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Data Sheet AD9763/AD9765/AD9767

Rev. G | Page 23 of 44

DAC TRANSFER FUNCTION

Both DACs in the AD9763/AD9765/AD9767 provide comple-

mentary current outputs, I

OUTA

and I

OUTB

. I

OUTA

provides a near

full-scale current output (I

OUTFS

) when all bits are high (that is,

DAC CODE = 1024/4095/16,384 for the AD9763/AD9765/

AD9767, respectively), while I

OUTB

, the complementary output,

provides no current. The current output appearing at I

OUTA

and

OUTB

is a function of both the input code and I

OUTFS

. I

OUTA

for the

AD9763, AD9765, and AD9767, respectively, can be expressed as

OUTA

= (DAC CODE/1024) × I

OUTFS

(1)

OUTA

= (DAC CODE/4096) × I

OUTFS

OUTA

= (DAC CODE/16,384) × I

OUTFS

OUTB

for the AD9763, AD9765, and AD9767, respectively, can be

expressed as

OUTB

= ((1023 − DAC CODE)/1024) × I

OUTFS

(2)

OUTB

= ((4095 − DAC CODE)/4096) × I

OUTFS

OUTB

= ((16,383 − DAC CODE)/16,384) × I

OUTFS

where DAC CODE = 0 to 1024, 0 to 4095, or 0 to 16,384 (decimal

representation).

OUTFS

is a function of the reference current (I

REF

). This is nominally

set by a reference voltage (V

REFIO

) and an external resistor (R

SET

It can be expressed as

OUTFS

= 32 × I

REF

(3)

where I

REF

is set as discussed in the Setting the Full-Scale

Current section.

The two current outputs typically drive a resistive load directly

or via a transformer. If dc coupling is required, I

OUTA

and I

OUTB

should be directly connected to matching resistive loads (R

LOAD

)

that are tied to the analog common (ACOM). Note that R

LOAD

can represent the equivalent load resistance seen by I

OUTA

or I

OUTB

as is the case in a doubly terminated 50  or 75  cable. The single-

ended voltage output appearing at the I

OUTA

and I

OUTB

nodes is

OUTA

= I

OUTA

× R

LOAD

(5)

OUTB

= I

OUTB

× R

LOAD

(6)

Note that the full-scale value of V

OUTA

and V

OUTB

must not

exceed the specified output compliance range to maintain the

specified distortion and linearity performance.

DIFF

= (I

OUTA

− I

OUTB

) × R

LOAD

(7)

Equation 7 highlights some of the advantages of operating the

AD9763/AD9765/AD9767 differentially. First, the differential

operation helps cancel common-mode error sources associated

with I

OUTA

and I

OUTB

such as noise, distortion, and dc offsets.

Second, the differential code-dependent current and subsequent

voltage, V

DIFF

, is twice the value of the single-ended voltage

output (that is, V

OUTA

or V

OUTB

), thus providing twice the signal

power to the load.

The gain drift temperature performance for a single-ended

OUTA

and V

OUTB

) or differential output (V

DIFF

) of the

AD9763/AD9765/AD9767 can be enhanced by selecting

temperature tracking resistors for R

LOAD

and R

SET

due to their

ratiometric relationship.

ANALOG OUTPUTS

The complementary current outputs, I

OUTA

and I

OUTB

, in each

DAC can be configured for single-ended or differential

operation. I

OUTA

and I

OUTB

can be converted into complementary

single-ended voltage outputs, V

OUTA

and V

OUTB

, via a load

resistor (R

LOAD

) as described in Equation 5 through Equation 7.

The differential voltage (V

DIFF

) existing between V

OUTA

and V

OUTB

can be converted to a single-ended voltage via a transformer or

differential amplifier configuration. The ac performance of the

AD9763/AD9765/AD9767 is optimum and specified using a

differential transformer-coupled output in which the voltage

swing at I

OUTA

and I

OUTB

is limited to ±0.5 V. If a single-ended

unipolar output is desired, select IOUTA.

The distortion and noise performance of the AD9763/AD9765/

AD9767 can be enhanced when it is configured for differential

operation. The common-mode error sources of both I

OUTA

and

OUTB

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 waveform

increases. This is due to the first-order cancellation of various

dynamic common-mode distortion mechanisms, digital feed-

through, and noise.

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

former also provides the ability to deliver twice the reconstructed

signal power to the load, assuming no source termination. Because

the output currents of I

OUTA

and I

OUTB

are complementary, they

become additive when processed differentially. A properly selected

transformer allows the AD9763/AD9765/AD9767 to provide the

required power and voltage levels to different loads.

The output impedance of I

OUTA

and I

OUTB

is determined by the

equivalent parallel combination of the PMOS switches associated

with the current sources and is typically 100 k in parallel with

5 pF. It is also slightly dependent on the output voltage (that is,

OUTA

and V

OUTB

) due to the nature of a PMOS device. As a result,

maintaining I

OUTA

and/or I

OUTB

at a virtual ground via an I-V

op amp configuration results in the optimum dc linearity. Note that

the INL/DNL specifications for the AD9763/AD9765/AD9767 are

measured with I

OUTA

maintained at a virtual ground via an op amp.