Datasheet
17
21576512f
LTC2157-12/
LTC2156-12/LTC2155-12
CONVERTER OPERATION
The LTC2157-12/LTC2156-12/LTC2155-12 are two-
channel, 12-bit 250Msps/210Msps/170Msps A/D con-
verters that are powered by a single 1.8V supply. The
analog inputs must be driven differentially. The en-
code inputs should be driven differentially for optimal
performance. The digital outputs are double-data rate
LVDS. 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 must be driven differ-
entially around a common mode voltage set by the V
CM
output pin, which is nominally 0.435 • V
DD
. For the 1.5V
input range, the inputs should swing from V
CM
– 0.375V
to V
CM
+ 0.375V. There should be 180° phase difference
between the inputs.
The two channels are simultaneously sampled by a shared
encode circuit.
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 wide band noise from the drive circuitry. Figure 3
shows an example of an input RC filter. The RC compo-
nent values should be chosen based on the application’s
specific input frequency.
Transformer-Coupled Circuits
Figure 3 shows the analog input being driven by an RF
transformer with the common mode supplied through a
pair of resistors via the V
CM
pin.
At higher input frequencies a transmission line balun
transformer (Figures 4 and 5) has better balance, resulting
in lower A/D distortion.
Figure 2. Equivalent Input Circuit. Only One
of Two Analog Channels Is Shown
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
APPLICATIONS INFORMATION
Figure 4. Recommended Front-End Circuit for
Input Frequencies from 15MHz to 150MHz
2pF
R
ON
20Ω
R
ON
20Ω
V
DD
V
DD
LTC2157-12
A
IN
+
21576512 F02
2pF
V
DD
A
IN
–
ENC
–
ENC
+
2pF
2pF
1.2V
10k
25Ω
25Ω
4.7Ω
4.7Ω
10Ω
0.1µF
10pF
0.1µF
LTC2157-12
IN
0.1µF
T1
1:1
T1: MACOM ETC1-1T
21576512 F03
A
IN
+
A
IN
–
V
CM
45Ω
45Ω
100Ω
10Ω
4.7Ω
4.7Ω
0.1µF
0.1µF
IN
0.1µF
0.1µF
T1: MABA
007159-000000
T2: WBC1-1L
21576512 F04
LTC2157-12
A
IN
+
A
IN
–
V
CM