AD6676EBZ datasheet(56/90 Pages) AD

56 / 90 page

AD6676

Data Sheet

Rev. A | Page 56 of 90

IF FREQUENCY PLANNING

The Σ-Δ ADC can achieve exceptional SFDR performance over

a wide IF frequency range because its high oversampling ratio

prevents low order harmonics from aliasing into the IF pass band.

Higher order harmonics that do alias back are typically of much

lower magnitude, with the shuffling option further reducing

their levels. However, finite isolation between the Σ-Δ ADC and

the digital block causes additional spurious signals that are a

function of the output data rate, fDATA_IQ, and input frequency,

fIN. Specifically, the feedback DACs in the Σ-Δ ADC suffer from

digital contamination of its clock signal. Therefore, the same

equation used to predict spurious locations on high speed DACs

with digital interpolation filters applies.

Equation 15 defines this relationship with the spur location

falling at fMN.

fMN = ±(M × fDATA_IQ) ± (N × fIN)

(15)

where:

M is the digital induced harmonic content from internal clocks.

N is the harmonics from the Σ-Δ ADC.

When N = 0, signal independent spurs fall at integer multiples

of fDATA_IQ. Table 23 shows the measured M × fDATA_IQ spurious

levels (dBFS) for different IF frequencies and decimation factors

with fDATA_IQ equal to 100 MSPS and 200 MSPS. All of the M ×

fDATA_IQ regions display low spurious with the exception of

200 MHz. This is because a large portion of digital circuitry is

clocked at FADC/16 for DEC_MODES of 1 and 3 or FADC/12 for

DEC_MODES of 2 and 4. As a result, the M = 2 spur is dominant

when operating at the higher decimation factors of 32 and 24

whereas the M = 1 spur is dominant when operating at the

lower decimation factors of 16 and 12.

When N = 1, signal dependent spurs falls at integer multiples of

fDATA_IQ. These M × N spurs are called images because they have

a 1:1 relationship in amplitude and frequency with the input signal,

fIN. Note that the magnitude of some images can also vary slightly

between power cycles, due to different phase relationships among

internal clock dividers upon device initialization. Figure 136

shows a normalized image graph (relative to fDATA_IQ) showing

the image location relative for a given input frequency.

When N > 1, spurious content is often at lower magnitude than

other spurious thus often can be ignored. The exception is when

fIN falls below the IF pass band such that its lower order harmonics

may fall within the pass band (that is, IF/2 and IF/3).

4.5

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

NORMALIZED INPUT FREQUENCY

M = 6

M = 7

M = 8

M = 5

M = 3

M = 4

M = 2

M = 3

M = 1

Figure 136. Image Location for Different M Factors Normalized to fDATA_IQ

Table 23. Measured Spurious Levels at Different IFs Where

M × fDATA_IQ Falls On for fDATA_IQ of 100 MSPS and 200 MSPS

DATA_IQ

Spurious Levels (dBFS)

IF =

100 MHz

IF =

200 MHz

IF =

300 MHz

IF =

400 MHz

100 MSPS

DEC_MODE = 1

<−100

−81

<−110

−97

DEC_MODE = 2

−100

−79

<−110

N/A1

200 MSPS

DEC_MODE = 3

<−110

−81

<−110

−90

DEC_MODE = 4

<−110

−77

<−110

N/A1

1 N/A means not applicable.

Because the image spurs are also at low levels, the AD6676 offers a

wide range of suitable IFs for a given output data rate, fDATA_IQ.

Even IFs that are situated in a region where the worst M × fDATA_IQ

spurious condition described in Table 23 can be used because

they remain at a fixed location and remain signal independent.

Similar to the LO feedthrough issue in a direct conversion IQ

receiver, a slow digital tracking loop in the host processor can

be used to nullify it. Figure 137 and Figure 138 show a case where

the IF of 200 MHz was selected for an fDATA_IQ of 200 MSPS and

100 MSPS such that dominant spur falls exactly at the IF center.

As shown in Figure 136, the IF is positioned at a normalized

fDATA_IQ of 1 or 2 for 200 MSPS and 100 MSPS operation, thus

explaining why the image term is M = 2 or 4. Note that the image

spur is quite low for M = 2 and can be further improved by

selecting a higher decimation factor (DEC_MODE of 3 vs. 1)

that results in the M = 4 image.

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