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AD9775

Rev. E | Page 43 of 56

DIFFERENTIAL COUPLING USING AN OP AMP

An op amp can also be used to perform a differential-to-single-

ended conversion, as shown in Figure 99. This has the added

benefit of providing signal gain as well. In Figure 99, the

AD9775 is configured with two equal load resistors, RLOAD, of

25 Ω. The differential voltage developed across IOUTA and IOUTB is

converted to a single-ended signal via the differential op amp

configuration. An optional capacitor can be installed across

IOUTA and IOUTB, forming a real pole in a low-pass filter. The

addition of this capacitor also enhances the op amp distortion

performance by preventing the DAC fast slewing output from

overloading the input of the op amp.

IOUTA

IOUTB

COPT

ROPT

225

DAC

225

500

AVDD

AD8021

Figure 99. Op Amp-Coupled Output Circuit

The common-mode (and second-order distortion) rejection of

this configuration is typically determined by the resistor

matching. The op amp used must operate from a dual supply

because its output is approximately ±1.0 V. A high speed

amplifier, such as the AD8021, capable of preserving the

differential performance of the AD9775 while meeting other

system level objectives (such as cost and power) is

recommended. The op amp differential gain, its gain setting

resistor values, and full-scale output swing capabilities should

all be considered when optimizing this circuit. ROPT is only

necessary if level shifting is required on the op amp output. In

Figure 99, AVDD, which is the positive analog supply for both

the AD9775 and the op amp, is also used to level shift the

differential output of the AD9775 to midsupply, that is,

AVDD/2.

INTERFACING THE AD9775 WITH THE AD8345

QUADRATURE MODULATOR

The AD9775 architecture was defined to operate in a transmit

signal chain using an image reject architecture. A quadrature

modulator is also required in this application and should be

designed to meet the output characteristics of the DAC as much

as possible. The AD8345 from Analog Devices meets many of

the requirements for interfacing with the AD9775. As with any

DAC output interface, there are a number of issues that have to

be resolved. The following sections list some of these issues.

DAC Compliance Voltage/Input Common-Mode Range

The dynamic range of the AD9775 is optimal when the DAC

outputs swing between ±1.0 V. The input common-mode range

of the AD8345, at 0.7 V, allows optimum dynamic range to be

achieved in both components.

Gain/Offset Adjust

The matching of the DAC output to the common-mode input

of the AD8345 allows the two components to be dc-coupled,

with no level shifting necessary. The combined voltage offset of

the two parts can therefore be compensated for via the AD9775

programmable offset adjust. This allows excellent LO cancella-

tion at the AD8345 output. The programmable gain adjust

allows for optimal image rejection as well.

The AD9775 evaluation board includes an AD8345 and

recommended interface (Figure 104 and Figure 105). On the

output of the AD9775, R9 and R10 convert the DAC output

current to a voltage. R16 may be used to do a slight common-

mode shift if necessary. The (now voltage) signal is applied to a

low-pass reconstruction filter to reject DAC images. The

components installed on the AD9775 provide a 35 MHz cutoff

but may be changed to fit the application. A balun (Mini-

Circuits ADTL1-12) is used to cross the ground plane boundary

to the AD8345. Another balun (Mini-Circuits ETC1-1-13) is

used to couple the LO input of the AD8345. The interface

requires a low ac impedance return path from the AD8345, so a

single connection between the AD9775 and AD8345 ground

planes is recommended.

The performance of the AD9775 and AD8345 in an image reject

transmitter, reconstructing three W-CDMA carriers, can be seen in

Figure 100. The LO of the AD8345 in this application is 800 MHz.

Image rejection (50 dB) and LO feedthrough (−78 dBFS) have been

optimized with the programmable features of the AD9775. The

average output power of the digital waveform for this test was set

to −15 dBFS to account for the peak-to-average ratio of the

W-CDMA signal.

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

FREQUENCY (MHz)

782.5

762.5

802.5

822.5

842.5

Figure 100. AD9775/AD8345 Synthesizing a Three-Carrier

W-CDMA Signal at an LO of 800 MHz

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