clc5958 National Semiconductor Corporation, clc5958 Datasheet - Page 9

• Manufacturer Part Number: clc5958
• Description: 14-bit, 52msps A/d Converter
• Manufacturer: National Semiconductor Corporation

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 f
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
frequency, f
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
For example, assume the input frequency is 4.8671MHz
and the sample rate is 52MSPS. Then the sideband
offset is derived as follows:
rolls off 2dB per dB as the input amplitude is reduced.
is the calibration sideband coefficient. The value of
x 1024 f
n round
s
is the sideband offset, f
is the sample rate, and round(
32 f
/ f
f
s
s
in
a
f
f
in
in
sin x
nf
32
is the input
x
s
s
•
/4 and f
) denotes
s
/8.
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.
The sideband magnitude is a function of the sideband
offset, as illustrated in Figure 5.
9
Figure 5: Sideband Magnitude vs. Sideband Offset
Figure 4: Sideband Offset vs. Input Frequency
x 1024
a
800
700
600
500
400
300
200
100
-100
-105
-110
-115
-120
n round
f
-80
-85
-90
-95
0
100e
0
0
4.8671e
100
7.9e / 52e
-6
5
Input Frequency (MHz)
200
3
32 4.8671e
sin .489
6
300
.489
10
52e
f (KHz)
6
3 52e
400
6
32
15
0.489
500
6
96e
6
600
20
-6
3
700
7.9KHz
25
800
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80dBc