AD9856SLASHPCB datasheet(19/32 Pages) AD

19 / 32 page

AD9856

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REV. B

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a. CIC Frequency Response (R = 2, HBF 3 Bypass)

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b. CIC Frequency Response (R = 63, HBF 3 Bypass)

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 accept-

able. 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. Since 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 AD9856

control register description). 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. Obviously, it is desirable to operate

the DAC over its full-scale range in order to minimize the inher-

ent quantization effects associated with a DAC. Any significant

loss through the CIC stage will be reflected at the DAC output

as a reduction in SNR. The degradation in SNR can be over-

come by boosting the CIC output level. Table II (The CIC

Interpolation Filter Insertion Loss Table) tabulates insertion

loss as a function of R. The values are provided in linear and

decibel form, both with and without the factor-of-two gain

employed.

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

must 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, thereby compromising the validity of the ana-

log output signal.

Figure 31. CIC Filter Frequency Response (HB 3 Bypassed and R = 2, 63)

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d. Passband Detail (R = 63, HBF 3 Bypass)

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