Datasheet

19
21754312fa
LTC2175-12/
LTC2174-12/LTC2173-12
applicaTions inForMaTion
CONVERTER OPERATION
The LTC2175-12/LTC2174-12/LTC2173-12 are low power,
4-channel, 12-bit, 125Msps/105Msps/80Msps A/D con-
verters 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 differen-
tially around a common mode voltage set by the V
CM12
or V
CM34
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 four 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
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
Figure 2. Equivalent Input Circuit. Only One of the Four
Analog Channels Is Shown.
C
SAMPLE
3.5pF
R
ON
25Ω
R
ON
25Ω
V
DD
V
DD
LTC2175-12
A
IN
+
217512 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
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
LTC2175-12
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
217512 F03