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AD9114 View Datasheet(PDF) - Analog Devices

Part NameDescriptionManufacturer
AD9114 Dual, 8-/10-/12-/14-Bit Low Power Digital-to-Analog Converters ADI
Analog Devices ADI
AD9114 Datasheet PDF : 48 Pages
First Prev 41 42 43 44 45 46 47 48
AD9114/AD9115/AD9116/AD9117
Note that LO feedthrough compensation is independent of
phase compensation. However, gain compensation could 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
10 kΩ resistor to one side of the differential DAC output.
See the Auxiliary DACs 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 AD9114/AD9115/AD9116/AD9117 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 95
and Figure 96. Figure 95 shows the output spectrum of the
quadrature demodulator before gain and offset correction.
Figure 96 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
FREQUENCY (MHz)
452.5
Figure 95. AD9114/AD9115/AD9116/AD9117 and ADL5370 with a Single-
Tone Signal at 450 MHz, No Gain or LO Compensation
5
0
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–10
–15
–20
–25
–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
–95
447.5
449.0
450.0
451.0
FREQUENCY (MHz)
452.5
Figure 96. AD9114/AD9115/AD9116/AD9117 and ADL5370 with a Single-
Tone Signal at 450 MHz, Gain and LO Compensation Optimized
Rev. 0 | Page 46 of 48
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