AD9125BCPZ View Datasheet(PDF) - Analog Devices
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AD9125BCPZ Datasheet PDF : 56 Pages
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AD9125
Figure 68 shows the most basic DAC output circuitry. A pair of
resistors, RO, is used to convert each of the complementary output
currents to a differential voltage output, VOUT. Because the current
outputs of the DAC are high impedance, the differential driving
point impedance of the DAC outputs, ROUT, is equal to 2 × RO.
Figure 69 illustrates the output voltage waveforms.
IOUT1P
VIP +
RO
RO
IOUT1N
VOUTI
VIN –
IOUT2P
VQP +
RO
VOUTQ
RO
IOUT2N
VQN –
Figure 68. Basic Transmit DAC Output Circuit
VPEAK
VCM
0
VN
VP
–VPEAK
VOUT
Figure 69. Voltage Output Waveforms
The common-mode signal voltage, VCM, is calculated as
VCM
=
I FS
2
× RO
The peak output voltage, VPEAK, is calculated as
VPEAK = IFS × RO
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, IOUTFS, 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
10mA
20mA
–65
30mA
–70
–75
–80
–85
–90
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCMD (V)
Figure 70. IMD vs. Common-Mode Output Voltage (fOUT = 61 MHz,
RLOAD = 50 Ω differential, IFS = 10 mA, 20 mA, and 30 mA)
–50
10mA
20mA
–55
30mA
–60
–65
–70
–75
–80
–85
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
VCMD (V)
Figure 71. IMD vs. Common-Mode Output Voltage (fOUT = 161 MHz,
RLOAD = 50 Ω differential, IFS = 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.
AVDD3
AUX DAC
AUX DAC
CURRENT
DIRECTION
AUX DAC
SIGN
I DAC
IOUTP
IOUTN
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.
Rev. 0 | Page 43 of 56
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