AD9856SLASHPCB datasheet(23/37 Pages) AD

23 / 37 page

AD9856

Rev. C | Page 22 of 36

–150

–120

–90

–60

–30

020

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

–4.0

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

Figure 35. CIC Filter Frequency Response (R = 2, HFB 3 Bypassed)

Figure 36. Pass-Band Detail (R = 2, HFB 3 Bypassed)

–150

–120

–90

–60

–30

144

216

258

360

432

504

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

–4.0

–3.5

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

DISPLAYED FREQUENCY IS RELATIVE TO I/Q NYQ. BW

Figure 37. CIC Filter Frequency Response (R = 63, HFB 3 Bypassed)

Figure 38. Pass-Band Detail (R = 63, HFB 3 Bypassed)

The degree of the impact of the attenuation introduced by the

CIC filter over the Nyquist bandwidth of the data is application

specific. The user must decide how much attenuation is

acceptable. If less attenuation is desired, then additional

oversampling of the baseband data must be employed.

Alternatively, the user can precompensate the baseband data

before presenting it to the AD9856. That is, if the data is

precompensated through a filter that has a frequency response

characteristic, which is the inverse of the CIC filter response,

then the overall system response can be nearly perfectly

flattened over the bandwidth of the data.

Another issue to consider with the CIC filters is insertion loss.

Unfortunately, CIC insertion loss is not fixed, but is a function

of R, M, and N. Because M, and N are fixed for the AD9856, the

CIC insertion loss is a function of R only.

Interpolation rates that are an integer power-of-2 result in no

insertion loss. However, all noninteger power-of-2 interpolation

rates result in a specific amount of insertion loss.

To help overcome the insertion loss problem, the AD9856

provides the user a means to boost the gain through the CIC

stage by a factor of 2 (via the CIC Gain bit—see the Serial

Control Bus Register section). The reason for this feature is to

allow the user to take advantage of the full dynamic range of the

DAC, thus maximizing the signal-to-noise ratio (SNR) at the

output of the DAC stage. It is best to operate the DAC over its

full-scale range in order to minimize the inherent quantization

effects associated with a DAC. Any significant loss through the

CIC stage is reflected at the DAC output as a reduction in SNR.

The degradation in SNR can be overcome by boosting the CIC

output level. Table 6 tabulates insertion loss as a function of R.

The values are provided in linear and decibel form, both with

and without the factor-of-2 gain employed.

A word of caution: When the CIC Gain bit is active, ensure that

the data supplied to the AD9856 is scaled down to yield an

overall gain of unity (1) through the CIC filter stage. Gains in

excess of unity are likely to cause overflow errors in the data

path, compromising the validity of the analog output signal.

AD9856/PCB 数据表(PDF) 23 Page - Analog Devices

AD9856/PCB 数据表(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

数据表下载

链接网址

AD9856/PCB 数据表(PDF) 23 Page - Analog Devices

AD9856/PCB 数据表(HTML) 23 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

数据表 下载

链接网址

数据表下载