DAC5687IPZP datasheet(35/72 Pages) TI

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Dual Channel Real Upconversion

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

DAC5687

SLWS164B – FEBRUARY 2005 – REVISED JUNE 2005

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 Fre-

Bandwidth

Inverted?

quency

Low Pass

0 - 0.4 x fDATA

0 - 0.4 x

0.4 x fDATA

fDATA

High Pass

Low Pass

0.2 to 0.4 x fDATA

0.6 - 0.8 x

0.2 x fDATA

Yes

fDATA

High Pass

0.2 to 0.4 x fDATA

1.2 - 1.4x

0.2 x fDATA

fDATA

Low Pass

High Pass

0 - 0.4 x fDATA

1.6 - 2x

0.4 x fDATA

Yes

fDATA

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

2x interpolation, FIR3 limits the signal to ±100 MHz (0.4 x 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).

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