Datasheet

AD9125
Rev. 0 | Page 43 of 56
Figure 68 shows the most basic DAC output circuitry. A pair of
resistors, R
O
, is used to convert each of the complementary 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
09016-038
Figure 68. Basic Transmit DAC Output Circuit
V
PEAK
V
P
V
OUT
V
N
V
CM
0
–V
PEAK
09016-039
Figure 69. Voltage Output 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
OUTFS
, and the common-mode level of the output.
Figure 70 and Figure 71 show the IMD performance vs. the
common-mode output voltage at various full-scale currents and
output frequencies.
–60
–65
–70
–75
–80
–85
–90
011.21.00.80.60.40.2
IMD (dBc)
V
CMD
(V)
.4
20mA
10mA
30mA
09016-168
Figure 70. IMD vs. Common-Mode Output 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
CMD
(V)
.4
20mA
10mA
30mA
09016-169
Figure 71. IMD vs. Common-Mode Output Voltage (f
OUT
= 161 MHz,
R
LOAD
= 50 Ω differential, I
FS
= 10 mA, 20 mA, and 30 mA)
AUXILIARY DAC OPERATION
The AD9125 has two auxiliary DACs; one is associated with the
I path, and the other is 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 P or N output of the associated
transmit DAC. The auxiliary DAC structure is shown in Figure 72.
IOUTP
IOUTN
I DAC
AVDD3
AUX DAC
CURRENT
DIRECTION
AUX DAC
AUX DAC
SIGN
09016-040
Figure 72. Auxiliary DAC Structure
The control registers for controlling the I and Q auxiliary DACs are
in Register 0x42, Register 0x43, Register 0x46, and Register 0x47.