Datasheet

AD9122
Rev. B | Page 47 of 60
Figure 68 shows the most basic transmit DAC output circuitry.
A pair of resistors, R
O
, is used to convert each of the comple-
mentary output currents to a differential voltage output, V
OUT
.
Because the current outputs of the DAC are high impedance,
the differential driving point impedance of the DAC outputs,
R
OUT
, is equal to 2 × R
O
. Figure 69 illustrates the output voltage
waveforms.
R
O
R
O
V
IP
+
V
IN
–
V
OUTI
IOUT1P
IOUT1N
R
O
R
O
V
QP
+
V
QN
–
V
OUTQ
IOUT2P
IOUT2N
08281-038
Figure 68. Basic Transmit DAC Output Circuit
+
V
PEAK
V
CM
0
–V
PEAK
V
N
V
P
V
OUT
08281-039
Figure 69. Output Voltage Waveforms
The common-mode signal voltage, V
CM
, is calculated as
O
FS
CM
R
I
V ×=
2
The peak output voltage, V
PEAK
, is calculated as
V
PEAK
= I
FS
× R
O
With this circuit configuration, the single-ended peak voltage is
the same as the peak differential output voltage.
Transmit DAC Linear Output Signal Swing
To achieve optimum performance, the DAC outputs have a
linear output compliance voltage range that must be adhered
to. The linear output signal swing is dependent on the full-scale
output current, I
FS
, and the common-mode level of the output.
Figure 70 and Figure 71 show the IMD performance vs. the
output common-mode voltage at different full-scale currents
and output frequencies.
–
60
–65
–70
–75
–80
–85
–90
011.21.00.80.60.40.2
IMD (dBc)
V
CM
(V)
.4
I
FS
= 20mA
I
FS
= 10mA
I
FS
= 30mA
08281-168
Figure 70. IMD vs. Output Common-Mode Voltage (f
OUT
= 61 MHz,
R
LOAD
= 50 Ω Differential, I
FS
= 10 mA, 20 mA, and 30 mA)
–
50
–55
–60
–65
–70
–75
–80
–85
011.21.00.80.60.40.2
IMD (dBc)
V
CM
(V)
.4
I
FS
= 20mA
I
FS
= 10mA
I
FS
= 30mA
08281-169
Figure 71. IMD vs. Output Common-Mode Voltage (f
OUT
= 161 MHz,
R
LOAD
= 50 Ω Differential, I
FS
= 10 mA, 20 mA, and 30 mA)
AUXILIARY DAC OPERATION
The AD9122 has two auxiliary DACs: one associated with the
I path and one associated with the Q path. These auxiliary DACs
can be used to compensate for dc offsets in the transmitted signal.
Each auxiliary DAC has a single-ended current that can sink or
source current into either the positive (P) or negative (N) output
of the associated transmit DAC. The auxiliary DAC structure is
shown in Figure 72.
IOUT1P
IOUT1N
I DAC
V
B
I AUX DAC
CURRENT
DIRECTION
I AUX DAC[9:0]
I AUX DAC
SIGN
08281-040
Figure 72. Auxiliary DAC Structure