AD9083BBCZ datasheet(36/93 Pages) AD

36 / 93 page

AD9083

Data Sheet

Rev. 0 | Page 36 of 93

CASCADED INTEGRATOR COMB (CIC) FILTER

Data from each ADC is sent to the CIC filter. The CIC filter can

be bypassed, and the ADC data sent to the DDCs to decimate

the data. This is enabled by writing a 1 to the DECI_ADC_

DATA bit (Bit DB4) in the DP_CTRL register. In this mode,

the mixer is bypassed. Therefore, there is no frequency shifting

available.

The frequency response of the CIC filter built in to the AD9083

is identical to that of a second-order moving average filter of

Length N, where N is the decimation rate. The decimation rates

allowed are 4×, 8×, or 16×, based on the setting. The output of

the CIC filter is a single 16-bit data sample at a rate of fS/N

where fS is the ADC sample rate and N is the CIC decimation

ratio setting.

The response of the CIC filter under different decimation

settings is shown in Figure 78. The response is normalized to

the ADC sample frequency (fS). This normalization helps the

end user with frequency planning. For example, if the ADC fS is

2 GSPS, a fundamental frequency placed at 100 MHz incurs CIC

losses of 1.09 dB, 4.77 dB, and 25.17 dB for CIC decimation ratios

of 4×, 8×, and 16×, respectively. Therefore, for this application

use case, it is best to select a CIC decimation ratio of 4× and

rely on the DDC decimation to further decimate the data.

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0.05

0.10

0.15

0.20

0.25

NORMALIZED FREQUENCY (×

fS)

4× DECIMATION

8× DECIMATION

16× DECIMATION

Figure 78. CIC Filter Response for Decimation Ratio = 4×, 8×, and 16×;

Normalized to fS

Adjust the CIC droop with a programmable gain adjustment.

For each sample frequency, the tones are at different frequency

values and the value of droop correction required is different.

Figure 79 shows the droop characteristics to a finer detail.

–100

–90

–80

–70

–60

–50

–40

–30

–20

–10

0.150

0.045 0.060 0.075 0.090 0.105 0.120 0.135

0.030

0.015

NORMALIZED FREQUENCY (×

fS)

4× DECIMATION

8× DECIMATION

16× DECIMATION

Figure 79. Zoomed in Image of the CIC Filter Response Shown in Figure 78;

Normalized to fS

The AD9083 digital datapath has a block that can be used to

program the gain to compensate for the droop. Based on the

application and the input frequency, the user can program the

appropriate gain to compensate for the loss through the CIC

filter. The gain setting to compensate for the loss is calculated

as follows

Fractional Bit Calculation ≥ frac(10(−Droop/20)) × 210

For example, if the input frequency is 25 MHz, Table 18 shows

the gain setting needed to compensate for the droop for the

various CIC decimation ratios for various frequencies. As

shown in Figure 79, the CIC roll-off must be considered when

designing the AD9083 in a system. As a general rule, keep the

IF to within 20% of the output data rate. For example, if the

highest frequency in the system is 100 MHz, then a CIC

decimation of 4× is recommended. Conversely, if a CIC

decimation of 16× is chosen, keep the IF below 25 MHz.

The CIC gain is programmed as an unsigned 4.10 word. The

first four bits represent the integer portion of the gain, and the

following 10 bits represent the fraction portion of the gain.

Table 18. Example Gain Compensation Settings for various CIC filter Roll-Off Values; fS = 2 GHz

Frequency

(MHz)

4× Decimation

8× Decimation

CIC Droop (dB)

Gain Compensation (Unsigned 4.10)

CIC Droop (dB)

Gain Compensation (Unsigned 4.10)

−0.0669177

0001_00_0000_1000

−0.28

0001_00_0010_0010

−0.2687053

0001_00_0010_0000

−1.14

0001_00_1001_0000

AD9083BBCZ Datasheet(PDF) 36 Page - Analog Devices

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