AD9776A Analog Devices, AD9776A Datasheet - Page 45

• Manufacturer Part Number: AD9776A
• Description: Dual 12-Bit, 1 GSPS, Digital-to-Analog Converter
• Manufacturer: Analog Devices

Specifications of AD9776A

• Resolution (bits): 12bit
• Dac Update Rate: 1GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: Par

LO FEEDTHROUGH COMPENSATION
The LO feedthrough compensation is the most complex of all
three operations. This is due to the structure of the offset aux-
iliary DACs, as shown in Figure 78. To achieve LO feedthrough
compensation in a circuit, each of four outputs of these auxiliary
DACs can be connected through a 500 Ω resistor to ground
and through a 250 Ω resistor to one of the four quadrature
modulator signal inputs. 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 of the quadrature modulator signal inputs.
To achieve LO feedthrough compensation, the user should start
with the default conditions of the auxiliary DAC sign registers,
and then increment the magnitude of one or the other auxiliary
DAC output currents. 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
changing the sign of the auxiliary DAC being adjusted or
adjusting the output current of the other auxiliary DAC. It may
take practice before an effective algorithm is achieved.
Using the AD9776A/AD9778A/AD9779A evaluation board, the
LO feedthrough can typically be adjusted down to the noise
floor, although this is not stable over temperature.
AD9779A
AUX1_P
OUT1_P
OUT1_N
AUX1_N
AUX2_N
OUT2_N
OUT2_P
AUX2_P
Figure 79. Typical Use of Auxiliary DACs AC Coupling to
500Ω
500Ω
500Ω
500Ω
90
93
92
89
87
84
83
86
RBQN
RBQP
RBIP
RBIN
250Ω
250Ω
50Ω
50Ω
50Ω
50Ω
250Ω
250Ω
82pF
82pF
C1Q
C3Q
82pF
82pF
Quadrature Modulator
C1I
C3I
390nH
390nH
390nH
390nH
LNQ
LPQ
LPI
LNI
39pF
39pF
C2Q
C2I
RSLQ
100Ω
100Ω
RSLI
21
22
10
9
IBBP
IBBN
QBBN
QBBP
Rev. B | Page 45 of 56
RESULTS OF GAIN AND OFFSET CORRECTION
The results of gain and offset correction can be seen in Figure 80
and Figure 81. Figure 80 shows the output spectrum of the quad-
rature demodulator before gain and offset correction. Figure 81
shows the output spectrum after correction. The LO feedthrough
spur at 2.1 GHz has been suppressed to the noise level. This
result can be achieved by applying the correction, but the correc-
tion needs to be repeated after a large change in temperature.
Note that the gain matching improved the negative frequency
image rejection, but there is still a significant image present.
The remaining image is now due to phase mismatch in the
quadrature modulator. Phase mismatch can be distinguished
from gain mismatch by the shape of the image. Note that the
image in Figure 80 is relatively flat and the image in Figure 81
slopes down with frequency. Phase mismatch is frequency
dependent, so an image dominated by phase mismatch has
this sloping characteristic.
Figure 80. AD9779A and ADL5372 with a Multitone Signal at 2.1 GHz,
Figure 81. AD9779A and ADL5372 with a Multitone Signal at 2.1 GHz,
–100
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
0
REF LVL
0dBm
CENTER 2.1GHz
REF LVL
0dBm
CENTER 2.1GHz
Gain and LO Compensation Optimized
AD9776A/AD9778A/AD9779A
No Gain or LO Compensation
RBW
VBW
SWT
RBW
VBW
SWT
20MHz
20MHz
3kHz
3kHz
56s
20kHz
20kHz
1.25s
REF ATT
MIXER
UNIT
REF ATT
MIXER
UNIT
SPAN 200MHz
SPAN 200MHz
30dB
–40dBm
dBm
20dB
–40dBm
dBm