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Data Sheet

AD9171

DIGITAL DATAPATH

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Rev. C | 40 of 134

The full digital datapath of the AD9171 consists of channel data-

paths that are bypassable, as well as a main DAC datapath leading

up to the analog DAC core. There is a variety of different digital

feature blocks available within each of the channel and main DAC

datapaths.

TOTAL DATAPATH INTERPOLATION

The AD9171 contains two separate blocks of interpolation: one

located in each of the channelizer datapaths, and a second inter-

polation stage located in the main DAC datapath. The total interpo-

lation for the chip can be determined by multiplying the channel

interpolation factor by the main datapath interpolation factor. The

relationship between the DAC sample rate and input data rate is

shown in the following equation:

TotalInterpolation = (ChannelInterpolation × MainInterpolation)

fDATA = fDAC / (ChannelInterpolation × MainInterpolation)

Each of the various cascaded half-band filters has 80% signal

bandwidth capabilities. Therefore, if using interpolation (not bypass

or 1× total interpolation for both channel and main datapaths), the

available signal bandwidth (BW) with respect to the input data rate

is calculated as follows:

SignalBW = 0.8 × (fDATA/2)

where the total interpolation > 1.

SignalBW = (fDATA /2)

where the total interpolation = 1.

The interpolation values are programmed as shown in Table 29.

Table 29. Interpolation Factor Register Settings

Interpolation

Factor

Main Datapath, Register

0x111, Bits[7:4]

Channel Datapath, Register

0x111, Bits[3:0]

2×

Not applicable

0x2

3×

Not applicable

0x3

4×

Not applicable

0x4

6×

0x6

8×

0x8

Not applicable

Table 30. Interpolation Modes and Useable Bandwidth

Total Interpolation

Available Signal

Bandwidth

fDATA

12×, 16×, 18×, 24×, 32×,

36×, 48×, 64×

80% × fDATA /2

fDAC/total interpolation

Filter Performance

The interpolation filters interpolate between existing data in such

a way that they minimize changes in the incoming data while

suppressing the creation of interpolation images.

The usable bandwidth, as shown in Table 30 is defined as the

frequency band over which the filters have a pass-band ripple of

less than ±0.001 dB and an image rejection of greater than 85 dB.

The maximum pass-band amplitude of all filters is the same; they

are different in the illustration to improve understanding.

Figure 43. Band Responses of Total Interpolation Rates for 8×, 12×, 16×, 18×,

24×, 32×, 36×, 48×, and 64× at a 6 GHz DAC Rate, 16-Bit

NCO Only Mode

The AD9171 is capable of operating in a mode with only the NCO

enabled. In this mode, a single-tone sine wave is generated using

a programmable internal dc amplitude level that is injected into the

NCO block to modulate the data into a single tone. This internal dc

level is injected into either the channel or main datapaths through

various controls, as shown in Figure 44. Mode 3 can be used to

generate a single-channel NCO only mode of operation. It is not

necessary to bring up the JESD204B link in this mode. This mode

is a useful option to bring up a transmitter radio signal chain without

requiring a digital data source, because the device generates the

NCO data internally.

This mode can also be used in applications where a sine wave is

all that is required, such as in a local oscillator application. There

is an additional optional calibration NCO block that can be used as

part of the initial system calibration without needing to reprogram

the final main datapath NCO to the final system configuration. This

feature is discussed in more detail in the Calibration NCO section.

When the NCO test mode is enabled, the data source of the digital

datapath is the dc test data word, meaning that the JESD204B

link can be brought up and data can be transferred to the device

over the link, but the data is not presented to the DAC when in

this test mode. Connection to the SERDES data source is only

achieved by disabling this test mode. The SPI control bit for this

test mode can be set on the fly, but because disabling the mode

and switching to the SERDES datapath normally requires the lanes

and/or interpolation mode to also be set, on the fly setting or

resetting of the NCO only mode control bit is normally not practical.

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