AD9874EB datasheet(25/40 Pages) AD

25 / 40 page

REV. A

AD9874

–25–

The signal transfer function of the AD9874 possesses inherent

antialias filtering by virtue of the continuous-time portions of

the loop filter in the band-pass

modulator. Figure 13b

illustrates this property by plotting the nominal signal transfer

function of the ADC for frequencies up to 2fCLK. The notches

that naturally occur for all frequencies that alias to the fCLK/8

pass band are clearly visible. Even at the widest bandwidth setting,

the notches are deep enough to provide greater than 80 dB of

alias protection. Thus, the wideband IF filtering requirements

preceding the AD9874 will be determined mostly by the mixer’s

image band, which is offset from the desired IF input frequency

by fCLK/4 (i.e., 2 3 fCLK/8) rather than any aliasing associated

with the ADC.

–10

–20

–30

–50

–40

NORMALIZED FREQUENCY – RELATIVE TO

fOUT

–60

–70

–80

0.5

1.0

1.5

2.0

NOTCH AT ALL ALIAS FREQUENCIES

Figure 13b. Signal Transfer Function of the

Band-Pass

Modulator from 0 fCLK to 2 fCLK

Figure 13c shows the nominal signal transfer function magni-

tude for frequencies near the fCLK/8 pass band. The width of the

pass band determines the transfer function droop, but even at

the lowest oversampling ratio (48) where the pass band edges

are at

fCLK/192 (

0.005 fCLK), the gain variation is less than

0.5 dB. Note that the amount of attenuation offered by the

signal transfer function near fCLK/8 should also be considered

when determining the narrow-band IF filtering requirements

preceding the AD9874.

–0.10

–5

–10

NORMALIZED FREQUENCY – RELATIVE TO

fCLK

–15

–20

–0.05

0.05

0.10

Figure 13c. Magnitude of the ADC’s Signal

Transfer Function near fCLK/8

Tuning of the

modulator’s two continuous-time resonators

is essential in realizing the ADC’s full dynamic range and must

be performed upon system startup. To facilitate tuning of the

LC tank, a capacitor array is internally connected to the MXOP

and MXON pins. The capacitance of this array is program-

mable from 0 pF to 200 pF

20% and can be programmed

either automatically or manually via the SPI port. The capaci-

tors of the active RC resonator are similarly programmable.

Note that the AD9874 can be placed in and out of its standby

mode without retuning since the tuning codes are stored in the

SPI Registers.

When tuning the LC tank, the sampling clock frequency must

be stable and the LNA/mixer, LO synthesizer, and ADC must

all be placed in standby. Tuning is triggered when the ADC is

taken out of standby if the TUNE_LC bit of Register 0x1C has

been set. This bit will clear when the tuning operation is com-

plete (less than 6 ms). The tuning codes can be read from the

3-bit CAPL1 (0x1D) and the 6-bit CAPL0 (0x1E) registers.

In a similar manner, tuning of the RC resonator is activated if

the TUNE_RC bit of Register 0x1C is set when the ADC is

taken out of standby. This bit will clear when tuning is com-

plete. The tuning code can be read from the CAPR (0x1F)

the LC tank and the active RC resonator in succession. During

tuning, the ADC is not operational and neither data nor a clock

is available from the SSI port. Table X lists the recommended

sequence of the SPI commands for tuning the ADC, and Table XI

lists all of the SPI registers associated with band-pass -

ADC.

Table X. Tuning Sequence

Address Value

Comments

0x00

0x45

LO synthesizer, LNA/mixer, and ADC are

placed in standby.

0x1C

0x03

Set TUNE_LC and TUNE_RC. Wait for

CLK to stabilize if CLK synthesizer used.

0x00

0x44

Take the ADC out of standby. Wait for

0x1C to clear (<6 ms). LNA/mixer can now

be taken out of standby.

*If external CLK VCO or source used, the CLK oscillator must also be disabled.

Table XI. SPI Registers Associated with Band-Pass

ADC

Address

Bit

Default

(Hex)

Breakdown

Width

Value

Name

0x00

(7:0)

0xFF

STBY

0x1C

(1)

TUNE_LC

(0)

TUNE_RC

0x1D

(2:0)

CAPL1(2:0)

0x1E

(5:0)

0x00

CAPL1(5:0)

0x1F

(7:0)

0x00

CAPR

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