DAC08

Rev. C | Page 13 of 20

APPLICATION INFORMATION

REFERENCE AMPLIFIER SETUP

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

current is the product of a digital number and the input

reference 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

REF

FR

I

I

×

=

256

255

In positive reference applications, an external positive reference

(Pin 14) of the reference amplifier. Alternatively, a negative

reference case. This negative reference connection has the

advantage of a very high impedance presented at Pin 15. The

voltage at Pin 14 is equal to and tracks the voltage at Pin 15 due

to the high gain of the internal reference amplifier. R15 (nomi-

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

may be eliminated with only a minor increase in error.

Pin 15. The negative common-mode range of the reference

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

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

recommended. 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 bypas-

sed to ground with a 0.1 µF capacitor.

trimming can be accomplished by adjusting the value of R14, or

by using a potentiometer for R14. An improved method of full-

scale trimming that eliminates potentiometer T.C. effects is

shown in the recommended full-scale adjustment circuit

(Figure 27).

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.

REFERENCE AMPLIFIER COMPENSATION FOR

MULTIPLYING APPLICATIONS

AC reference applications require the reference amplifier to be

compensated using a capacitor from Pin 16 to V−. The value of

this capacitor depends on the impedance presented to Pin 14;

for R14 values of 1.0 kΩ, 2.5 kΩ, and 5.0 kΩ, minimum values

For fastest response to a pulse, low values of R14 enabling small

such as a transistor current source, none of the preceding values

suffice, and the amplifier must be heavily compensated, which

decreases overall bandwidth and slew rate. For R14 = 1 kΩ and

Operation with pulse inputs to the reference amplifier can be

accommodated by an alternate compensation scheme. This

technique provides lowest full-scale transition times. An internal

clamp allows quick recovery of the reference amplifier from a

occurs in 120 ns when the equivalent impedance at Pin 14 is

LOGIC INPUTS

The DAC08 design incorporates a unique logic input circuit

that enables direct interface to all popular logic families and

provides maximum noise immunity. This feature is made

possible by the large input swing capability, 2 µA logic input

current, and completely adjustable logic threshold voltage. For

V− = −15 V, the logic inputs may swing between −10 V and

+18 V. This enables direct interface with 15 V CMOS logic, even

when the DAC08 is powered from a 5 V supply. Minimum

input logic swing and minimum logic threshold voltage are

given by

The logic threshold may be adjusted over a wide range by placing

an appropriate voltage at the logic threshold control pin (Pin 1,

For TTL and DTL interface, simply ground Pin 1. When

other logic families, see Figure 32. For general set-up of the

logic control circuit, note that Pin 1 sources 100 µA typical;

external circuitry should be designed to accommodate this

current.