DAC0830LCM datasheet(16/28 Pages) NSC

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DAC0830 Series Application Hints (Continued)

This configuration offers several useful application advan-

tages. Since the output is a voltage, an external op amp is

not necessarily required but the output impedance of the

DAC is fairly high (equal to the specified reference input

resistance of 10 k

Ω to 20 kΩ) so an op amp may be used for

buffering purposes. Some of the advantages of this mode

are illustrated in

Figures 12, 13, 14, 15.

There are two important things to keep in mind when using

this DAC in the voltage switching mode. The applied refer-

ence voltage must be positive since there are internal para-

sitic diodes from ground to the I

OUT1 and IOUT2 terminals

which would turn on if the applied reference went negative.

There is also a dependence of conversion linearity and gain

error on the voltage difference between V

CC and the voltage

applied to the normal current output terminals. This is a

result of the voltage drive requirements of the ladder

switches. To ensure that all 8 switches turn on sufficiently (so

as not to add significant resistance to any leg of the ladder

and thereby introduce additional linearity and gain errors) it

is recommended that the applied reference voltage be kept

less than +5V

DC and VCC be at least 9V more positive than

REF. These restrictions ensure less than 0.1% linearity and

gain error change.

Figures 16, 17, 18 characterize the ef-

fects of bringing V

REF and VCC closer together as well as

typical temperature performance of this voltage switching

configuration.

00560811

FIGURE 10. Adding Full-Scale Adjustment

00560812

FIGURE 11. Voltage Mode Switching

00560841

•

Voltage switching mode eliminates output signal inver-

sion and therefore a need for a negative power supply.

•

Zero code output voltage is limited by the low level output

saturation voltage of the op amp. The 2 k

Ω pull-down

resistor helps to reduce this voltage.

•

OS of the op amp has no effect on DAC linearity.

FIGURE 12. Single Supply DAC

DAC0830LCM Datasheet(PDF) 16 Page - National Semiconductor (TI)

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