AD6674 datasheet(61/97 Pages) AD

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AD6674

Data Sheet

Rev. C | Page 60 of 96

VARIABLE DYNAMIC RANGE (VDR)

The AD6674 features a VDR digital processing block to allow

up to a 14-bit dynamic range to be maintained in a subset of the

Nyquist band. Across the full Nyquist band, a minimum 9-bit

dynamic range is available at all times. This operation is suitable

for applications such as DPD processing. The harmonic perfor-

mance of the receiver is unaffected by this feature. When

enabled, VDR does not contribute loss to the input signal but

operates by effectively changing the output resolution at the

output pins. This feature can be independently controlled per

channel via the SPI.

The VDR block operates in either complex or real mode. In

complex mode, VDR has selectable bandwidths of 25% and 43%

of the output sample rate. In real mode, the bandwidth of

operation is limited to 25% of the output sample rate. The

bandwidth and mode of the VDR operation are selected by

setting the appropriate bits in Register 0x430.

When the VDR block is enabled, input signals that violate a

defined mask (signified by gray shaded areas in Figure 119)

result in the reduction of the output resolution of the AD6674.

The VDR block analyzes the peak value of the aggregate signal

level in the disallowed zones to determine the reduction of the

output resolution. To indicate that the AD6674 is reducing

output, the resolution VDR punish bits and/or a VDR high/low

resolution bit can optionally be inserted into the output data

stream as control bits by programming the appropriate value

into Register 0x559 and Register 0x55A. Up to two control bits

can be used without the need to change the converter resolution

parameter, N. Up to three control bits can be used, but if using

three, the converter resolution parameter, N, must be changed

to 13. The VDR high/low resolution bit can be programmed

into either of the three available control bits and simply

indicates if VDR is reducing output resolution (bit value is a 1),

or if full resolution is available (bit value is a 0). Enable the two

punish bits to give a clearer indication of the available

resolution of the sample. To decode these two bits, see Table 25.

Table 25. VDR Reduced Output Resolution Values

VDR Punish Bits[1:0]

Output Resolution (Bits)

12 or 11

10 or 9

The frequency zones of the mask are defined by the bandwidth

mode selected in Register 0x430. The upper amplitude limit for

input signals located in these frequency zones is −30 dBFS. If

the input signal level in the disallowed frequency zones goes

above an amplitude level of –30 dBFS (into the gray shaded

areas), the VDR block triggers a reduction in the output

resolution, as shown in Figure 119. The VDR block engages and

begins limiting output resolution gradually as the signal

amplitudes increase in the mask regions. As the signal

amplitude level increases into the mask regions, the output

resolution is gradually lowered. For every 6 dB increase in

signal level above −30 dBFS, one bit of output resolution is

discarded from the output data by the VDR block, as shown in

Table 26. These zones can be tuned within the Nyquist band by

setting Bits[3:0] in Register 0x434 to determine the VDR center

frequency (fVDR). The VDR center frequency in complex mode

can be adjusted from 1/16 fS to 15/16 fS in 1/16 fS steps. In real

mode, fVDR can be adjusted from 1/8 fS to 3/8 fS in 1/16 fS steps.

Table 26. VDR Reduced Output Resolution Values

Signal Amplitude Violating Defined

VDR Mask

Output Resolution

(Bits)

Amplitude ≤ −30 dBFS

−30 dBFS < amplitude ≤ −24 dBFS

−24 dBFS < amplitude ≤ −18 dBFS

−18 dBFS < amplitude ≤ −12 dBFS

−12 dBFS < amplitude ≤ −6 dBFS

−6 dBFS < amplitude ≤ 0 dBFS

dBFS

–30

INTERMODULATION PRODUCTS < –30dBFS

INTERMODULATION PRODUCTS > –30dBFS

Figure 119. VDR Operation—Reduction in Output Resolution

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