DAC5687MPZPEP datasheet(37/75 Pages) TI1

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Dual Channel Real Upconversion

Limitations on Signal BW and Final Output Frequency in X4L and X8 Modes

DAC5687-EP

SGLS333 – JUNE 2006

Table 8. Digital Filter Taps (continued)

FIR1 and FIR3

FIR2

FIR4 (Invsinc)

Tap

Coeff

Tap

Coeff

Tap

Coeff

12, 40

13, 39

619

14, 38

15, 37

–971

16, 36

17, 35

1490

18, 34

19, 33

–2288

20, 32

21, 31

3649

22, 30

23, 29

–6628

24, 28

25, 27

20750

32768

The DAC5687 can be used in a dual channel mode with real upconversion by mixing with a 1, –1,

… sequence

in the signal chain to invert the spectrum. This mixing mode maintains isolation of the A and B channels. There

are two points of mixing: in X4L mode, the FIR1 output is inverted (high-pass mode) by setting registers hpla

and hplb to 1 and the FIR3 output is inverted by setting CMIX to fDAC/2. In X8 mode, the output of FIR1 is

inverted by setting hpla and hplb to 1 and the FIR3 output is inverted by setting CMIX to fDAC/2. In X2 and X4

modes, the output of FIR3 is inverted by setting CMIX to fDAC/2.

The wide bandwidth of FIR3 (40% passband) in X4L mode provides options for setting four different frequency

ranges, listed in Table 9. For example, with fDATA = 125 MSPS (fDAC = 500 MSPS), setting FIR1/FIR3 to High

Pass/High Pass respectively will upconvert a signal between 25 and 50 MHz to 150 to 175 MHz. With the High

Pass/Low Pass and Low Pass/High Pass setting the upconvertered signal will be spectrally inverted.

Table 9. X4L Mode High-Pass/Low-Pass Options

FIR1

FIR3

Input Frequency

Output Frequency

Bandwidth

Inverted?

Low Pass

0 – 0.4

× f

DATA

0 – 0.4

× f

DATA

0.4

× f

DATA

High Pass

Low Pass

0.2 to 0.4

× f

DATA

0.6 – 0.8

× f

DATA

0.2

× f

DATA

Yes

High Pass

0.2 to 0.4

× f

DATA

1.2 – 1.4

× f

DATA

0.2

× f

DATA

Low Pass

High Pass

0 – 0.4

× f

DATA

1.6 – 2

× f

DATA

0.4

× f

DATA

Yes

For very wide bandwidth signals, the FIR3 pass-band (0 – 0.4

× F

DAC/2) can limit the range of the final output

frequency. For example in X4L FMIX CMIX mode (4x interpolation with FMIX after FIR1), at the maximum input

data rate FIN = 125 MSPS the input signal can be ±50 MHz before running into the transition band of FIR1. After

× interpolation, FIR3 limits the signal to ±100 MHz (0.4 × 250 MHz). Therefore, at the maximum signal

bandwidth, FMIX can mix up to 50 MHz and still fall within the passband of FIR3. This results in gaps in the final

output frequency between FMIX alone (0 MHz to 50 MHz) and FMIX + CMIX with fDAC/4 (75 MHz to 175 MHz)

and FMIX + fDAC/2 (200 MHz to 250 MHz).

In practice, it may be possible to extend the signal into the FIR3 transition band. Referring to Figure 37 in the

Digital Filter section above, if 0.5 dB of attenuation at the edge of the signal can be tolerated, then the signal can

be extended up to 0.44

× F

IN. This would extend the range of FMIX in the example to 60 MHz.

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