DAC5687IPZP datasheet(36/79 Pages) TI

36 / 79 page

www.ti.com

Dual-Channel Real Upconversion

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

Fine Mixer (FMIX)

Frequency

Accumulator

RESET

CLK

PHSTR

Phase

Look-Up

Table

sin

cos

fNCO_CLK

B0026-02

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

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, upconverts a signal between 25 MHz and 50 MHz to a signal between 150 MHz

and 175 MHz. With the High Pass/Low Pass and Low Pass/High Pass settings, the upconvertered signal is

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 (4

× interpolation with FMIX after FIR1), at the maximum input

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

After 2

× 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 pass band 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 + CMIX with 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 36 in the

preceding FIR Filters section, 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.

The fine mixer block FMIX uses a numerically controlled oscillator (NCO) with a 32-bit frequency register

freq(31:0) and a 16-bit phase register phase(15:0) to provide sin and cos for mixing. The NCO tuning frequency

is programmed in registers 0x09 through 0x0C. Phase offset is programmed in registers 0x0D and 0x0E. A block

diagram of the NCO is shown in Figure 39.

Figure 39. Block Diagram of the NCO

Submit Documentation Feedback

Product Folder Link(s): DAC5687

DAC5687IPZP データシート(PDF) 36 Page - Texas Instruments

DAC5687IPZP データシート(HTML) 36 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

データシートダウンロード

リンク URL

DAC5687IPZP データシート(PDF) 36 Page - Texas Instruments

DAC5687IPZP データシート(HTML) 36 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79

データシート ダウンロード

リンク URL

データシートダウンロード