DAC5687IPZP datasheet(36/72 Pages) TI

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Fine Mixer (FMIX)

Frequency

Accumulator

RESET

CLK

PHSTR

Phase

Look-Up

Table

sin

cos

fDAC

B0026-01

NCO

freq

NCO_CLK

232

for freq

v 231

NCO

(freq * 232)

NCO_CLK

232

for freq

u 231

DAC5687

SLWS164B – FEBRUARY 2005 – REVISED JUNE 2005

In practice, it may be possible to extend the signal into the FIR3 transition band. Referring to Figure 36 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 x FIN. 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 0xD and 0xE. A

block-diagram of the NCO is shown in Figure 39.

Figure 39. Block-Diagram of the NCO

Synchronization of the NCO occurs by resetting the NCO accumulator to zero with assertion of PHSTR. See the

Fine Mixer Synchronization section below. Frequency word freq in the frequency register is added to the

accumulator every clock cycle. The output frequency of the NCO is

where fNCO_CLK is the clock frequency of the NCO circuit. In X4 mode, the NCO clock frequency is the same as

the DAC sample rate fDAC. The maximum clock frequency the NCO can operate at is 320 MHz – in X4 FMIX

mode, where FMIX operates at the DAC update rate, the DAC updated rate will be limited to 320 MSPS. In X2,

X4L and X8 modes, the NCO circuit is followed by a further 2x interpolation and so fNCO_CLK = fDAC/2 and

operates at fDAC = 500 MHz.

Treating channels A and B as a complex vector I + I x Q where I(t) = A(t) and Q(t) = B(t), the output of FMIX

IOUT(t) and QOUT(t) is

IOUT(t) = (IIN(t)cos(2πfNCOt + δ) – QIN(t)sin(2πfNCOt + δ)) x 2

(NCO_GAIN – 1)

QOUT(t) = (IIN(t)sin(2πfNCOt + δ) + QIN(t)cos(2πfNCOt + δ)) x 2

(NCO_GAIN – 1)

Where t is the time since the last resetting of the NCO accumulator,

δ is the initial accumulator value and

NCO_GAIN, bit 6 in register CONFIG2, is either 0 or 1.

δ is given by

δ = 2π x phase/216.

The maximum output amplitude of FMIX occurs if IIN(t) and QIN(t) are simultaneously full scale amplitude and the

sine and cosine arguments 2

πf

NCOt + δ = (2N-1) x π/4 (N = 1, 2, ...). With NCO_GAIN = 0, the gain through FMIX

is sqrt(2)/2 or – 3 dB. This loss in signal power is in most cases undesirable, and it is recommended that the gain

function of the QMC block be used to increase the signal by 3 dB to 0 dBFS by setting qmca_gain and

qmcb_gain each to 1446 (decimal).

With NCO_GAIN = 1, the gain through FMIX is sqrt(2) or + 3 dB, which can cause clipping of the signal if IIN(t)

and QIN(t) are simultaneously near full scale amplitude and should therefore be used with caution.

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