DAC08 Datasheet, PDF(9/12 Page) NXP Semiconductors – 8-Bit high-speed multiplying D/A converter

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DAC08 Datasheet, PDF (9/12 Pages) NXP Semiconductors – 8-Bit high-speed multiplying D/A converter

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DAC08

Figure 26. Positive Low Impedance

Output Operation

Figure 27. Negative Low Impedance

Output Operation

Figure 28. Interfacing With Various Logic Families

APPLICATIONS INFORMATION

REFERENCE AMPLIFIER SET-UP

The DAC08 is a multiplying D/A converter in which the output

current is the product of a digital number and the input refer-

ence current. The reference current may be fixed or may vary

from nearly zero to +4.0 mA. The full-scale output current is a

linear function of the reference current and is given by:

IFR =

255

256

Ã IREF, where IREF = I14.

In positive reference applications, an external positive reference

voltage forces current through R14 into the VREF(+) terminal

(pin 14) of the reference amplifier. Alternatively, a negative ref-

erence may be applied to VREF(â) at pin 15; reference current

flows from ground through R14 into VREF(+) as in the positive

reference case. This negative reference connection has the ad-

vantage of a very high impedance presented at pin 15. The volt-

age at pin 14 is equal to and tracks the voltage at pin 15 due to

the high gain of the internal reference amplifier. R15 (nominally

equal to R14) is used to cancel bias current errors; R15 may be

eliminated with only a minor increase in error.

Bipolar references may be accommodated by offsetting VREF or

pin 15. The negative common-mode range of the reference am-

plifier is given by: VCMâ = Vâ plus (IREF Ã 1 kâ¦) plus 2.5 V. The

positive common-mode range is V+ less 1.5 V.

When a dc reference is used, a reference bypass capacitor is rec-

ommended. A 5.0 V TTL logic supply is not recommended as a

reference. If a regulated power supply is used as a reference, R14

should be split into two resistors with the junction bypassed to

ground with a 0.1 ÂµF capacitor.

For most applications the tight relationship between IREF and

IFS will eliminate the need for trimming IREF. If required,

full-scale trimming may be accomplished by adjusting the value

of R14, or by using a potentiometer for R14. An improved

method of full-scale trimming which eliminates potentiometer

T.C. effects is shown in the recommended full-scale adjustment

circuit.

Using lower values of reference current reduces negative power

supply current and increases reference amplifier negative common-

mode range. The recommended range for operation with a dc

reference current is +0.2 mA to +4.0 mA.

REV. A

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