AD9957 datasheet(13/38 Pages) AD

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PRELIMINARY TECHNICAL DATA

AD9957

Rev. PrF | Page 13 of 38

delivered to the input of the AD9957 may be formatted as either

twos-complement or unsigned binary (see the Data Format bit

in the register map). Furthermore, in BFI mode, the order of

the bit sequence can be set to either "MSB First" or "LSB First"

(via the BlackFin Bit Order bit in the register map).

Inverse CCI Filter

The inverse CCI (cascaded comb integrator) filter precompen-

sates the data to offset the slight attenuation gradient imposed

by the CCI filter (see the Programmable (2× to 63×) CCI Inter-

polating filter section). Data entering the first half-band filter

occupies a maximum band width of one-half fIQ as defined by

Nyquist (where fIQ is the sample rate at the input of the first

half-band filter). This is shown graphically in Figure 7.

If the CCI filter is employed, the inband attenuation gradient

could pose a problem for those applications requiring an ex-

tremely flat pass band. For example, if the spectrum of the data

as supplied to the AD9957 occupies a significant portion of the

one-half fDATA region, the higher frequencies of the data spec-

trum receives slightly more attenuation than the lower frequen-

cies (the worst-case overall droop from f=0 to ½fDATA is < 0.8

dB). The Inverse CCI filter has a response characteristic that is

the inverse of the CCI filter response over the ½fIQ region.

Figure 7. CCI Filter Response

The product of the two responses yields in an extremely flat

pass band (±0.05 dB over the base band Nyquist band width),

thereby eliminating the inband attenuation gradient introduced

by the CCI filter. The cost is a slight attenuation of the input

signal of approximately 0.5 dB for a CCI interpolation rate of 2

and 0.8 dB for interpolation rates of 3 to 63.

The Inverse CCI filter can be bypassed using the appropriate bit

in the register map. Even if it is enabled, it is automatically by-

passed if the CCI interpolation rate is 1×. When the Inverse

CCI filter is bypassed, power to the stage is turned off to reduce

power consumption.

Fixed interpolator (4x)

This block is a fixed 4× rate interpolator. It is implemented as a

cascade of two half-band filters. Together, the two half-band

filters provide a factor of four increase in the sampling rate,

while preserving the spectrum of the base band signal applied at

the input. Both half-band filters are linear phase filters, so that

virtually no phase distortion is introduced within the pass band

of the filters. Their combined insertion loss is 0.01 dB, thus

preserving the relative amplitude of the input signal.

The half-band filters are designed so that their composite per-

formance yields a usable pass band of 40% of the input sample

rate (0.2 on the frequency scale below). Within that pass band,

the ripple does not exceed 0.002 dB. The stop band extends

from 60% to 200% of the input sample rate (0.3 to 1.0 on the

frequency scale) and offers a minimum of 85 dB attenuation.

Figure 8 and Figure 9 show the composite response of the two

half-band filters.

FREQUENCY

0.2

0.4

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

–130

–140

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.3

0.2

–85

Figure 8. Half-Band 1 and 2 Frequency Response; Frequency

Relative to HB1 Output Sample Rate

0.05

0.10

0.15

0.20

0.25

0.010

0.008

0.006

0.004

0.002

–0.002

–0.004

–0.006

–0.008

–0.010

RELATIVE FREQUENCY (HB1 OUTPUT SAMPLE RATE = 1)

Figure 9. Combined Half-Band 1 and 2 Pass Band Detail;

Frequency Relative to HB1 Output Sample Rate

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