ltc2182 Linear Technology Corporation, ltc2182 Datasheet - Page 19

• Manufacturer Part Number: ltc2182
• Description: 16-bit, 65msps/ 40msps/25msps Low Power Dual Adcs
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2182/LTC2181/LTC2180 are low power, two-
channel, 16-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, or single ended for lower
power consumption. 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.
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
V
input range, the inputs should swing from V
to V
between the inputs.
CM2
ENC
ENC
A
A
IN
IN
+
–
+
–
CM
Figure 2. Equivalent Input Circuit. Only One of the Two
Analog Channels Is Shown
output pins, which are nominally V
LTC2182
+ 0.5V. There should be 180° phase difference
10
10Ω
1.2V
1.2V
10k
10k
V
V
DD
DD
V
DD
C
1.8pF
C
1.8pF
PARASITIC
PARASITIC
DD
15
15
R
R
ON
ON
/2. For the 2V
C
C
SAMPLE
SAMPLE
CM
5pF
5pF
218210 F02
– 0.5V
CM1
or
The two channels are simultaneously sampled by a shared
encode circuit (Figure 2).
Single-Ended Input
For applications less sensitive to harmonic distortion, the
A
centered around V
to V
to 2.2µF . With a single-ended input the harmonic distortion
and INL will degrade, but the noise and DNL will remain
unchanged.
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
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
tap is biased with V
DC level. At higher input frequencies a transmission line
balun transformer (Figure 4 to Figure 6) has better balance,
resulting in lower A/D distortion.
LTC2182/LTC2181/LTC2180
IN
ANALOG
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz
+
CM
INPUT
input can be driven single-ended with a 1V
and the V
0.1µF
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
1:1
T1
CM
CM
CM
bypass capacitor should be increased
. The A
, setting the A/D input at its optimal
25
25
0.1µF
IN
50
25
25
–
input should be connected
0.1µF
12pF
V
A
A
CM
IN
IN
+
–
LTC2182
P-P
218210
19
F03
signal
218210f