kad2708c Kenet Inc., kad2708c Datasheet - Page 13

• Manufacturer Part Number: kad2708c
• Description: 8-bit, 275msps Analog-to-digital Converter
• Manufacturer: Kenet Inc.

KAD2708C 8-Bit, 275MSPS Analog-to-Digital Converter
The value of the termination resistor should be deter-
mined based on the desired impedance. The differ-
ential input impedance of the KAD2708 is 10MΩ.
A differential amplifier can be used in applications
that require dc coupling, at the expense of reduced
dynamic performance. In this configuration the am-
plifier will typically reduce the achievable SNR and
distortion performance. A typical differential amplifier
configuration is shown in Figure 20.
Clock Input
The clock input circuit is a differential pair (see Figure
24). Driving these inputs with a high level (up to 1.8V
on each input) sine or square wave will provide the
lowest jitter performance. The recommended drive
circuit is shown in Figure 21. The clock inputs can be
driven single-ended, but this is not recommended as
performance will suffer.
The CLKDIV pin is a 1.8V CMOS control pin (input)
that selects whether the input clock frequency is
passed directly to the ADC or divided by two. Apply-
ing a low level will divide by two; 1.8V applied (or left
floating) will not divide.
Use of the clock divider is optional. The KAD2708C's
ADC requires a clock with 50% duty cycle for opti-
mum performance. If such a clock is not available,
one option is to generate twice the desired sampling
Rev 1.1
Figure 21. Recommended Clock drive
Figure 20. Differential Amplifier Input
PP
rate, then use the KAD2708C's divide-by-2 to gener-
ate a 50%-duty-cycle clock. The divider only uses the
rising edge of the clock, so 50% clock duty cycle is
assured .
Jitter
In a sampled data system, clock jitter directly im-
pacts the achievable SNR performance. The theoreti-
cal relationship between clock jitter and maximum
SNR is shown in Equation 1 and is illustrated in Figure
22.
This relationship shows the SNR that would be
achieved if clock jitter were the only non-ideal fac-
tor. In reality, achievable SNR is limited by internal
factors such as differential nonlinearity, aperture jitter
and thermal noise.
Any internal aperture jitter combines with the input
clock jitter, in a root-sum-square fashion since they
are not statistically correlated, and this determines
the total jitter in the system. The total jitter, combined
with other noise sources, then determines the achiev-
able SNR.
Where tj is the RMS uncertainty in the sampling instant.
100
95
90
85
80
75
70
65
60
55
50
1
SNR
Figure 22. SNR vs. Clock Jitter
Table 3. CLKDIV Pin Settings
tj=100ps
CLKDIV Pin
AVDD
=
AVSS
20
Equation 1.
10
log
Input Frequency - MHz
tj=10ps
10
Divide Ratio
⎛
⎜ ⎜
⎝
tj=1ps
2
π
tj=0.1ps
2
1
1
f
100
IN
t
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