LTC2264-14_15 Datasheet, PDF (18/32 Pages) Linear Technology – 14-Bit, 65Msps/40Msps/ 25Msps Low Power Dual ADCs

LTC2265-14/
LTC2264-14/LTC2263-14
APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2265-14/LTC2264-14/LTC2263-14 are low power,
2-channel, 14-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. To minimize
the number of data lines, the digital outputs are serial LVDS.
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 differentially
around a common mode voltage set by the V_CM1 or V_CM2
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 two 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 switching
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
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
tap is biased with V_CM, setting the A/D input at its opti-
mal DC level. At higher input frequencies a transmission
line balun transformer (Figures 4 to 6) has better balance,
resulting in lower A/D distortion.
LTC2265-14
V_DD
10Ω
A_IN+
V_DD
10Ω
A_IN–
V_DD
C_PARASITIC
1.8pF
C_PARASITIC
1.8pF
R_ON
C_SAMPLE
3.5pF
25Ω
C_SAMPLE
R_{ON 3.5pF}
25Ω
ENC⁺
1.2V
10k
ENC^–
10k
1.2V
226514 F02
Figure 2. Equivalent Input Circuit. Only One of
the Two Analog Channels Is Shown.
50Ω
V_CM
ANALOG
INPUT
0.1µF T1
1:1
25Ω
25Ω 0.1µF
25Ω _25Ω
0.1µF
A_IN+
LTC2265-14
12pF
A_IN–
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
226514 F03
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
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