13
2271f
LTC2271
APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2271 is a low power, 2-channel, 16-bit, 20Msps
A/D converter that is powered by a single 1.8V supply. The
analog inputs must be driven differentially. The encode
input can be driven differentially or single ended for lower
power consumption. To minimize the number of data lines
the digital outputs are serial LVDS. Each channel outputs
one bit at a time (1-lane mode), two bits at a time (2-lane
mode) or four bits at a time (4-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 2.1V input range, the inputs should swing from
V
CM
– 525mV to V
CM
+ 525mV. There should be 180°
phase difference between the inputs.
C
SAMPLE
17pF
R
ON
24
R
ON
24
V
DD
V
DD
LTC2271
A
IN
+
2271 F02
C
SAMPLE
17pF
V
DD
A
IN
–
ENC
–
ENC
+
1.2V
10k
1.2V
10k
C
PARASITIC
1.8pF
C
PARASITIC
1.8pF
10
10Ω
Figure 2. Equivalent Input Circuit. Only One of Two Analog Channels Is Shown
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 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
CM
, setting the A/D input at its optimal
DC level. At higher input frequencies a transmission line
balun transformer (Figures 4 to 5) has better balance,
resulting in lower A/D distortion.
