DAC5674IPHPG4【PDF 10/39 页】IC DAC 14BIT 400MSPS 48-HTQFP

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DAC5674

SLWS148A SEPTEMBER 2003 REVISED OCTOBER 2005

www.ti.com

High-Pass/High-Pass 4

y Interpolation Filter Operation

Figure 19 shows the high-pass/high-pass filter configuration. The transition bands of filter 1 and filter 2 allow

for the placement of the fundamental IF between 0.2

…0.4Fdata. In this configuration, the user can select the

images at Fdata + IF and 3Fdata – IF. For Fdata = 100 MSPS and a fundamental IF of 0.4 × Fdata = 40 MHz, this

would translate into images located at 140 MHz and 260 MHz. Note that this is the equivalent of mixing a 60-MHz

analog IF signal with a 200-MHz sine wave.

Input Spectrum

Output of DUC

1st 2x

interpolation

filter

Spectrum after

2x interpolation

2nd 2x

interpolation

filter

Spectrum after

4x interpolation

Fdata

Fdac

80 dB of

attenuation

Unwanted images

removed by HPF

80 dB of

attenuation

80 dB of

attenuation

Unwanted images

removed by HPF

Figure 19. High-Pass/High-Pass 4

y Interpolation Filter Operation

DAC Sinx/x Output Attenuation

The output frequency spectrum of the DACs shows some inherent attenuation due to their sample-and-hold

nature. The output of the DAC is normally seen as the signal sample held over the sampling time in a stair-step

manner. In the time domain, this step-like output can be thought of as an impulse sample of some value

convolved with a unit-square pulse with a duration of the sampling time. In the frequency domain, this translates

to the frequency response of the discretely sampled signal multiplied by the sinx/x frequency response function

of the square pulse. The sinx/x function has a null at every integer multiple of the sampling rate.

This is shown in Figure 20 for various data rates at 4

× interpolation.

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