AD9768 datasheet(4/4 Pages) AD

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AD9768

REV. A

–4–

If the load resistor (RL) has a value of 50 ohms, if RSET has a

value of 220 ohms, and if VRET is 0 V, the center of the VM

voltage will be –0.6 V; and it can vary from –0.1 V to –1.1 V.

Typically, the frequency of these variations has an upper limit of

250 kHz when operating in the voltage multiplying mode; that

frequency is the 3 dB point of the bandwidth of the internal

reference amplifier.

The combined effects of variations in VM and changes in digital

input values are shown in Figure 3, IOUT vs. Multiplying Volt-

age. In this illustration, the ordinate of the graph is expressed in

terms of milliamps of IOUT current at Pin 13. VOUT, of course,

will be a function of the value of RL chosen by the user.

Figure 3. IOUT vs. Multiplying Voltage

The negative value of VM on the horizontal axis is shown start-

ing at approximately –0.1 V, rather than 0 V, because the

AD9768 must have some small value of voltage applied to per-

form a multiplying function. For the conditions shown in the

figure, output current starts to become nonlinear at approxi-

mately 20 mA because of the maximum 30 mA output drive

capabilities of the device. Different values for RSET and RL

would alter the point where limiting first appears.

CURRENT MULTIPLYING MODE

The AD9768 D/A converter can be operated at markedly higher

multiplying rates when operated in a current-multiplying mode,

as contrasted with the voltage multiplying mode. Refer to Figure

4, Multiplying AD9768SD (Current Mode).

Figure 4. Multiplying AD9768SD (Current Mode)

In this mode, the internal reference amplifier and its inherent

frequency limitations are replaced by a current source comprised

of U1 and associated circuits. These circuits supply a unipolar

current IM which is one-fourth the full-scale output current

(with digital “1” applied to all inputs) and set current flow

through the load resistor.

VIN is some voltage chosen by the user for his particular applica-

tion; the value of this voltage is based in part on the size of the

load resistor and the 0 mA to 5 mA range of IM. VIN can have

frequency components as high as 40 MHz. VADJ and RADJ pro-

vide an offset adjustment to compensate for the dc component

of VIN to assure IM is always a unipolar current between 0 mA

and 5 mA. The values of the required voltages and resistors can

be calculated using the equations which are part of Figure 4.

Refer to Figure 5, IOUT vs. Multiplying Current.

Figure 5. IOUT vs. Multiplying Current

As shown, IM can vary over the range of 0 mA to 5 mA; a value

of approximately 0.3 mA may be the practical lower limit because

of nonlinearities at extremely small current levels. These changes

in IM are combined with variations in digital inputs, producing

complex changes in the output current (at pin 13) and in VOUT.

The “rounding” of the current curve in the graph is the result of

IOUT approaching the 30 mA maximum drive capabilities of the

AD9768 and needs to be taken into account to assure optimum

performance in the selected application.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

Ceramic (D-18)

LCC (E-20A)

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