AD9864 datasheet(30/44 Pages) AD

30 / 44 page

AD9864

Rev. 0 | Page 30 of 44

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.250

0.125

–1

–2

–3

PASS-BAND GAIN FREQUENCY = 1.2dB

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.50

0.25

–20

–40

–60

–80

–100

–120

MIN ALIAS ATTN = 97.2dB

Figure 54. Pass-Band Frequency Response of the Decimator for K = 0

Figure 57. Folded Decimator Frequency Response for K = 1

NORMALIZED FREQUENCY (RELATIVE TO

fOUT)

0.250

0.125

–1

–2

–3

PASS-BAND GAIN VARIATION = 0.9dB

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 58. The AGC control circuitry provides a

high degree of programmability, allowing users to optimize the AGC

response as well as the AD9864’s dynamic range for a given applica-

tion. The VGA is programmable over a 12 dB range and implemented

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

ADC’s full scale 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 (i.e., 0.67 mA) is drawn from VDDI and VDDF as the VGA

changes from 0 dB to 12 dB attenuation.

Figure 55. 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

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 AD9864’s effective noise figure 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 (cen-

tered about fCLK/8) and remaining relatively independent of gain

setting. As a result, phase modulated signals should experience mini-

mal 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 will still be affected by the AGC settings.

Figure 56. Folded Decimator Frequency Response for K = 0

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