AD9772AST datasheet(16/30 Pages) AD

16 / 30 page

REV. 0

AD9772

–16–

The current output appearing at IOUTA and IOUTB is a func-

tion of both the input code and IOUTFS and can be expressed as:

IOUTA = (DAC CODE/16384)

× I

OUTFS

(1)

IOUTB = (16383 – DAC CODE)/16384

× I

OUTFS

(2)

where DAC CODE = 0 to 16383 (i.e., Decimal Representation).

As previously mentioned, IOUTFS is a function of the reference

current IREF, which is nominally set by a reference voltage

VREFIO, and external resistor, RSET. It can be expressed as:

IOUTFS = 32

× I

REF

(3)

where

IREF = VREFIO/RSET

(4)

The two current outputs will typically drive a resistive load

directly or via a transformer. If dc coupling is required, IOUTA

and IOUTB should be directly connected to matching resistive

loads, RLOAD, that are tied to analog common, ACOM. Note

that RLOAD may represent the equivalent load resistance seen by

IOUTA or IOUTB as would be the case in a doubly terminated

Ω or 75 Ω cable. The single-ended voltage output appearing

at the IOUTA and IOUTB nodes is simply:

VOUTA = IOUTA

× R

LOAD

(5)

VOUTB = IOUTB

× R

LOAD

(6)

Note that the full-scale value of VOUTA and VOUTB should not

exceed the specified output compliance range of 1.25 V to pre-

vent signal compression. To maintain optimum distortion and

linearity performance, the maximum voltages at VOUTA and

VOUTB should not exceed

±500 mV p-p.

The differential voltage, VDIFF, appearing across IOUTA and

IOUTB, is:

VDIFF = (IOUTA – IOUTB)

× R

LOAD

(7)

Substituting the values of IOUTA, IOUTB and IREF; VDIFF can

be expressed as:

VDIFF = [(2 DAC CODE – 16383)/16384]

(32 RLOAD/RSET)

× V

REFIO

(8)

The last two equations highlight some of the advantages of

operating the AD9772 differentially. First, the differential

operation will help cancel common-mode error sources such as

noise, distortion and dc offsets associated with IOUTA and

IOUTB. Second, the differential code-dependent current and

subsequent voltage, VDIFF, is twice the value of the single-ended

voltage output (i.e., VOUTA or VOUTB), thus providing twice the

signal power to the load.

Note that the gain drift temperature performance for a single-

ended (VOUTA and VOUTB) or differential output (VDIFF) of

the AD9772 can be enhanced by selecting temperature tracking

resistors for RLOAD and RSET due to their ratiometric relation-

ship as shown in Equation 8.

REFERENCE OPERATION

The AD9772 contains an internal 1.20 V bandgap reference

that can easily be disabled and overridden by an external

reference. REFIO serves as either an output or input, depending

on whether the internal or external reference is selected. If

REFLO is tied to ACOM, as shown in Figure 32, the internal

reference is activated, and REFIO provides a 1.20 V output. In

this case, the internal reference must be compensated externally

with a ceramic chip capacitor of 0.1

µF or greater from REFIO

to REFLO. If any additional loading is required, REFIO should

be buffered with an external amplifier having an input bias cur-

rent less than 100 nA.

+1.2V REF

REFIO

FSADJ

CURRENT

SOURCE

ARRAY

250pF

REFLO

AVDD

AD9772

0.1 F

ADDITIONAL

LOAD

OPTIONAL

EXTERNAL

REF BUFFER

+2.7V TO +3.6VA

Figure 32. Internal Reference Configuration

The internal reference can be disabled by connecting REFLO to

AVDD. In this case, an external 1.2 V reference such as the

AD1580 may then be applied to REFIO as shown in Figure 33.

The external reference may provide either a fixed reference

voltage to enhance accuracy and drift performance or a varying

reference voltage for gain control. Note that the 0.1

µF compen-

sation capacitor is not required since the internal reference is

disabled, and the high input impedance of REFIO minimizes

any loading of the external reference.

+1.2V REF

REFIO

FSADJ

CURRENT

SOURCE

ARRAY

250pF

REFLO

AVDD

AD9772

AD1580

+2.7 TO +3.6VA

REFERENCE

CONTROL

AMPLIFIER

RSET

IREF =

VREFIO/RSET

10k

VREFIO

Figure 33. External Reference Configuration

REFERENCE CONTROL AMPLIFIER

The AD9772 also contains an internal control amplifier that is

used to regulate the DAC’s full-scale output current, IOUTFS.

The control amplifier is configured as a V-I converter, as shown

in Figure 33, such that its current output, IREF, is determined by

the ratio of the VREFIO and an external resistor, RSET, as stated

in Equation 4. IREF is copied over to the segmented current

sources with the proper scaling factor to set IOUTFS as stated in

Equation 3.

The control amplifier allows a wide (10:1) adjustment span of

IOUTFS over a 2 mA to 20 mA range by setting IREF between

62.5

µA and 625 µA. The wide adjustment span of I

OUTFS

provides several application benefits. The first benefit relates

directly to the power dissipation of the AD9772’s DAC, which

is proportional to IOUTFS (refer to the Power Dissipation sec-

tion). The second benefit relates to the 20 dB adjustment, which

is useful for system gain control purposes.

IREF can be controlled using the single-supply circuit shown in

Figure 34 for a fixed RSET. In this example, the internal refer-

ence is disabled, and the voltage of REFIO is varied over its

compliance range of 1.25 V to 0.10 V. REFIO can be driven

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