DAC08

Data Sheet

Rev. D | Page 14 of 21

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

of the reference amplifier. Alternatively, a negative reference may be

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 (nominally equal to R14) cancels 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 must be split into two resistors with the junction bypassed

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 200 Ω

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

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 must be designed to accommodate this current.

Fastest settling times are obtained when Pin 1 sees a low

impedance. If Pin 1 is connected to a 1 kΩ divider, for example,

it must be bypassed to ground by a 0.01 μF capacitor.

ANALOG OUTPUT CURRENTS

Both true and complemented output sink currents are provided

a 1 (logic high) is applied to each logic input. As the binary count

increases, the sink current at Pin 4 increases proportionally, in

the fashion of a positive logic DAC. When a 0 is applied to any

input bit, that current is turned off at Pin 4 and turned on at Pin 2.

logic DAC. Both outputs may be used simultaneously.