AD768ACHIPS datasheet(6/20 Pages) AD

6 / 20 page

REV. B

–6–

AD768

IREFIN – mA

550

500

300

1.0

7.0

2.0

3.0

4.0

5.0

6.0

450

400

350

Figure 3. Power Dissipation vs. IREFIN Current

Note the AD768 is optimized for operation at an input current

of 5 mA. Both linearity and dynamic performance at other input

currents may be somewhat degraded. Figure 4 shows typical dc

linearity over a range of input currents. Figure 5 shows typical

SFDR (to Nyquist) performance over a range of input currents

and CLOCK input rates for a 1 MHz output frequency.

IREFIN – mA

1.0

7.0

2.0

3.0

4.0

5.0

6.0

INL

DNL

Figure 4. INL/DNL vs. IREFIN Current

IREFIN – mA

–85

–55

1.0

7.0

2.0

3.0

4.0

5.0

6.0

–80

–75

–70

–65

–60

CLOCK = 10 MSPS

CLOCK = 20 MSPS

CLOCK = 30 MSPS

CLOCK = 40 MSPS

Figure 5. SFDR (to Nyquist) vs. IREFIN @ FOUT = 1 MHz

DAC TRANSFER FUNCTION

The AD768 may be used in either current-output mode with the

output connected to a virtual ground, or voltage-output mode

with the output connected to a resistive load.

In current output mode,

IOUT = (DAC CODE/65536)

× (I

REFIN

× 4)

In voltage output mode,

VOUT = IOUT

× R

LOAD RLAD

where:

DAC CODE is the decimal representation of the DAC inputs;

an integer between 0 and 65535.

IREFIN is the current applied at the IREFIN pin, determined by

VREF/RREF.

Substituting for IOUT and IREFIN,

VOUT = –VREF

× (DAC CODE/65536) × 4 × [(R

LOAD RLAD)/RREF]

These equations clarify an important aspect of the AD768

transfer function; the full-scale current output of the DAC is

proportional to a current input. The voltage output is then a

function of the ratio of (RLOAD RLAD)/RREF, allowing for cancel-

lation of resistor drift by selection of resistors with matched

characteristics.

REFERENCE INPUT

The IREFIN pin is a current input node with low impedance to

REFCOM. This input current sets the magnitude of the DAC

current sources such that the full-scale output current is exactly

four times the current applied at IREFIN. For the nominal in-

put current of 5 mA, the nominal full-scale output current is

20 mA.

The 5 mA reference input current can be generated from the

on-chip 2.5 V reference with an external resistor of 500

Ω from

REFOUT to IREFIN. If desired, a variety of external reference

voltages may be used based on the selection of an appropriate

resistor. However, to maintain stability of the reference ampli-

fier, the external impedance at IREFIN must be kept below

Ω.

REFCOM

IREFIN

VEE

IFB

5mA

Figure 2. Equivalent Reference Input Circuit

The IREFIN current can be varied from 1 mA to 7 mA which

subsequently will result in a proportional change in the DAC

full-scale. Since the operating currents within the DAC vary

with IREFIN, so does the power dissipation. Figure 3 illustrates

that relationship.

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