AD9146 datasheet(39/52 Pages) AD

39 / 52 page

AD9146

Rev. 0 | Page 39 of 52

Figure 52 shows the most basic transmit DAC output circuitry.

A pair of resistors, RO, is used to convert each of the comple-

mentary 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 53 illustrates the output voltage

waveforms.

VIP +

VIN –

VOUTI

IOUT1P

IOUT1N

VQP +

VQN –

VOUTQ

IOUT2P

IOUT2N

Figure 52. Basic Transmit DAC Output Circuit

+VPEAK

VOUT

VCM

–VPEAK

Figure 53. Output Voltage Waveforms

The common-mode signal voltage, VCM, is calculated as

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, IFS, and the common-mode level of the output.

Figure 54 shows the IMD performance vs. the output common-

mode voltage at different full-scale currents.

–85

–80

–75

–70

–65

–60

–55

–50

0.2

0.4

0.6

0.8

1.0

1.2

1.4

IFS = 10mA

IFS = 20mA

IFS = 30mA

VCM (V)

Figure 54. IMD vs. Output Common-Mode Voltage (fOUT = 161 MHz,

RLOAD = 50 Ω

Differential, IFS = 10 mA, 20 mA, and 30 mA)

AUXILIARY DAC OPERATION

The AD9146 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) out-

put of the associated transmit DAC. The auxiliary DAC

structure is shown in Figure 55.

IOUT1P

IOUT1N

I DAC

I AUX DAC

CURRENT

DIRECTION

I AUX DAC[9:0]

I AUX DAC

SIGN

Figure 55. Auxiliary DAC Structure

The control registers for the I and Q auxiliary DACs are

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