AD9768

REV. A

–3–

speed, high performance device: optimum use requires careful

attention to all design details, including the layout of the circuit

in which the converter is used.

CONVENTIONAL AD9768SD

Refer to Figure 1, Conventional AD9768SD.

source discussed above)

Figure 1. Conventional AD9768SD

the user for his application. The necessary equations for

calculating precise values for each are part of Figure 1. As

VOLTAGE MULTIPLYING MODE

In addition to its use as an ultra-high speed current output D/A

converter, the AD9768 can also be used as a two-quadrant

multiplying D/A in either a voltage mode or a current mode.

Refer to Figure 2, Multiplying AD9768 (Voltage Mode).

When operating in this mode, the analog output of the AD9768

is influenced by the digital inputs and an external multiplying

the place of the internal reference used when the D/A is

operating in a conventional manner.

Figure 2. Multiplying AD9768 (Voltage Mode)

amount of this current is part of the equation which establishes

user for his application. As it is when operating the D/A in a

voltage and can be varied over a range which is approximately 1

volt peak-to-peak.

THEORY OF OPERATION

Refer to the AD9768SD schematic.

The transistors pictured on the bottom of the diagram, con-

nected to paired transistors in the middle of the schematic, are

current sources which are always “on”. The paired transistors

are differential current switches, designed to steer current from

Digital inputs applied to Pins 1-8 determine which transistors

will be operating in each pair and establish what current will

flow at Pins 13 and 14.

The transistor on the extreme left of the schematic is a base

reference for the paired current switches and is used to assure

the switches will be centered around an ECL voltage swing. The

diodes connected to the base of this transistor are temperature

compensation devices for the base reference circuit.

There are three different current sources in the AD9768 D/A.

The eight transistors shown on the bottom of the schematic are

structured as two identical groups of four current sources, each

of which is binarily weighted. The MSB group, comprised of the

four on the right, is connected to the LSB group through a 15:1

current divider made up of two 50

Ω and two 750 Ω resistor

networks. The geometry of the AD9768 guarantees the binary

weighing ratios among the 100, 200, 400 and 800 resistors in

each emitter circuit are correct.

The resistor values which are shown indicate the ratios among

the resistors, and not their nominal values.

The third current source is a single transistor, pictured in the

lower left portion of the schematic with its collector connected

on the eight current sources; it works in conjunction with the

the reference amplifier. Current flowing through this transistor

When the AD9768 is operating as a conventional current-output

the inputs to the on-board reference amplifier shown in the

schematic. The other input to this amplifier is the on-chip

reference voltage of –1.26 volts.

The output of the reference amplifier adjusts the current-source

base reference voltage at Pin 17; this, in turn, adjusts the value

–1.26 volts of the on-chip reference supply.

To maintain good stability in the internal loop reference

amplifier, a ceramic chip capacitor with a nominal value of

0.01

µF should be connected to Pin 17 COMPENSATION;

minimum recommended value for this capacitor is 3900 pF.

the performance of the AD9768 D/A converter, as it can with

any current-output converter. The design and use of the

perature coefficients.

DIGITAL GROUND (Pin 11) and ANALOG RETURN (Pin

12) are normally connected together; this connection should be

made as close as possible to the device case to minimize possible

noise problems. The AD9768 D/A is similar to any other high-