AD9857SLASHPCB datasheet(19/40 Pages) AD

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AD9857

Rev. C| Page 19 of 40

tDH

TxENABLE

PDCLK

D<13:0>

tDH

tDS

Figure 21. 14-Bit Parallel Port Timing Diagram—Quadrature Modulation Mode

tDH

TxENABLE

PDCLK

D<13:0>

IK– 1

tDH

tDS

Figure 22. 14-Bit Parallel Port Timing Diagram—Interpolating DAC Mode

Table 5. Parallel Data Bus Timing

Symbol

Definition

Minimum

tDS

Data Setup Time

4 ns

tDH

Data Hold Time

0 ns

INVERSE CIC FILTER

The inverse cascaded integrator comb (CIC) filter precompen-

sates the data to offset the slight attenuation gradient imposed

by the CIC filter. See the Programmable (2× to 63×) CIC

Interpolating Filter section. The I (or Q) data entering the first

half-band filter occupies a maximum bandwidth of one-half

fDATA as defined by Nyquist (where fDATA is the sample rate at the

input of the first half-band filter). This is shown graphically in

Figure 23.

INBAND

ATTENUATION

GRADIENT

CIC FILTER RESPONSE

fDATA/2

fDATA

4fDATA

Figure 23. CIC Filter Response

If the CIC filter is employed, the inband attenuation gradient

could pose a problem for those applications requiring an

extremely flat pass band. For example, if the spectrum of the

data as supplied to the AD9857 I or Q path occupies a

significant portion of the one-half fDATA region, the higher

frequencies of the data spectrum receives slightly more

attenuation than the lower frequencies (the worst-case overall

droop from f = 0 to one-half fDATA is < 0.8 dB). This may not be

acceptable in certain applications. The inverse CIC filter has a

response characteristic that is the inverse of the CIC filter

response over the one-half fDATA region.

The net result is that the product of the two responses yields in

an extremely flat pass band, thereby eliminating the inband

attenuation gradient introduced by the CIC filter. The price

to be paid is a slight attenuation of the input signal of approx-

imately 0.5 dB for a CIC interpolation rate of 2 and 0.8 dB for

interpolation rates of 3 to 63.

The inverse CIC filter is implemented as a digital FIR filter

with a response characteristic that is the inverse of the

programmable CIC interpolator. The product of the two

responses yields a nearly flat response over the baseband

Nyquist bandwidth. The inverse CIC filter provides frequency

compensation that yields a response flatness of ±0.05 dB over

the baseband Nyquist bandwidth, allowing the AD9857 to

provide excellent SNR over its performance range.

The inverse CIC filter can be bypassed by setting Control

interpolation rate is 1×. Whenever this stage is bypassed, power

to the stage is shutoff, thereby reducing power dissipation.

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