AD9744 datasheet(14/32 Pages) AD

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AD9744

Rev. B | Page 14 of 32

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 IOUTFS pro-

vides several benefits. The first relates directly to the power

dissipation of the AD9744, which is proportional to IOUTFS

(refer to the Power Dissipation section). The second relates to

the 20 dB adjustment, which is useful for system gain control

purposes.

The small signal bandwidth of the reference control amplifier is

approximately 500 kHz and can be used for low frequency small

signal multiplying applications.

DAC TRANSFER FUNCTION

Both DACs in the AD9744 provide complementary current

outputs, IOUTA and IOUTB. IOUTA provides a near full-scale

current output, IOUTFS, when all bits are high (that is, DAC

CODE = 16383), while IOUTB, the complementary output,

provides no current. The current output appearing at IOUTA

and IOUTB is a function of both the input code and IOUTFS and

can be expressed as

(

)

OUTFS

CODE

DAC

IOUTA

16384

(1)

(

)

OUTFS

CODE

DAC

IOUTB

−

/16384

16383

(2)

where DAC CODE = 0 to 16383 (that is, decimal representation).

As mentioned previously, 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

REF

OUTFS

= 32

(3)

where

SET

REFIO

REF

(4)

The two current outputs will typically drive a resistive load di-

rectly 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

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

at the IOUTA and IOUTB nodes is simply

LOAD

OUTA

IOUTA

(5)

LOAD

OUTB

IOUTB

(6)

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

exceed the specified output compliance range to maintain speci-

fied distortion and linearity performance.

(

)

LOAD

DIFF

IOUTB

IOUTA

−

(7)

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

expressed as

(

)

[

]

()

REFIO

SET

LOAD

DIFF

CODE

DAC

−

16384

16383

(8)

Equation 7 and Equation 8 highlight some of the advantages

of operating the AD9744 differentially. First, the differential

operation helps cancel common-mode error sources associated

with IOUTA and IOUTB, such as noise, distortion, and dc

offsets. Second, the differential code dependent current and

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

voltage output (that is, 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

AD9744 can be enhanced by selecting temperature tracking

resistors for RLOAD and RSET due to their ratiometric relationship,

as shown in Equation 8.

ANALOG OUTPUTS

The complementary current outputs in each DAC, IOUTA,

and IOUTB may be configured for single-ended or differential

operation. IOUTA and IOUTB can be converted into comple-

mentary single-ended voltage outputs, VOUTA and VOUTB, via a

load resistor, RLOAD, as described in the DAC Transfer Function

section by Equation 5 through Equation 8. The differential

voltage, VDIFF, existing between VOUTA and VOUTB, can also be

converted to a single-ended voltage via a transformer or

differential amplifier configuration. The ac performance of the

AD9744 is optimum and specified using a differential trans-

former-coupled output in which the voltage swing at IOUTA

and IOUTB is limited to ±0.5 V.

The distortion and noise performance of the AD9744 can be

enhanced when it is configured for differential operation. The

common-mode error sources of both IOUTA and IOUTB can

be significantly reduced by the common-mode rejection of a

transformer or differential amplifier. These common-mode

error sources include even-order distortion products and noise.

The enhancement in distortion performance becomes more

significant as the frequency content of the reconstructed wave-

form increases and/or its amplitude decreases. This is due to the

first-order cancellation of various dynamic common-mode

distortion mechanisms, digital feedthrough, and noise.

Performing a differential-to-single-ended conversion via a

transformer also provides the ability to deliver twice the

reconstructed signal power to the load (assuming no source

termination). Since the output currents of IOUTA and IOUTB

are complementary, they become additive when processed

differentially. A properly selected transformer will allow the

AD9744 to provide the required power and voltage levels to

different loads.

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