LTC2209 Linear Technology Corporation, LTC2209 Datasheet - Page 19

• Manufacturer Part Number: LTC2209
• Description: 160Msps ADC
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
input change is large, such as the change seen with input
frequencies near Nyquist, then a larger charging glitch
will be seen.
Common Mode Bias
The ADC sample-and-hold circuit requires differential
drive to achieve specifi ed performance. Each input should
swing ±0.5625V for the 2.25V range (PGA = 0) or ±0.375V
for the 1.5V range (PGA = 1), around a common mode
voltage of 1.25V. The V
to provide the common mode bias level. V
directly to the center tap of a transformer to set the DC
input level or as a reference level to an op amp differential
driver circuit. The V
close to the ADC with 2.2µF or greater.
Input Drive Impedance
As with all high performance, high speed ADCs the dy-
namic performance of the LTC2209 can be infl uenced
by the input drive circuitry, particularly the second and
third harmonics. Source impedance and input reactance
can infl uence SFDR. At the falling edge of ENC the
sample and hold circuit will connect the 4.6pF sampling
capacitor to the input pin and start the sampling period.
The sampling period ends when ENC rises, holding the
sampled input on the sampling capacitor. Ideally, the
input circuitry should be fast enough to fully charge
the sampling capacitor during the sampling period
1/(2F encode); however, this is not always possible and the
incomplete settling may degrade the SFDR. The sampling
glitch has been designed to be as linear as possible to
minimize the effects of incomplete settling.
INPUT DRIVE CIRCUITS
Input Filtering
A fi rst order RC low pass fi lter at the input of the ADC can
serve two functions: limit the noise from input circuitry and
provide isolation from ADC S/H switching. The LTC2209
has a very broadband S/H circuit, DC to 700MHz; it can
be used in a wide range of applications; therefore, it is not
possible to provide a single recommended RC fi lter.
CM
CM
pin must be bypassed to ground
output pin (Pin 3) is designed
CM
can be tied
Figures 3, 4a and 4b show three examples of input RC
fi ltering at three ranges of input frequencies. In general
it is desirable to make the capacitors as large as can be
tolerated—this will help suppress random noise as well
as noise coupled from the digital circuitry. The LTC2209
does not require any input fi lter to achieve data sheet
specifi cations; however, no fi ltering will put more stringent
noise requirements on the input drive circuitry.
Transformer Coupled Circuits
Figure 3 shows the LTC2209 being driven by an RF trans-
former with a center-tapped secondary. The secondary
center tap is DC biased with V
signal at its optimum DC level. Figure 3 shows a 1:1 turns
ratio transformer. Other turns ratios can be used; however,
as the turns ratio increases so does the impedance seen by
the ADC. Source impedance greater than 50Ω can reduce
the input bandwidth and increase high frequency distor-
tion. A disadvantage of using a transformer is the loss of
low frequency response. Most small RF transformers have
poor performance at frequencies below 1MHz.
Center-tapped transformers provide a convenient means
of DC biasing the secondary; however, they often show
poor balance at high input frequencies, resulting in large
2nd order harmonics.
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
Figure 3. Single-Ended to Differential Conversion
Using a Transformer. Recommended for Input
Frequencies from 5MHz to 100MHz
T1
0.1µF
10Ω
10Ω
5Ω
35Ω
35Ω
8.2pF
2.2µF
CM
, setting the ADC input
8.2pF
8.2pF
5Ω
5Ω
LTC2209
V
A
A
CM
IN
IN
+
–
LTC2209
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