Linear Technology LTC2170-12, Manual, Page: 20 / 34

LTC2172-12/
LTC2171-12/LTC2170-12
20
21721012fb
CONVERTER OPERATION
The LTC2172-12/LTC2171-12/LTC2170-12 are low power,
4-channel, 12-bit, 65Msps/40Msps/25Msps A/D convert-
ers 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 performance,
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) or one bit at a time (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
applicaTions inForMaTion
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 a 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 lowpass filter right
at the analog inputs. This lowpass filter isolates the
drive circuitry from the A/D sample-and-hold switch-
ing and 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 ap-
plication’s input frequency.
Figure 2. Equivalent Input Circuit. Only One of
the Four Analog Channels Is Shown.
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
C
SAMPLE
3.5pF
R
ON
25Ω
R
ON
25Ω
V
DD
V
DD
LTC2172-12
A
IN
+
217212 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
25Ω
25Ω
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2172-12
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
217212 F03