AD9122 datasheet(2/9 Pages) AD

2 / 9 page

CN-0205

Circuit Note

Rev. 0 | Page 2 of 9

TOP VIEW

AD9122

DAC

ADL5375

MODULATOR

BOTTOM VIEW

FILTER

Figure 2. AD9122-M5375-EBZ Evaluation Board for Circuit Implementation

The DAC’s full-scale output current (IFS) is programmable from

10 mA to 30 mA. The nominal and default value is 20 mA. In

this configuration, the DAC outputs swing from 0 mA to 20 mA

across each of the four ground-referenced 50 Ω resistors (RB =

RBIP = RBIN = RBQP = RBQN). This establishes the 500 mV

dc bias level and a full-scale voltage swing of 2 V p-p differential

on each output pair (with no load). This 2 V p-p voltage swing

can be adjusted by the RL (RL = RSLI = RSLQ) shunt resistors

without affecting the 500 mV bias level. The resulting

differential peak-to-peak swing at the I/Q modulator input

is given by the equation

[

]

[

]

SIGNAL

Note that the relatively high differential input impedance of the

ADL5375 (typically >60 kΩ) can be ignored when calculating

this signal level. Figure 3 shows the relationship between the

peak-to-peak voltage swing and RL when 50 Ω bias-setting

resistors are used.

The ADL5375-05 and AD9122 are well matched in terms of

dynamic range and gain. As a result, there is no need for any

active gain between the devices. The I/Q modulator drive level

can be fine tuned as needed by adjusting the value of RL as

described above. For most applications, a value of 100 Ω for

RL is recommended. This results in a full-scale signal level of

1 V p-p (DAC output at 0 dBFS).

AD9122 Datasheet(PDF) 2 Page - Analog Devices