AD9864-EBZ datasheet(30/48 Pages) AD

30 / 48 page

Data Sheet

AD9864

Rev. A | Page 29 of 47

A 16 dB step attenuator is also included within the LNA/mixer

circuitry to prevent large signals (that is, > −18 dBm) from

overdriving the Σ-Δ modulator. In such instances, the Σ-Δ

modulator becomes unstable, thus severely desensitizing the

receiver. The 16 dB step attenuator can be invoked by setting

the ATTEN bit (Register 0x03, Bit 7), causing the mixer gain to

be reduced by 16 dB. The 16 dB step attenuator can be used in

applications where a potential target or blocker signal could

exceed the IF input clip point. Although the LNA is driven into

compression, it may still be possible to recover the desired signal

if it is FM. See Table 19 for the gain compression characteristics

of the LNA and mixer with the 16 dB attenuator enabled.

Table 19. SPI Registers Associated with LNA/Mixer

Address (Hex)

Bit(s)

Width

Default Value

Name

0x00

[7:0]

0xFF

STBY

0x03

ATTEN

BAND-PASS Σ-∆ ADC

The ADC of the AD9864 is shown in Figure 47. The ADC

contains a sixth-order, multibit band-pass Σ-Δ modulator that

achieves very high instantaneous dynamic range over a narrow

frequency band. The loop filter of the band-pass Σ-Δ modulator

consists of two continuous-time resonators followed by a discrete

time resonator, with each resonator stage contributing a pair of

complex poles. The first resonator is an external LC tank, while

the second is an on-chip active RC filter. The output of the LC

resonator is ac-coupled to the second resonator input via 100 pF

capacitors. The center frequencies of these two continuous-time

resonators must be tuned to fCLK/8 for the ADC to function

properly. The center frequency of the discrete time resonator

automatically scales with fCLK, thus no tuning is required.

TO DIGITAL

FILTER

RESO-

NATOR

NINE-

LEVEL

FLASH

ESL

GAIN

CONTROL

MIXER

OUTPUT

EXTERNAL

fCLK = 13 MSPS TO 26 MSPS

MXOP

MXON

IF2P

IF2N

RESO-

NATOR

DAC1

Figure 47. Equivalent Circuit of Sixth-Order Band-Pass Σ-∆ Modulator

Figure 48 shows the measured power spectral density measured

at the output of the undecimated band-pass Σ-Δ modulator.

Note that the wide dynamic range achieved at the center

frequency, fCLK/8, is achieved once the LC and RC resonators of

the Σ-Δ modulator have been successfully tuned. The out-of-

band noise is removed by the decimation filters following

quadrature mixer.

–10

–20

–30

–50

–40

FREQUENCY (MHz)

–60

–70

–80

–90

–100

–2dBFS OUTPUT

fCLK = 18MHz

NBW = 3.3kHz

Figure 48. Measured Undecimated Spectral Output of Σ-∆

Modulator ADC with fCLK = 18 MSPS and Noise Bandwidth of 3.3 kHz

The signal transfer function of the AD9864 possesses inherent

anti-alias filtering by virtue of the continuous time portions of

the loop filter in the band-pass Σ-Δ modulator. Figure 49

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 AD9864 are determined mostly by

the image band of the mixer, which is offset from the desired IF

input frequency by fCLK/4 (that is, 2 × 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 49. Signal Transfer Function of the Band-Pass Σ-∆

Modulator from 0 fCLK to 2fCLK

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