AD6674 datasheet(58/97 Pages) AD

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Data Sheet

AD6674

Rev. C | Page 57 of 96

NOISE SHAPING REQUANTIZER (NSR)

When operating the AD6674 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. On the AD6674-750 and AD6674-1000, this

filter is nonbypassable when the NSR is enabled. The filter can

be optionally enabled on the AD6674-500 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 Register 0x41E.

DECIMATING HALF-BAND FILTER

The AD6674 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 1000 MHz,

this reduces the output sample rate to 500 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 1000 MSPS, the filter provides a maximum

usable bandwidth of 197.5 MHz.

Half-Band Filter Coefficients

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

low power consumption due to its polyphase implementation.

Table 24 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 24 are 0s.

Table 24. 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 is designed to provide approxi-

mately 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 response of the half-band filter in low-pass

mode is shown in Figure 111 for an input sample clock of

1000 MHz. 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 1000 MHz, the output sample rate is 500 MSPS and fS/2 =

250 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 111. 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 112 shows the response of the half-band

filter in high-pass mode with an input sample clock of 1000 MHz.

In high-pass mode, operation is allowed in the second and third

Nyquist zones, which includes frequencies from fS/2 to 3 fS/2,

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

input clock of 1000 MHz, the output sample rate is 500 MSPS,

fS/2 = 250 MHz, and 3 fS/2 = 750 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 112. High-Pass Half-Band Filter Response

NSR OVERVIEW

The AD6674 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 AD6674.

The NSR feature can be independently controlled per channel

via the SPI.

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