AD9115-EBZ datasheet(43/48 Pages) AD

43 / 48 page

AD9114/AD9115/AD9116/AD9117

Rev. 0 | Page 43 of 48

APPLICATIONS INFORMATION

OUTPUT CONFIGURATIONS

The following sections illustrate some typical output configu-

rations for the AD9114/AD9115/AD9116/AD9117. Unless

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

20 mA. For applications requiring the optimum dynamic

performance, a differential output configuration is suggested.

A differential output configuration can consist of either an RF

transformer or a differential op amp configuration. The trans-

former configuration provides the optimum high frequency

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 88. 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.

AD9114/AD9115/

AD9116/AD9117

IOUTN

IOUTP

OPTIONAL RDIFF

RLOAD

Figure 88. 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

AD9114/AD9115/AD9116/AD9117.

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 89. The AD9114/AD9115/AD9116/AD9117 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

AD9114/AD9115/

AD9116/AD9117

IOUTP

IOUTN 29

RFB

VOUT

REFIO 34

AVSS 25

–

ADA4899-1

–5V

Figure 89. Single-Supply Single-Ended Buffer

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