AD9714-EBZ datasheet(43/48 Pages) AD

43 / 48 page

AD9714/AD9715/AD9716/AD9717

Rev. 0 | Page 43 of 48

APPLICATIONS INFORMATION

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output confi-

gurations for the AD9714/AD9715/AD9716/AD9717. Unless

otherwise noted, it is assumed that IOUTFS is set to a nominal

2 mA. For applications requiring the optimum dynamic perfor-

mance, a differential output configuration is suggested. A

differential output configuration can consist of either an

RF transformer or a differential op amp configuration. The

transformer configuration provides the optimum high fre-

quency performance and is recommended for any application

that allows ac coupling. The differential op amp configuration

is suitable for applications requiring dc coupling, signal gain,

and/or a low output impedance.

A single-ended output is suitable for applications where low

cost and low power consumption are primary concerns.

DIFFERENTIAL COUPLING USING A TRANSFORMER

An RF transformer can be used to perform a differential-to-

single-ended signal conversion, as shown in Figure 89. The

distortion performance of a transformer typically exceeds

that available from standard op amps, particularly at higher

frequencies. Transformer coupling provides excellent rejection

of common-mode distortion (that is, even-order harmonics)

over a wide frequency range. It also provides electrical isolation

and can deliver voltage gain without adding noise. Transformers

with different impedance ratios can also be used for impedance

matching purposes. The main disadvantages of transformer

coupling are low frequency roll-off, lack-of-power gain, and

high output impedance.

AD9714/AD9715/

AD9716/AD9717

IOUTN

IOUTP

OPTIONAL RDIFF

RLOAD

Figure 89. Differential Output Using a Transformer

The center tap on the primary side of the transformer must be

connected to a voltage that keeps the voltages on IOUTP and

IOUTN within the output common-mode voltage range of the

device. Note that the dc component of the DAC output current

is equal to IOUTFS and flows out of both IOUTP and IOUTN. The

center tap of the transformer should provide a path for this dc

current. In most applications, AGND provides the most conve-

nient voltage for the transformer center tap. The complementary

voltages appearing at IOUTP and IOUTN (that is, VIOUTP and

VIOUTN) swing symmetrically around AGND and should be

maintained with the specified output compliance range of the

AD9714/AD9715/AD9716/AD9717.

A differential resistor, RDIFF, can be inserted in applications

where the output of the transformer is connected to the load,

RLOAD, via a passive reconstruction filter or cable. RDIFF, as

reflected by the transformer, is chosen to provide a source

termination that results in a low VSWR. Note that approx-

imately half the signal power is dissipated across RDIFF.

SINGLE-ENDED BUFFERED OUTPUT USING

AN OP AMP

An op amp such as the ADA4899-1 can be used to perform

a single-ended current-to-voltage conversion, as shown in

Figure 90. The AD9714/AD9715/AD9716/AD9717 are config-

ured with a pair of series resistors, RS, off each output. For best

distortion performance, RS should be set to 0 Ω. The feedback

resistor, RFB, determines the peak-to-peak signal swing by the

formula

VOUT = RFB × IFS

The common-mode voltage of the output is determined by the

formula

REF

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

The maximum and minimum voltages out of the amplifier are,

respectively,

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

REF

MAX

VMIN = VMAX – IFS × RFB

+5V

AD9714/AD9715/

AD9716/AD9717

IOUTP

IOUTN 29

RFB

VOUT

REFIO 34

AVSS 25

–

ADA4899-1

–5V

Figure 90. Single-Supply Single-Ended Buffer

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