AD9714BCPZ datasheet(44/80 Pages) AD

44 / 80 page

AD9714/AD9715/AD9716/AD9717

Data Sheet

Rev. B | Page 44 of 80

DAC TRANSFER FUNCTION

The AD9714/AD9715/AD9716/AD9717 provide two differen-

tial current outputs, IOUTP/IOUTN and QOUTP/QOUTN.

IOUTP and QOUTP provide a near full-scale current output,

IxOUTFS, when all bits are high (that is, DAC CODE = 2N − 1,

where N = 8, 10, 12, or 14 for the AD9714, AD9715, AD9716,

and AD9717, respectively), while IOUTN and QOUTN, the

complementary outputs, provide no current. The current

outputs appearing at the positive DAC outputs, IOUTP and

QOUTP, and at the negative DAC outputs, IOUTN and QOUTN,

are a function of both the input code and IxOUTFS and can be

expressed as follows:

IOUTP

= (IDAC CODE/2

N) × IIOUTFS

(1)

QOUTP

= (QDAC CODE/2

N) × IQOUTFS

IOUTN

= ((2

N − 1) − IDAC CODE)/2N × IIOUTFS

(2)

QOUTN

= ((2

N − 1) − QDAC CODE)/2N × IQOUTFS

where:

IDAC CODE

and QDAC CODE = 0 to 2N − 1 (that is, decimal

representation).

IIOUTFS

and IQOUTFS are functions of the reference currents, IIREF

and IQREF, respectively, which are nominally set by a reference

voltage, VREFIO, and external resistors, IRSET and QRSET, respec-

tively. IIOUTFS and IQOUTFS can be expressed as follows:

IIOUTFS

= 32 × IIREF

(3)

IQOUTFS

= 32 × IQREF

where:

IIREF

= VREFIO/IRSET

(4)

IQREF

= VREFIO/QRSET

IIOUTFS

= 32 × VREFIO/IRSET

(5)

IQOUTFS = 32 × VREFIO/QRSET

A differential pair (IOUTP/IOUTN or QOUTP/QOUTN)

typically drives a resistive load directly or via a transformer. If

dc coupling is required, the differential pair (IOUTP/IOUTN or

QOUTP/QOUTN) should be connected to matching resistive

loads, xRLOAD, that are tied to analog common, AVSS. The

single-ended voltage output appearing at the positive and

negative nodes is

VIOUTP

= IOUTP × IRLOAD

(6)

VQOUTP

= QOUTP × QRLOAD

VIOUTN

= IOUTN × IRLOAD

(7)

VQOUTN

= QOUTN × QRLOAD

To achieve the maximum output compliance of 1 V at the

nominal 4 mA output current, IRLOAD = QRLOAD must be set

to 250 Ω.

Substituting the values of IOUTP, IOUTN, and IxREF, VIDIFF can

be expressed as

VIDIFF

= {(2 × IDAC CODE – (2

N − 1))/2N} ×

(8)

(32 × VREFIO/IRSET) × IRLOAD

Equation 8 highlights some of the advantages of operating the

AD9714/AD9715/AD9716/AD9717 differentially. First, the

differential operation helps cancel common-mode error sources

associated with IOUTP and IOUTN, such as noise, distortion,

and dc offsets. Second, the differential code-dependent current and

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

voltage output (that is, VIOUTP or VIOUTN), thus providing twice

the signal power to the load. Note that the gain drift temperature

performance for a single-ended output (VIOUTP and VIOUTN) or

differential output (VIDIFF) of the AD9714/AD9715/AD9716/

AD9717 can be enhanced by selecting temperature-tracking

resistors for xRLOAD and xRSET because of their ratiometric

relationship, as shown in Equation 8.

ANALOG OUTPUT

The complementary current outputs in each DAC, IOUTP/

IOUTN and QOUTP/QOUTN, can be configured for single-

ended or differential operation. IOUTP/IOUTN and QOUTP/

QOUTN can be converted into complementary single-ended

voltage outputs, VIOUTP and VIOUTN, as well as VQOUTP and VQOUTN

via a load resistor, xRLOAD, as described in the DAC Transfer

Function section by Equation 6 through Equation 8. The differen-

tial voltages, VIDIFF and VQDIFF, existing between VIOUTP and VIOUTN,

and VQOUTP and VQOUTN, can also be converted to a single-ended

voltage via a transformer or a differential amplifier configuration.

The ac performance of the AD9714/AD9715/AD9716/AD9717

is optimum and is specified using a differential transformer-

coupled output in which the voltage swing at IOUTP and IOUTN

is limited to ±0.5 V. The distortion and noise performance of

the AD9714/AD9715/AD9716/AD9717 can be enhanced when

it is configured for differential operation. The common-mode

error sources of both IOUTP/IOUTN and QOUTP/QOUTN

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 increases. 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). Because

the output currents of IOUTP/IOUTN and QOUTP/QOUTN

are complementary, they become additive when processed

differentially.

AD9714BCPZ Datasheet(PDF) 44 Page - Analog Devices

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