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AD9122

Rev. 0 | Page 39 of 56

DATAPATH CONFIGURATION EXAMPLE

8× Interpolation Without NCO

Given the following:

fDATA = 100 MSPS

8× interpolation

fBW = 75 MHz

fCENTER = 100 MHz

The desired 75 MHz of bandwidth is 75% of fDATA. In this case,

the ratio of fOUT/fDATA = 100/100 = 1.0. From Figure 57, the band-

width supported at fDATA is 0.8, which verifies that the AD9122

supports the bandwidth required in this configuration.

The signal center frequency is fDATA, and assuming the input signal

is at baseband, the frequency shift required is also fDATA. Choosing

the third row (highlighted by the superscripted number two) of the

IF column from Table 20 selects filter modes that give a center

frequency of fDATA and a frequency translation of fDATA. The selected

modes for the three half-band filters are: HB1, Mode 2; HB2,

Mode 2; and HB3, Mode 1. Figure 58 shows how the signal

propagates through the interpolation filters.

Because 2 × fIN1 = fIN2 and 2 × fIN2 = fIN3, the signal appears

frequency scaled by ½ into each consecutive stage. The output

signal band spans 0.15 to 0.35 of fIN3 (400 MHz). Therefore,

the output frequency supported is 60 MHz to 140 MHz, which

covers the 75 MHz bandwidth centered at 100 MHz, as desired.

4× Interpolation With NCO

Given the following:

fDATA = 250 MSPS

4× interpolation

fBW = 140 MHz

fCENTER = 175 MHz

The desired 140 MHz of bandwidth is 56% of fDATA. As shown

in Figure 56, the value at 0.7 × fDATA is 0.6. This is calculated as

0.8 − 2(0.7 − 0.6) = 0.6. This verifies that the AD9122 supports

a bandwidth of 60% of fDATA, which exceeds the required 56%.

The signal center frequency is 0.7 × fDATA, and assuming the

input signal is at baseband, the frequency shift required is also

0.7 × fDATA. Choosing the second row in the IF column in the

4× interpolation section in Table 20 selects the filter modes that

give a center frequency of fDATA/2 and no frequency translation.

The selected modes for the three half-band filters are HB1,

Mode 1; HB2, Mode 1; and HB3, bypassed.

Because Mode 1 of HB1 was selected, the premodulation block

should be enabled. This provides fDATA/2 modulation, which

centers the baseband input data at the center frequency of HB1.

The digital modulator can be used to provide the final frequency

translation of 0.2 × fDATA to place the output signal at 0.7 × fDATA,

as desired.

The formula for calculating the FTW of the NCO is:

NCO

CARRIER

FTW

where:

fCARRIER = 0.2 × fDATA.

fNCO = 2 × fDATA. Therefore, FTW = 232/10.

–0.5

0.5

HB1

0.1

0.4

0.6

1.5

2.0

fIN1

1.0

–0.5

0.5

HB3

0.2

–0.2

0.3

0.7

0.15

0.35

1.5

2.0

fIN3

1.0

–0.5

0.5

HB2

0.25

0.75

0.3

0.7

1.25

1.75

1.5

2.0

fIN2

1.0

Figure 58. Signal Propagation for 8× Interpolation (fDATA Modulation)

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