AD9864 datasheet(34/48 Pages) AD

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Data Sheet

AD9864

Rev. A | Page 33 of 47

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.250

0.125

–1

–2

–3

PASS-BAND GAIN VARIATION = 0.9dB

Figure 56. Pass-Band Frequency Response of the Decimator for K = 1

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.50

0.25

–20

–40

–60

–80

–100

–120

MIN ALIAS ATTN = 87.7dB

Figure 57. Folded Decimator Frequency Response for K = 0

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.50

0.25

–20

–40

–60

–80

–100

–120

MIN ALIAS ATTN = 97.2dB

Figure 58. Folded Decimator Frequency Response for K = 1

VARIABLE GAIN AMPLIFIER OPERATION WITH

AUTOMATIC GAIN CONTROL

The AD9864 contains both a variable gain amplifier (VGA) and

a digital VGA (DVGA) along with all of the necessary signal

estimation and control circuitry required to implement automatic

gain control (AGC), as shown in Figure 59. The AGC control

circuitry provides a high degree of programmability, allowing

users to optimize the AGC response as well as the dynamic

range of the AD9864 for a given application. The VGA is

programmable over a 12 dB range and implemented within the

ADC by adjusting its full-scale reference level. Increasing the

full scale of the ADC is equivalent to attenuating the signal. An

additional 12 dB of digital gain range is achieved by scaling the

output of the decimation filter in the DVGA. Note that a slight

increase in the supply current (0.67 mA) is drawn from VDDI

and VDDF as the VGA changes from 0 dB to 12 dB attenuation.

The purpose of the VGA is to extend the usable dynamic range

of the AD9864 by allowing the ADC to digitize a desired signal

over a large input power range as well as recover a low level

signal in the presence of larger unfiltered interferers without

saturating or clipping the ADC. The DVGA is most useful in

extending the dynamic range in narrow-band applications

requiring a 16-bit I and Q data format. In these applications,

quantization noise resulting from internal truncation to 16 bits

as well as external 16-bit fixed point post processing can degrade

the effective noise figure of the AD9864 by 1 dB or more.

The DVGA is enabled by writing a 1 to the AGCV field. The

VGA (and the DVGA) can operate in either a user controlled

variable gain mode or automatic gain control (AGC) mode. It is

worth noting that the VGA imparts negligible phase error upon

the desired signal as its gain is varied over a 12 dB range. This is

due to the bandwidth of the VGA being far greater than the

down converted desired signal (centered about fCLK/8) and

remaining relatively independent of gain setting. As a result,

phase modulated signals experience minimal phase error as the

AGC varies the VGA gain while tracking an interferer or the

desired signal under fading conditions. Note that the envelope

of the signal is still affected by the AGC settings.

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