Datasheet

13
215814f
LTC2158-14
CONVERTER OPERATION
The LTC2158-14 is a two-channel, 14-bit 310Msps
A/D converter 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 1.32V
input range, the inputs should swing from V
CM
– 0.33V to
V
CM
+ 0.33V. 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.
2pF
R
ON
20Ω
R
ON
20Ω
V
DD
V
DD
LTC2158-14
A
IN
+
215814 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
LTC2158-14
IN
0.1µF
T1: MACOM ETC1-1T
215814 F03
A
IN
+
A
IN
–
V
CM
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
45Ω
45Ω
10Ω
4.7Ω
4.7Ω
0.1µF
0.1µF
100Ω
IN
0.1µF
0.1µF
T1: MABA
007159-000000
T2: WBC1-1L
215814 F04
LTC2158-14
A
IN
+
A
IN
–
V
CM