CLC5958PCASM datasheet(9/13 Pages) NSC

9 / 13 page

http://www.national.com

Routing Output Data Lines

It is recommended that the ground plane be removed

under the data output lines to minimize the capacitive

loading of these lines. In some systems this may not be

permissible because of EMI considerations.

Harmonics and Clock Spurious

Harmonics are created by non-linearity in the track-and-

hold and the quantizer. Harmonics that arise from

repetitive non-linearities in the quantizer may be reduced

by the application of a dither signal.

Transformers

and

baluns

can

contribute

harmonic

distortion, particularly at low frequencies where trans-

former operation relies on magnetic ﬂux in the core. If a

transformer is used to perform single-ended to differential

conversion at the input, care should be taken in the

selection of the transformer.

The clock is internally divided by the CLC5958 in order to

generate internal control signals. These divided clocks

can contribute spurious energy, principally at fs/4 and fs/8.

The clock spurious is typically less than -90dBFS.

Calibration Sidebands

The CLC5958 incorporates on-board calibration. The

calibration process creates low level sideband spurious

close to the carrier and near DC for some input

frequencies. In most applications these sidebands will

not be an issue. The sidebands add negligible power to

the carrier and therefore do not reduce sensitivity in

receiver applications. Also, the sidebands never fall in

adjacent channels with any appreciable power. They may

be visible in some very narrow-band applications, and so

are documented here for completeness.

The offset of the sidebands relative to the carrier and rel-

ative to DC is derived using the equations:

where f

∆ is the sideband offset, f

is the input

frequency, fs is the sample rate, and round(•) denotes

integer rounding. The magnitude of the sideband relative

to the carrier for a full scale input tone is approximated by

the equations:

where a∆ is the sideband magnitude relative to the input,

and

α is the calibration sideband coefﬁcient. The value of

α rolls off 2dB per dB as the input amplitude is reduced.

For example, assume the input frequency is 4.8671MHz

and the sample rate is 52MSPS. Then the sideband

offset is derived as follows:

If the input is a full scale input, then the magnitude of the

sidebands is derived as:

The sidebands roll off rapidly with increasing sideband

offset. For example, if the sideband is offset 200KHz from

the carrier (in an adjacent GSM channel) as opposed to

the 7.9KHz offset from the previous example, the side-

band magnitude is reduced to -116dBc.

Figure 4 shows how the sideband offset frequency

varies with input frequency at a sample rate of 52MSPS.

Figure 4: Sideband Offset vs. Input Frequency

The sideband magnitude is a function of the sideband

offset, as illustrated in Figure 5.

Figure 5: Sideband Magnitude vs. Sideband Offset

f∆ (KHz)

-80

-90

-120

100

200

300

400

500

600

800

-95

-100

-105

-110

-115

700

-85

round

32 f









=−

∆

1024 f

/ f

sin x

()

πα

∆∆

round

32 4.8671e

52e

4.8671e

3 52e

7.9KHz

∗









=− ∗

∆

1024

7.9e / 52e

0.489

100e

sin .489

.489

96e

80dBc

-6

=∗

()

== −

∆

Input Frequency (MHz)

800

700

600

500

200

100

400

300

CLC5958PCASM Datenblatt(PDF) 9 Page - National Semiconductor (TI)

CLC5958PCASM Datenblatt(HTML) 9 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13

Datenblatt Download

Link URL