AD9864 Analog Devices, AD9864 Datasheet - Page 30

• Manufacturer Part Number: AD9864
• Description: IF Digitizing Subsystem
• Manufacturer: Analog Devices

Specifications of AD9864

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 18MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni,SE-Uni
• Adc Architecture: Sigma-Delta
• Pkg Type: CSP

AD9864
Figure 54. Pass-Band Frequency Response of the Decimator for K = 0
Figure 55. Pass-Band Frequency Response of the Decimator for K = 1
–100
–120
–20
–40
–60
–80
–1
–2
–3
–1
–2
–3
3
2
1
0
3
2
1
0
0
0
0
Figure 56. Folded Decimator Frequency Response for K = 0
0
MIN ALIAS ATTN = 87.7dB
NORMALIZED FREQUENCY (RELATIVE TO
NORMALIZED FREQUENCY (RELATIVE TO
NORMALIZED FREQUENCY (RELATIVE TO
PASS-BAND GAIN FREQUENCY = 1.2dB
PASS-BAND GAIN VARIATION = 0.9dB
0.125
0.125
0.25
f
f
OUT
OUT
f
OUT
)
)
)
0.250
0.250
0.50
Rev. 0 | Page 30 of 44
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.
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 f
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.
–100
–120
–20
–40
–60
–80
0
Figure 57. Folded Decimator Frequency Response for K = 1
0
MIN ALIAS ATTN = 97.2dB
CLK
/8) and remaining relatively independent of gain
NORMALIZED FREQUENCY (RELATIVE TO
0.25
f
OUT
)
0.50