AD9786 datasheet(32/60 Pages) AD

32 / 60 page

AD9786

Rev. 0 | Page 32 of 60

Interpolation Modes

Table 28. Interpolation Modes

INTERP[1]

INTERP[0]

Mode

No Interpolation

×2 Interpolation

×4 Interpolation

×8 Interpolation

Interpolation is the process of increasing the number of points

in a time domain waveform by approximating points between

the input data points, on a uniform time grid. This produces a

higher output data rate. Applied to an interpolation DAC, a

digital interpolation filter is used to approximate the inter-

polated points, having an output data rate increased by the

interpolation factor. Interpolation filter responses are achieved

by cascading individual digital filter banks, whose filter

coefficients are given in Table 23, Table 24, and Table 25. Filter

responses are shown in Figure 57, which shows the interpola-

tion filters of the AD9786 under different interpolation rates,

normalized to the input data rate, fSIN.

The digital filter’s frequency domain response exhibits

symmetry about half the output data rate and dc. It will cause

images of the input data to be shaped by the interpolation

filter’s frequency response. This has the advantage of causing

input data images, which fall in the stop band of the digital filter

to be rejected by the stop-band attenuation of the interpolation

filter; input data images falling in the interpolation filter’s pass

band will be passed. In band-limited applications, the images at

the output of the DAC must be limited by an analog reconstruc-

tion filter. The complexity of the analog reconstruction filter is

determined by the proximity of the closest image to the

required signal band. Higher interpolation rates yield larger

stop-band regions, suppressing more input images and resulting

in a much relaxed analog reconstruction filter.

A DAC shapes its output with a sinc function, having a null at

the sampling frequency of the DAC. The higher the DAC sam-

pling rate compared to the input signal bandwidth, the less the

DAC sinc function will shape the output. The higher the

interpolation rate, the more input data images fall in the

interpolation filter stop band and are rejected; the bandwidth

between passed images is larger with higher interpolation

factors. The sinc function shaping is also reduced with a higher

interpolation factor.

Table 29. Sinc Shaping at Band Edge of Interpolation Filters

Mode

Sinc Shaping

at 0.43fSIN (dB)

Bandwidth to First Image

No Interpolation

–2.8241

fSIN

×2 Interpolation

–0.6708

2fSIN

×4 Interpolation

–0.1657

4fSIN

×8 Interpolation

–0.0413

8fSIN

AD9786 Datasheet(PDF) 32 Page - Analog Devices

AD9786 Datasheet(HTML) 32 Page - Analog Devices

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

Datasheet Download

Link URL