AD6679 datasheet(59/81 Pages) AD

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Data Sheet

AD6679

Rev. B | Page 59 of 81

VARIABLE DYNAMIC RANGE (VDR)

The AD6679 features a variable dynamic range (VDR) digital

processing block to allow up to 14-bit dynamic range to be

maintained in a subset of the Nyquist band. Across the full

Nyquist band, a minimum of a 9-bit dynamic range is available

at all times. This operation is suitable for applications such as

digital predistortion processing (DPD). The harmonic

performance 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 80)

result in the reduction of the output resolution of the AD6679.

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 AD6679 is reducing

output, the VDR punish bit or a VDR high/low resolution bit

can optionally be on the STATUS±/OVR± pins by programming

the appropriate value into Register 0x559. The VDR high/low

resolution bit can alternatively be programmed to output on the

STATUS± pins 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). These VDR high/low resolution and VDR punish

bits can be decoded by using Table 27. Note that only one can

be output at a given time.

Table 27. VDR Reduced Output Resolution Values

VDR Punish Bit

VDR High/Low

Resolution Bit

Output Resolution

(Bits)

Not applicable

14 or 13

Not applicable

≤12

Not applicable

≤13

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 80. 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 28. 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 28. 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 80. VDR Operation—Reduction in Output Resolution

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