AD9714 datasheet(46/48 Pages) AD

46 / 48 page

AD9714/AD9715/AD9716/AD9717

Rev. 0 | Page 46 of 48

Note that LO feedthrough compensation is independent of

phase compensation. However, gain compensation can affect

the LO compensation because the gain compensation may

change the common-mode level of the signal. The dc offset of

some modulators is common-mode level dependent. Therefore,

it is recommended that the gain adjustment be performed prior

to LO compensation.

LO FEEDTHROUGH COMPENSATION

To achieve LO feedthrough compensation in a circuit, each

output of the two AUXDACs must be connected through a

100 kΩ resistor to one side of the differential DAC output. See

the Auxiliary DAC

s section for details of how to use AUXDACs.

The purpose of these connections is to drive a very small amount

of current into the nodes at the quadrature modulator inputs,

therefore adding a slight dc bias to one or the other of the

quadrature modulator signal inputs.

To achieve LO feedthrough compensation, the user should

start with the default conditions of the AUXDAC registers,

then increment the magnitude of one or the other AUXDAC

output voltages. While this is being done, the amplitude of the

LO feedthrough at the quadrature modulator output should be

sensed. If the LO feedthrough amplitude increases, try either

decreasing the output voltage of the AUXDAC being adjusted,

or try adjusting the output voltage of the other AUXDAC. It

may take practice before an effective algorithm is achieved.

Using the AD9714/AD9715/AD9716/AD9717 evaluation

board, the LO feedthrough can typically be adjusted down to

the noise floor, although this is not stable over temperature.

RESULTS OF GAIN AND OFFSET CORRECTION

The results of gain and offset correction can be seen in Figure 96

and Figure 97. Figure 96 shows the output spectrum of the

quadrature demodulator before gain and offset correction.

Figure 97 shows the output spectrum after correction. The

LO feedthrough spur at 450 MHz has been suppressed to the

noise level. This result can be achieved by applying the correc-

tion, but the correction needs to be repeated after a large change

in temperature.

Note that gain matching improves the negative frequency image

rejection, but it is also related to the phase mismatch in the

quadrature modulator. It can be improved by adjusting the

relative phase between the two quadrature signals at the digital

side or properly designing the low-pass filter between the DACs

and quadrature modulators. Phase mismatch is frequency depen-

dent, so routines have to be developed to adjust it if wideband

signals are desired.

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447.5

449.0

450.0

451.0

452.5

FREQUENCY (MHz)

Figure 96. AD9714/AD9715/AD9716/AD9717 and ADL5370 with a Single-

Tone Signal at 450 MHz, No Gain or LO Compensation

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–45

–55

–65

–75

–85

–95

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–20

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–50

–60

–70

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447.5

449.0

450.0

451.0

452.5

FREQUENCY (MHz)

Figure 97. AD9714/AD9715/AD9716/AD9717 and ADL5370 with a Single-

Tone Signal at 450 MHz, Gain and LO Compensation Optimized

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