AD9122 Analog Devices, AD9122 Datasheet - Page 47

• Manufacturer Part Number: AD9122
• Manufacturer: Analog Devices

Specifications of AD9122

• Resolution (bits): 16bit
• Dac Update Rate: 1.23GSPS
• Dac Settling Time: n/a
• Max Pos Supply (v): +3.47V
• Single-supply: No
• Dac Type: Current Out
• Dac Input Format: LVDS,Par

Figure 68 shows the most basic transmit DAC output circuitry.
A pair of resistors, R
mentary output currents to a differential voltage output, V
Because the current outputs of the DAC are high impedance,
the differential driving point impedance of the DAC outputs,
R
waveforms.
The common-mode signal voltage, V
The peak output voltage, V
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
Figure 70 and Figure 71 show the IMD performance vs. the
output common-mode voltage at different full-scale currents
and output frequencies.
OUT
+V
–V
, is equal to 2 × R
V
V
PEAK
PEAK
PEAK
V
CM
CM
0
=
= I
I
2
Figure 68. Basic Transmit DAC Output Circuit
FS
FS
× R
×
Figure 69. Output Voltage Waveforms
FS
R
, and the common-mode level of the output.
V
O
O
N
O
, is used to convert each of the comple-
O
. Figure 69 illustrates the output voltage
PEAK
IOUT1P
IOUT1N
IOUT2P
IOUT2N
, is calculated as
R
R
R
R
O
O
O
O
V
P
CM
, is calculated as
V
V
V
V
IP
IN
QP
QN
V
V
V
OUTQ
OUTI
OUT
+
–
+
–
OUT
Rev. B | Page 47 of 60
.
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.
Figure 71. IMD vs. Output Common-Mode Voltage (f
Figure 70. IMD vs. Output Common-Mode Voltage (f
–60
–65
–70
–75
–80
–85
–90
–50
–55
–60
–65
–70
–75
–80
–85
R
R
0
0
LOAD
LOAD
= 50 Ω Differential, I
= 50 Ω Differential, I
I AUX DAC[9:0]
0.2
0.2
I
I
I
I
I
I
FS
FS
FS
FS
FS
FS
DIRECTION
I AUX DAC
I AUX DAC
Figure 72. Auxiliary DAC Structure
CURRENT
= 10mA
= 20mA
= 30mA
= 10mA
= 20mA
= 30mA
I DAC
SIGN
0.4
0.4
0.6
0.6
V
FS
FS
B
V
V
= 10 mA, 20 mA, and 30 mA)
= 10 mA, 20 mA, and 30 mA)
CM
CM
(V)
(V)
0.8
0.8
IOUT1P
IOUT1N
1.0
1.0
OUT
OUT
1.2
1.2
= 161 MHz,
= 61 MHz,
AD9122
1
1
.4
.4