation from sources of RFI

and EMI. The key to elimination of RF radiation or pickup

is small loop area. Signal leads and their return conductors

should be kept close together such that they present a small

flux-capture cross section for any external field.

FIGURE 9. Alternate Connection for Ground Sensing at the

Load (Current Output Models).

5

6

7

DAC

R

1

R

2

3

R

R

R

R

R

Sense

Output

To System Ground

L

R

DAC708

Ω

4k

Ω

2k

Ω

10k

2mA

+1%

0 to

2mA

Micro-

Processor

Interface

L

2

CC

DD

4

3

R

R

R

R

1µF

+

1µF

+

1µF

+

+V

V

±V

CC

Digital

Common

Analog

Common

Alternate Ground

Sense Connection

System

Ground

DD

V

Supply

Supply

Sense

Output

Digital

Common

Analog Common

DAC707/709

FIGURE 7. DAC707/709 Bipolar Output Circuit (Voltage

Out).

1

R

Ω

2.45k

Ω

10k

Micro-

Processor

Interface

L

2

CC

DD

3

R

R

R

1µF

+

1µF

+

1µF

+

+V

V

±V

CC

Digital

Common

Analog

Common

Alternate Ground

Sense Connection

System

Ground

DD

V

Supply

Supply

Sense

Output

Digital

Common

Analog Common

B

R

1

RF

DAC708

4

R

FIGURE 8. DAC708 Bipolar Output Circuit (with External

Op Amp).