DAC5687IPZP datasheet(37/79 Pages) TI

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NCO +

freq

NCO_CLK

232

for freq

v 231

NCO +

(freq * 232)

NCO_CLK

232

for freq

u 231

Coarse Mixer (CMIX)

DAC5687

SLWS164E – FEBRUARY 2005 – REVISED SEPTEMBER 2006

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

following NCO Synchronization section. 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 is limited to 320 MSPS. In X2, X4L

and X8 modes, the NCO circuit is followed by a further 2

× interpolation and so f

NCO_CLK = fDAC/2 and operates at

fDAC = 500 MHz.

Treating channels A and B as a complex vector I + I

× 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)) × 2

(NCO_GAIN – 1)

QOUT(t) = (IIN(t)sin(2πfNCOt + δ) + QIN(t)cos(2πfNCOt + δ)) × 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 × phase(15:0)/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) × 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.

The coarse mixer block provides mixing capability at the DAC output rate with fixed frequencies of fS/2 or fS/4.

The coarse mixer output phase sequence is selected by the cm_mode(3:0) bits in register CONFIG2 and is

shown in Table 10.

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