AD9873JS datasheet(27/39 Pages) AD

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REV. 0

AD9873

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Figure 10d. Cascaded Filter Passband Detail (N = 3)

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Figure 12b. SINC Compensated Response

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Figure 12a. SINC and ISF Filter Response

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Figure 11d. Cascaded Filter Passband Detail (N = 4)

Inverse SINC Filter (ISF)

The AD9873 transmit section is almost entirely digital. The input

“signal” is made up of a time series of digital data words. These

data words propagate through the device as numbers. Ultimately,

this number stream must be converted to an analog signal. To this

end, the AD9873 incorporates an integrated DAC. The output

waveform of the DAC is the familiar “staircase” pattern typical

of a signal that is sampled and quantized. The staircase pattern

is a result of the finite time that the DAC holds a quantized level

until the next sampling instant. This is known as a zero-order hold

function. The spectrum of the zero-order hold function is the

familiar SIN(x)/x, or SINC, envelope.

The series of digital data words presented at the input of the DAC

represent an impulse stream. It is the spectrum of this impulse

stream, which is the characteristic of the desired output signal.

Due to the zero-order hold effect of the DAC, however, the output

spectrum is the product of the zero-order hold spectrum (the

SINC envelope) and the Fourier transform of the impulse stream.

Thus, there is an intrinsic distortion in the output spectrum,

which follows the SINC response.

The SINC response is deterministic and totally predictable. Thus,

it is possible to predistort the input data stream in a manner,

which compensates for the SINC envelope distortion. This can

be accomplished by means of an ISF. The ISF incorporated on

the AD9873 is a 5-tap, linear phase FIR filter. Its frequency

response characteristic is the inverse of the SINC envelope and

it equalizes the SINC droop up to 0.6 times the Nyquist fre-

quency. Figure 12a and Figure 12b show the effectiveness of the

ISF in correcting for the SINC distortion. Figure 12a includes a

graph of the SINC envelope and ISF response while Figure 12b

shows the SYSTEM response (which is the product of the SINC

and ISF responses). It should be mentioned at this point that

the ISF exhibits an insertion loss of 1.4 dB. Thus, signal levels

at the output of the AD9873 with the ISF bypassed are 1.4 dB

higher than with the ISF engaged. However, for modulated

output signals, which have a relatively wide bandwidth, the ben-

efits of the SINC compensation usually outweighed the 1.4 dB

loss in output level. The decision of whether or not to use the

ISF is an application specific system design issue.

AD9873JS Datasheet(PDF) 27 Page - Analog Devices

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