AD9765AST datasheet(23/44 Pages) AD

23 / 44 page

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, IOUTA and IOUTB. IOUTA provides a near

full-scale current output (IOUTFS) when all bits are high (that is,

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

AD9767, respectively), 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. IOUTA for the

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

IOUTA = (DAC CODE/1024) × IOUTFS

(1)

IOUTA = (DAC CODE/4096) × IOUTFS

IOUTA = (DAC CODE/16,384) × IOUTFS

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

expressed as

IOUTB = ((1023 − DAC CODE)/1024) × IOUTFS

(2)

IOUTB = ((4095 − DAC CODE)/4096) × IOUTFS

IOUTB = ((16,383 − DAC CODE)/16,384) × IOUTFS

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

representation).

IOUTFS is a function of the reference current (IREF). This is nominally

set by a reference voltage (VREFIO) and an external resistor (RSET).

It can be expressed as

IOUTFS = 32 × IREF

(3)

where IREF 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, IOUTA and IOUTB

should be directly connected to matching resistive loads (RLOAD)

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

can represent the equivalent load resistance seen by IOUTA or IOUTB,

as is 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

VOUTA = IOUTA × RLOAD

(5)

VOUTB = IOUTB × RLOAD

(6)

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

exceed the specified output compliance range to maintain the

specified distortion and linearity performance.

VDIFF = (IOUTA − IOUTB) × RLOAD

(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 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.

The gain drift temperature performance for a single-ended

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

AD9763/AD9765/AD9767 can be enhanced by selecting

temperature tracking resistors for RLOAD and RSET due to their

ratiometric relationship.

ANALOG OUTPUTS

The complementary current outputs, IOUTA and IOUTB, in each

DAC can be configured for single-ended or differential

operation. IOUTA and IOUTB can be converted into complementary

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

resistor (RLOAD) as described in Equation 5 through Equation 7.

The differential voltage (VDIFF) existing between VOUTA and VOUTB

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 IOUTA and IOUTB 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 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 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 IOUTA and IOUTB 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 IOUTA and IOUTB is determined by the

equivalent parallel combination of the PMOS switches associated

with the current sources and is typically 100 kΩ in parallel with

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

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

maintaining IOUTA and/or IOUTB 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 IOUTA maintained at a virtual ground via an op amp.

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