SPT
7
8/1/00
SPT7851
CLOCK
The SPT7851 accepts a low voltage CMOS logic level at
the CLK input. The duty cycle of the clock should be kept as
close to 50% as possible. Because consecutive stages in
the ADC are clocked in opposite phase to each other, a non-
50% duty cycle reduces the settling time available for every
other stage and thus could potentially cause a degradation
of dynamic performance.
For optimal performance at high input frequencies, the clock
should have low jitter and fast edges. The rise/fall times
should be kept shorter than 2 ns. Overshoot and undershoot
should be avoided. Clock jitter causes the noise floor to rise
proportional to the input frequency. Because jitter can be
caused by crosstalk on the PC board, it is recommended
that the clock trace be kept as short as possible and stan-
dard transmission line practices be followed.
DIGITAL OUTPUTS
The digital output data appears in an offset binary code at
3.3 V CMOS logic levels. A negative full scale input results
in an all zeros output code (000…0). A positive full scale
PACKAGE OUTLINE
44L TQFP
INCHES
MILLIMETERS
SYMBOL
MIN
MAX
MIN
MAX
A
0.472 Typ
12.00 Typ
B
0.394 Typ
10.00 Typ
C
0.394 Typ
10.00 Typ
D
0.472 Typ
12.00 Typ
E
0.031 Typ
0.80 Typ
F
0.012
0.018
0.300
0.45
G
0.053
0.057
1.35
1.45
H
0.002
0.006
0.05
0.15
I
0.018
0.030
0.450
0.750
J
0.039 Typ
1.00 Typ
K
0-7
°
0-7
°
input results in an all 1’s code (111…1). The output data is
available 7.5 clock cycles after the data is sampled. The in-
put signal is sampled on the high to low transition of the
input clock. Output data should be latched on the low to high
clock transition as shown in figure 1, the Timing Diagram.
The output data is invalid for the first 20 clock cycles after
the device is powered up.
EVALUATION BOARD
The EB7851 Evaluation Board is available to aid designers
in demonstrating the full performance capability of the
SPT7851. The board includes an on-board clock driver, ad-
justable voltage references, adjustable bias current circuits,
single-to-differential input buffers with adjustable levels, a
single-to-differential transformer (1:1), digital output buffers
and 3.3/5 V adjustable logic outputs. An application note
(AN7851) is also available which describes the operation of
the evaluation board and provides an example of the recom-
mended power and ground layout and signal routing. Con-
tact the factory for price and availability.
Index
A
B
C
D
Pin 1
E
F
G
H
I
J
K
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