Datasheet

LTM9011-14/
LTM9010-14/LTM9009-14
22
9009101114fa
applicaTions inForMaTion
CONVERTER OPERATION
The LTM9011-14/LTM9010-14/LTM9009-14 are low
power, 8-channel, 14-bit, 125Msps/105Msps/80Msps A/D
converters that are powered by a single 1.8V supply. The
analog inputs should be driven differentially. The encode
input can be driven differentially for optimal jitter perfor-
mance, or single-ended for lower power consumption. The
digital outputs are serial LVDS to minimize the number
of data lines. Each channel outputs two bits at a time
(2-lane mode). At lower sampling rates there is a one bit
per channel option (1-lane mode). Many additional features
can be chosen by programming the mode control registers
through a serial SPI port.
ANALOG INPUT
The analog inputs are differential CMOS sample-and-hold
circuits (Figure 2). The inputs should be driven differentially
around a common mode voltage set by the appropriate
V
CM
output pins, which are nominally V
DD
/2. For the 2V
input range, the inputs should swing from V
CM
– 0.5V
to V
CM
+ 0.5V. There should be 180° phase difference
between the inputs.
The eight channels are simultaneously sampled by a shared
encode circuit (Figure 2).
INPUT DRIVE CIRCUITS
Input Filtering
If possible, there should be an RC low pass filter right at
the analog inputs. This
lowpass filter isolates the drive
cir
cuitry from the A/D sample-and-hold switching, and also
limits wideband noise from the drive circuitry. Figure 3
shows an example of an input RC filter. The RC component
values should be chosen based on the application’s input
frequency.
Transformer Coupled Circuits
Figure 3 shows the analog input being driven by an RF
transformer with a center-tapped secondary. The center
tap is biased with V
CM
, setting the A/D input at its opti-
mal DC level. At higher input frequencies a transmission
line balun transformer (Figures 4 to 6) has better balance,
resulting in lower A/D distortion.
Figure 2. Equivalent Input Circuit. Only One
of the Eight Analog Channels Is Shown
C
SAMPLE
3.5pF
R
ON
25Ω
R
ON
25Ω
V
DD
V
DD
LTM9011-14
A
IN
+
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C
SAMPLE
3.5pF
V
DD
A
IN
–
ENC
–
ENC
+
1.2V
10k
1.2V
10k
C
PARASITIC
1.8pF
C
PARASITIC
1.8pF
10Ω
10Ω
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
25Ω
25Ω
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTM9011-14
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
9009101114 F03