LTC2173-14 Linear Technology Corporation, LTC2173-14 Datasheet - Page 19

• Manufacturer Part Number: LTC2173-14
• Description: (LTC217x-14) 125Msps Low Power Quad ADCs
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2175-14/LTC2174-14/LTC2173-14 are low power,
4-channel, 14-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
or V
ENC
ENC
A
A
IN
IN
Figure 2. Equivalent Input Circuit. Only One of the Four
Analog Channels is Shown.
+
–
+
–
CM34
LTC2175-14
10Ω
10Ω
output pins, which are nominally V
1.2V
1.2V
V
V
10k
10k
DD
DD
V
DD
C
1.8pF
C
1.8pF
PARASITIC
PARASITIC
25Ω
25Ω
R
R
ON
ON
DD
C
C
SAMPLE
SAMPLE
3.5pF
3.5pF
/2. For the
217514 F02
CM12
2V input range, the inputs should swing from V
to V
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 fi lter right at
the analog inputs. This lowpass fi lter 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 fi lter. 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 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz.
ANALOG
CM
INPUT
LTC2174-14/LTC2173-14
+ 0.5V. There should be 180° phase difference
0.1μF
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
1:1
T1
25Ω
25Ω
0.1μF
50Ω
25Ω
25Ω
LTC2175-14/
0.1μF
12pF
V
A
A
CM
IN
IN
+
–
LTC2175-14
CM
217514 F03
19
21754314p
– 0.5V