AD9864 datasheet(33/48 Pages) AD

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AD9864

Data Sheet

Rev. A | Page 32 of 47

DECIMATION FILTER

COS

SIN

DATA

FROM Σ-Δ

MODULATOR

DEC1

SINC4

FILTER

DEC2

SINC4

FILTER

M + 1

DEC3

FIR

FILTER

COMPLEX

DATA TO

SSI PORT

Figure 52. Decimation Filter Architecture

The decimation filter shown in Figure 52 consists of an fCLK/8

complex mixer and a cascade of three linear phase FIR filters:

DEC1, DEC2, and DEC3. DEC1 downsamples by a factor of 12

using a fourth-order comb filter. DEC2 also uses a fourth-order

comb filter, but its decimation factor is set by the M field of

decimate-by-4 FIR filter, depending on the value of the K bit

within Register 0x07. Thus, the composite decimation factor

can be set to either 60 × M or 48 × M for K equal to 0 or 1,

respectively.

The output data rate (fOUT) is equal to the modulator clock

frequency (fCLK) divided by the decimation factor of the digital

filter. Due to the transition region associated with the frequency

response of the decimation filter, the decimation factor must be

selected so that fOUT is equal to or greater than twice the signal

bandwidth, which ensures low amplitude ripple in the pass

band along with the ability to provide further application-

specific digital filtering prior to demodulation.

Figure 53 shows the response of the decimation filter at a

decimation factor of 900 (K = 0, M = 14) and a sampling clock

frequency of 18 MHz. In this example, the output data rate (fOUT)

is 20 kSPS, with a usable complex signal bandwidth of 10 kHz

centered around dc. As this figure shows, the first and second

alias bands (occurring at even integer multiples of fOUT/2) have

the least attenuation but provide at least 88 dB of attenuation.

Note that signals falling around frequency offsets that are odd

integer multiples of fOUT/2 (that is, 10 kHz, 30 kHz, and 50 kHz)

fall back into the transition band of the digital filter.

FREQUENCY (kHz)

–40

–100

–20

–60

–80

100

FOLD-

ING

POINT

±5.0kHz PASS BAND

–120

120

–88dB

–101dB

–103dB

Figure 53. Decimation Filter Frequency Response for

fOUT = 20 kSPS (fCLK = 18 MHz, OSR = 900)

Figure 54 shows the response of the decimation filter with a

decimation factor of 48 and a sampling clock rate of 26 MHz.

The alias attenuation is at least 94 dB and occurs for frequencies

at the edges of the fourth alias band. The difference between

the alias attenuation characteristics of Figure 53 and those of

Figure 54 is due to the fact that the third decimation stage

decimates by a factor of 5 for Figure 53 compared with a factor

of 4 for Figure 54.

–40

–100

–20

–60

–80

–120

FREQUENCY (MHz)

EQUENCY

1.5

0.5

1.0

2.0

2.5

±135.466kHz PASS BAND

–98dB

–115dB

–94dB

Figure 54. Decimation Filter Frequency Response for

fOUT = 541.666 kSPS (fCLK = 26 MHz, OSR = 48)

Figure 55 and Figure 56 show expanded views of the pass band

for the two possible configurations of the third decimation

filter. When decimating by 60n (K = 0), the pass-band gain

variation is 1.2 dB; when decimating by 48n (K = 1), the pass-

band gain variation is 0.9 dB. Normalization of full scale at

band center is accurate to within 0.14 dB across all decimation

modes. Figure 57 and Figure 58 show the folded frequency

response of the decimator for K = 0 and K = 1, respectively.

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.250

0.125

–1

–2

–3

PASS-BAND GAIN FREQUENCY = 1.2dB

Figure 55. Pass-Band Frequency Response of the Decimator for K = 0

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