AD9166 datasheet(49/138 Pages) AD

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Data Sheet

AD9166

Rev. 0 | Page 49 of 138

MAIN DIGITAL DATAPATH

2×

3×

JESD

2×,

4×,

8×

NCO

INV

SINC

2×

Figure 65. Block Diagram of the Main Digital Datapath

The block diagram in Figure 65 shows the functionality of the

main digital datapath. The digital processing includes an input

interpolation block with choice of bypass 1×, 2×, or 3× interpo-

lation, three additional 2× half-band interpolation filters, a final

2× NRZ mode interpolator filter, FIR85, that can be bypassed,

and a quadrature modulator that consists of a 48-bit NCO and

an inverse sinc block.

All of the interpolation filters accept I and Q data streams as a

complex data stream. Similarly, the quadrature modulator and

inverse sinc function also accept input data as a complex data

stream. Thus, any use of the digital datapath functions requires

the input data to be a complex data stream.

In bypass mode (1× interpolation), the input data stream is

expected to be real data.

Table 33. Pipeline Delay (Latency) for Various Datapath

Configurations

Mode

FIR85

Filter

Bandwidth

Inverse

Sinc

NCO

Pipeline Delay1

(fCLK Cycles)

NCO only

N/A2

Yes

1× (Bypass)

N/A2

113

1× (Bypass)

N/A2

Yes

137

2×

80%

155

2×

90%

176

2×

Yes

80%

202

2×

80%

Yes

185

2×

Yes

80%

Yes

239

2×

Yes

80%

Yes

279

3×

80%

168

3×

90%

202

4×

80%

308

6×

80%

332

8×

80%

602

12×

80%

674

16×

80%

1188

24×

80%

1272

1 The pipeline delay given is a representative number, and may vary by a cycle

or two based on the internal handoff timing conditions at startup.

2 N/A means not applicable.

The pipeline delay changes based on the digital datapath

functions that are selected. See Table 33 for examples of the

pipeline delay per block. These delays are in addition to the

JESD204B latency.

DATA FORMAT

The input data format for all modes on the AD9166 is 16-bit,

twos complement. The digital datapath and the DAC decoder

operate in twos complement format. The DAC is a current

steering DAC and cannot represent 0. The DAC must either

source or sink current. As a result, when the 0 of twos

complement is represented in the DAC, it is a +1, and all the

positive values thereafter are shifted by +1. This mapping error

introduces a ½ LSB shift in the DAC output. The leakage can

become apparent when using the NCO to shift a signal that is

above or below 0 Hz when synthesized. The NCO frequency is

seen as a small spur at the NCO frequency tuning word.

To avoid the NCO frequency leakage, operate the DAC with a

slight digital backoff of one or several codes, and then add 1 to

all values in the data stream. These actions remove the NCO

frequency leakage but cause a ½ LSB dc offset. This small dc

offset is benign to the DAC and does not affect most applications

because the DAC output is ac-coupled through dc blocking

capacitors.

INTERPOLATION FILTERS

The main digital path contains five half-band interpolation

filters, plus a final half-band interpolation filter that is used in

2× NRZ mode. The filters are cascaded, as shown in Figure 65.

The first pair of filters is a 2× (HB2) or 3× (HB3) filter. Each of

these filters has two options for bandwidth, 80% or 90%. The

80% filters are lower power than the 90% filters. The filters

default to the lower power 80% bandwidth. To select the filter

bandwidth as 90%, program the FILT_BW bit in the

DATAPATH_CFG register to 1 (Register 0x111, Bit 4 = 0b1).

Following the first pair of filters is a series of 2× half-band

filters, each of which halves the usable bandwidth of the

previous one. HB4 has 45%, HB5 has 22.5%, and HB6 has

11.25% of the fDATA bandwidth.

The final half-band filter, FIR85, is used in 2× NRZ mode.

FIR85 is clocked at the 2 × fCLK rate and has a usable bandwidth

of 45% of the fCLK rate. The FIR85 filter is a complex filter, and

therefore the bandwidth is centered at 0 Hz. The FIR85 filter is

used in conjunction with the complex interpolation modes to

allow doubling the DAC update rate, thus moving the image

spur further from the signal. Table 34 shows how to select each

available interpolation mode, their usable bandwidths, and their

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