Datasheet

LTC2261-12
LTC2260-12/LTC2259-12
18
226112fc
For more information www.linear.com/LTC2261-12
CONVERTER OPERATION
The LTC2261-12/LTC2260-12/LTC2259-12 are low power
12-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 or single ended for lower power consump
-
tion. The digital outputs can be CMOS, double-data rate
CMOS (to halve the number of output lines), or double-data
rate LVDS (to reduce digital noise in the system.) Many
additional features can be chosen by programming the
mode control registers through a serial SPI port. See the
Serial Programming Mode section.
ANALOG INPUT
The analog input is a differential CMOS sample-and-hold
circuit (Figure 2). The inputs should be driven differentially
around a common mode voltage set by the V
CM
output
pin, which is 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.
INPUT DRIVE CIRCUITS
Input Filtering
If possible, there should be an RC lowpass filter right at
the analog inputs. This lowpass filter isolates the drive
circuitry 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 optimal
DC level. At higher input frequencies a transmission line
C
SAMPLE
3.5pF
R
ON
25Ω
R
ON
25Ω
V
DD
V
DD
LTC2261-12
A
IN
+
226112 F02
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
25Ω
25Ω
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2261-12
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
226112 F03
Figure 2. Equivalent Input Circuit
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
applicaTions inForMaTion