KAD2710C Datasheet, PDF (13/16 Pages) List of Unclassifed Manufacturers – 10-Bit, 275MSPS Analog-to-Digital Converter

KAD2710C
0.01µF
Analog
In
ADT1-1WT
50OΩ
ADT1-1WT
0.1µF
KAD2710
VCM
FIGURE 23. TRANSFORMER INPUT FOR GENERAL
APPLICATIONS
Analog ^1nF
Input
1nF
ADTL1-12
ADTL1-12
0.1µF
KAD2710
VCM
FIGURE 24. TRANSMISSION-LINE TRANSFORMER INPUT
FOR HIGH IF APPLICATIONS
A back-to-back transformer scheme is used to improve
common-mode rejection, which keeps the common-mode
level of the input matched to V_CM. The value of the shunt
resistor should be determined based on the desired load
impedance.
The sample and hold circuit design uses a switched
capacitor input stage, which creates current spikes when the
sampling capacitance is reconnected to the input voltage.
This creates a disturbance at the input which must settle
before the next sampling point. Lower source impedance will
result in faster settling and improved performance. Therefore
a 1:1 transformer and low shunt resistance are
recommended for optimal performance.
A differential amplifier can be used in applications that
require dc coupling. In this configuration the amplifier will
typically determine the achievable SNR and distortion. A
typical differential amplifier circuit is shown in Figure 25.
69.8OΩ
100OΩ
Analog
Input
0.22µF
100OΩ
49.9OΩ
69.8OΩ
348OΩ
CM
348OΩ
25OΩ
217ΩO
25OΩ
0.1µF
KAD2710
VCM
FIGURE 25. DIFFERENTIAL AMPLIFIER INPUT
Clock Input
The sample clock input circuit is a differential pair (see
Figure 29). Driving these inputs with a high level (up to
1.8V_P-P on each input) sine or square wave will provide the
lowest jitter performance.
The recommended drive circuit is shown in Figure 26. The
clock can be driven single-ended, but this will reduce the
edge rate and may impact SNR performance.
1kOΩ
1kOΩ
AVDD2
1nF
Clock
1nF
Input
TC4-1W
200OΩ
CLKP
CLKN
FIGURE 26. RECOMMENDED CLOCK DRIVE
Use of the clock divider is optional. The KAD2710C's ADC
requires a clock with 50% duty cycle for optimum
performance. If such a clock is not available, one option is to
generate twice the desired sampling rate and use the
KAD2710C's divide-by-2 setting. This frequency divider uses
the rising edge of the clock, so 50% clock duty cycle is
assured. Table 2 describes the CLKDIV connection.
TABLE 2. CLKDIV PIN SETTINGS
CLKDIV PIN
DIVIDE RATIO
AVSS
2
AVDD
1
CLKDIV is internally pulled low, so a pull-up resistor or logic
driver must be connected for undivided clock.
Jitter
In a sampled data system, clock jitter directly impacts the
achievable SNR performance. The theoretical relationship
between clock jitter (t_J) and SNR is shown in Equation 1 and
is illustrated in Figure 27.
SNR = 20 log_{10⎝⎛2}----_π----_f-¹-_I--_N----_t--_J-_⎠⎞
(EQ. 1)
Where t_J is the RMS uncertainty in the sampling instant.
10 0
95
90
85
80
75
70
65
60
55
50
1
tj=0.1p s
tj=1 ps
tj=1 00p s
tj=1 0p s
10
10 0
Input Frequency - MHz
FIGURE 27. SNR vs CLOCK JITTER
1 4 Bits
1 2 Bits
10 Bits
10 00
13
FN6814.0
December 5, 2008