Datasheet

LTC2142-12/
LTC2141-12/LTC2140-12
20
21421012fa
CONVERTER OPERATION
The LTC2142-12/LTC2141-12/LTC2140-12 are low
power, 2-channel, 12-bit, 65Msps/40Msps/25Msps 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 consumption. 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.
ANALOG INPUT
The analog inputs are differential CMOS sample-and-hold
circuits (Figure 2). The inputs should be driven differen-
tially around a common mode voltage set by the V
CM1
or
V
CM2
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 two channels are simultaneously sampled by a shared
encode circuit (Figure 2).
Single-Ended Input
For applications less sensitive to harmonic distortion, the
A
IN
+
input can be driven single-ended with a 1V
P-P
signal
centered around V
CM
. The A
IN
–
input should be connected
to V
CM
. With a single-ended input the harmonic distortion
and INL will degrade, but the noise and DNL will remain
unchanged.
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 opti-
mal DC level. At higher input frequencies, a transmission
line balun transformer (Figure 4 to Figure 6) has better
balance, resulting in lower A/D distortion.
C
SAMPLE
5pF
R
ON
15
R
ON
15
V
DD
V
DD
LTC2142-12
A
IN
+
21421012 F02
C
SAMPLE
5pF
V
DD
A
IN
–
ENC
–
ENC
+
1.2V
10k
1.2V
10k
C
PARASITIC
1.8pF
C
PARASITIC
1.8pF
10
10Ω
25
25
25
25
50
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2142-12
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
21421012 F03
Figure 2. Equivalent Input Circuit. Only One of the Two
Analog Channels Is Shown
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
APPLICATIONS INFORMATION