AD6684 datasheet(49/107 Pages) AD

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Data Sheet

AD6684

Rev. 0 | Page 49 of 107

NOISE SHAPING REQUANTIZER (NSR)

When operating the AD6684 with the NSR enabled, a decimating

half-band filter that is optimized at certain input frequency bands

can also be enabled. This filter offers the user the flexibility in

signal bandwidth processing and image rejection. Careful

frequency planning can offer advantages in analog filtering

preceding the ADC. The filter can function either in high-pass

or low-pass mode. The filter can be optionally enabled on the

AD6684 when the NSR is enabled. When operating with the

NSR enabled, the decimating half-band filter mode (low pass or

high pass) is selected by setting Bit 7 in Register 0x041E. When the

decimating half-band filter is enabled, the chip decimation ratio

(register value = 0x01).

DECIMATING HALF-BAND FILTER

The AD6684 optional decimating half-band filter reduces the

input sample rate by a factor of 2 while rejecting aliases that fall

into the band of interest. For an input sample clock of 500 MHz,

this filter reduces the output sample rate to 250 MSPS. This filter is

designed to provide >40 dB of alias protection for 39.5% of the

output sample rate (79% of the Nyquist band). For an ADC sample

rate of 500 MSPS, the filter provides a maximum usable

bandwidth of 98.75 MHz.

Half-Band Filter Coefficients

The 19-tap, symmetrical, fixed coefficient half-band filter has

low power consumption due to its polyphase implementation.

Table 23 lists the coefficients of the half-band filter in low-pass

mode. In high-pass mode, Coefficient C9 is multiplied by −1.

The decimal coefficients used in the implementation and the

decimal equivalent values of the coefficients are listed.

Coefficients not listed in Table 23 are 0s.

Table 23. Fixed Coefficients for Half-Band Filter

Coefficient

Number

Decimal

Coefficient

Quantized

Coefficient (12-Bit)

0.012207

C2, C16

−0.022949

−47

C4, C14

0.045410

C6, C12

−0.094726

−194

C8, C10

0.314453

644

0.500000

1024

Half-Band Filter Features

The half-band decimating filter provides approximately 39.5%

of the output sample rate in usable bandwidth (19.75% of the

input sample clock). The filter provides >40 dB of rejection. The

normalized response of the half-band filter in low-pass mode is

shown in Figure 82. In low-pass mode, operation is allowed in

the first Nyquist zone, which includes frequencies of up to fS/2,

where fS is the decimated sample rate. For example, with an

input clock of 500 MHz, the output sample rate is 250 MSPS

and fS/2 = 125 MHz.

–80

–70

–60

–50

–40

–30

–20

–10

0.05 0.10 0.15 0.20 0.25

NORMALIZED FREQUENCY (×

RAD/SAMPLE)

0.30 0.35 0.40 0.45 0.50

Figure 82. Low-Pass, Half-Band Filter Response

The half-band filter can also be used in high-pass mode. The

usable bandwidth remains at 39.5% of the output sample rate

(19.75% of the input sample clock), which is the same as in low-

pass mode). Figure 83 shows the normalized response of the

half-band filter in high-pass mode. In high-pass mode, operation

is allowed in the second and third Nyquist zones, which includes

frequencies from fS/2 to 3fS/2, where fS is the decimated sample

rate. For example, with an input clock of 500 MHz, the output

sample rate is 250 MSPS, fS/2 = 125 MHz, and 3fS/2 = 375 MHz.

–80

–70

–60

–50

–40

–30

–20

–10

0.1

0.2

0.3

0.4

0.5

NORMALIZED FREQUENCY (× π RAD/SAMPLE)

0.6

0.7

0.8

0.9

1.0

Figure 83. High-Pass, Half-Band Filter Response

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