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AD6679

Data Sheet

Rev. B | Page 56 of 81

NOISE SHAPING REQUANTIZER (NSR)

When operating the AD6679 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 process 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 AD6679 when the NSR is enabled. When

operating with NSR enabled, the decimating half-band filter

mode (low pass or high pass) is selected by setting Bit 7 in

DECIMATING HALF-BAND FILTER

The AD6679 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 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 26 lists the coefficients of the half-band filter in low-pass

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

The normalized coefficients used in the implementation and

the decimal equivalent values of the coefficients are listed.

Coefficients not listed in Table 26 are 0s.

Table 26. Fixed Coefficients for Half-Band Filter

Coefficient

Number

Normalized

Coefficient

Decimal 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 72. 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 0.150 0.20

0.25

NORMALIZED FREQUENCY (×

RAD/SAMPLE)

0.30

0.35

0.40

0.45

0.50

Figure 72. Low-Pass Half-Band Filter Response

The half-band filter can also be utilized 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 73 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 73. High-Pass Half-Band Filter Response

NSR OVERVIEW

The AD6679 features an NSR to allow higher than 9-bit SNR to

be maintained in a subset of the Nyquist band. The harmonic

performance of the receiver is unaffected by the NSR feature.

When enabled, the NSR contributes an additional 3.0 dB of loss

to the input signal, such that a 0 dBFS input is reduced to

−3.0 dBFS at the output pins. This loss does not degrade the SNR

performance of the AD6679.

The NSR feature can be independently controlled per channel

via the SPI.

Two different bandwidth modes are provided; select the mode

from the SPI port. In each of the two modes, the center frequency

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