AD9957 Analog Devices, AD9957 Datasheet - Page 13

• Manufacturer Part Number: AD9957
• Description: 1 GSPS Quadrature Digital Upconverter
• Manufacturer: Analog Devices

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DataSheet
4
PRELIMINARY TECHNICAL DATA
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 f
Nyquist (where f
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 f
trum receives slightly more attenuation than the lower frequen-
cies (the worst-case overall droop from f=0 to ½f
dB). The Inverse CCI filter has a response characteristic that is
the inverse of the CCI filter response over the ½f
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.
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.com
DATA
region, the higher frequencies of the data spec-
IQ
Figure 7. CCI Filter Response
is the sample rate at the input of the first
IQ
as defined by
IQ
DATA
region.
is < 0.8
Rev. PrF | Page 13 of 38
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.
Figure 8. Half-Band 1 and 2 Frequency Response; Frequency
Figure 9. Combined Half-Band 1 and 2 Pass Band Detail;
–0.002
–0.004
–0.006
–0.008
–0.010
0.010
0.008
0.006
0.004
0.002
–100
–110
–120
–130
–140
–10
–20
–30
–40
–50
–60
–70
–80
–90
10
0
0
Frequency Relative to HB1 Output Sample Rate
0
0
0.2
RELATIVE FREQUENCY (HB1 OUTPUT SAMPLE RATE = 1)
0.2
Relative to HB1 Output Sample Rate
0.3
0.05
0.4
0.6
0.10
0.8
FREQUENCY
1.0
1.2
0.15
1.4
1.6
0.20
1.8
AD9957
–85
2.0
0.25