ltc2258cuj-12 Linear Technology Corporation, ltc2258cuj-12 Datasheet - Page 17

• Manufacturer Part Number: ltc2258cuj-12
• Description: 12-bit, 65/40/2 5msps Ultralow Power 1.8v Adcs
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2258-12/LTC2257-12/LTC2256-12 are low power
12-bit 65Msps/40Msps/25Msps A/D converters that are
powered by a single 1.8V supply. The analog inputs should
be driven differentially. The encode input can be driven
differentially or single-ended for lower power consump-
tion. The digital outputs can be CMOS, double data rate
CMOS (to halve the number of output lines), or double
data rate LVDS (to reduce digital noise in the system.)
Many additional features can be chosen by programming
the mode control registers through a serial SPI port. See
the Serial Programming Mode section.
ANALOG INPUT
The analog input is a differential CMOS sample-and-hold
circuit (Figure 2). The inputs should be driven differentially
around a common mode voltage set by the V
pin, which is nominally V
ENC
ENC
A
A
IN
IN
+
–
+
–
LTC2258-12
10Ω
10Ω
1.2V
1.2V
Figure 2. Equivalent Input Circuit
V
V
10k
10k
DD
DD
V
DD
DD
C
1.8pF
C
1.8pF
/2. For the 2V input range, the
PARASITIC
PARASITIC
R
25Ω
R
25Ω
ON
ON
C
C
CM
SAMPLE
SAMPLE
3.5pF
3.5pF
output
225812 F02
inputs should swing from V
should be 180° phase difference between the inputs.
INPUT DRIVE CIRCUITS
Input fi ltering
If possible, there should be an RC lowpass 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
tap is biased with V
DC level. At higher input frequencies a transmission line
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
ANALOG
INPUT
LTC2257-12/LTC2256-12
0.1μF
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
1:1
T1
CM
, setting the A/D input at its optimal
25Ω
25Ω
CM
0.1μF
50Ω
25Ω
25Ω
– 0.5V to V
LTC2258-12
0.1μF
12pF
V
A
A
CM
IN
IN
+
–
CM
LTC2258-12
+ 0.5V. There
225812 F03
17
225812p