1 5 -Bit , 1 0 0 Ms p s ADC w it h -7 7 .7 d BFS
No is e Flo o r fo r Ba s e b a n d Ap p lic a t io n s
BACK-TO-BACK DIODE
AV DV DRV
CC
CC
CC
0.1µF
T2-1T–KK81
50Ω
50Ω
INP
D0–D14
MAX1429
15
INN
0.1µF
0.01µF 0.1µF
0.01µF
CLKP
GND
CLKN
Figure 5. Transformer-Coupled Clock Input Configuration
held to 3V
differential or less. If a larger amplitude
P-P
Ap p lic a t io n s In fo rm a t io n
signal is provided (to maximize the zero-crossing slew
rate), then the diodes serve to limit the differential sig-
nal swing at the clock inputs.
Diffe re n t ia l, AC-Co u p le d Clo c k In p u t
The clock inputs to the MAX1429 are designed to be
driven with an AC-coupled differential signal, and best
p e rforma nc e is a c hie ve d und e r the s e c ond itions .
However, it is often the case that the available clock
source is single ended. Figure 5 demonstrates one
method for converting a single-ended clock signal into
a differential signal through a transformer. In this exam-
ple, the transformer turns ratio from the primary to sec-
ond a ry s id e is 1:1.414. The imp e d a nc e ra tio from
primary to secondary is the square of the turns ratio, or
1:2, so that terminating the secondary side with a 100Ω
differential resistance results in a 50Ω load looking into
the primary side of the transformer. The termination
resistor in this example comprises the series combina-
tion of two 50Ω resistors with their common node AC-
coupled to ground. Alternatively, a single 100Ω resistor
across the two inputs with no common-mode connec-
tion could be employed.
Any differential mode noise coupled to the clock inputs
translates to clock jitter and degrades the SNR perfor-
mance of the MAX1429. Any differential mode coupling
of the analog input signal into the clock inputs results in
harmonic distortion. Consequently, it is important that
the clock lines be well isolated from the analog signal
input and from the digital outputs. See the PC Board
Layout Considerations sections for more discussion on
noise coupling.
Diffe re n t ia l, AC-Co u p le d An a lo g In p u t
The analog inputs (INP and INN) are designed to be dri-
ven with a differential AC-coupled signal. It is extremely
important that these inputs be accurately balanced. Any
common-mode signal applied to these inputs degrade
even-order distortion terms. Therefore, any attempt at
driving these inputs in a single-ended fashion results in
significant even-order distortion terms.
In the example of Figure 5, the secondary side of the
transformer is coupled directly to the clock inputs.
Since the clock inputs are self-biasing, the center tap of
the transformer must be AC-coupled to ground or left
floating. If the center tap of the secondary were DC-
coupled to ground, then it would be necessary to add
blocking capacitors in series with the clock inputs.
Figure 6 presents one method for converting a single-
ended signal to a balanced differential signal using a
transformer. The primary-to-secondary turns ratio in this
example is 1:1.414. The impedance ratio is the square
of the turns ratio, so in this example, the impedance
ratio is 1:2. In order to achieve a 50Ω input impedance
at the primary side of the transformer, the secondary
side is terminated with a 112Ω differential load. This
load, in shunt with the differential input resistance of the
MAX1429, results in a 100Ω differential load on the sec-
ondary side. It is reasonable to use a larger transformer
turns ratio in order to achieve a larger signal step-up,
and this may be desirable in order to relax the drive
requirements for the circuitry driving the MAX1429.
Clock jitter is generally improved if the clock signal has
a hig h s le w ra te a t the time of its ze ro c ros s ing .
Therefore, if a sinusoidal source is used to drive the
clock inputs, it is desirable that the clock amplitude be
as large as possible to maximize the zero-crossing
slew rate. The back-to-back Schottky diodes shown in
Figure 5 are not required as long as the input signal is
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