ltc2195 Linear Technology Corporation, ltc2195 Datasheet - Page 17

• Manufacturer Part Number: ltc2195
• Description: Ltc2195/ltc2194/ltc2193 - 16-bit, 125/105/80msps Low Power Dual Adcs
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
CONVERTER OPERATION
The LTC2195/LTC2194/LTC2193 are low power, 2-channel,
16-bit, 125/105/80Msps 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 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
four bits at a time (4-lane mode). At lower sampling rates
there is a one bit per channel option (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
For the 2V input range, the inputs should swing from
V
difference between the inputs.
The two channels are simultaneously sampled by a shared
encode circuit (Figure 2).
CM1
CM
– 0.5V to V
or V
CM2
output pins, which are nominally V
CM
+ 0.5V. There should be 180° phase
Figure 2. Equivalent Input Circuit. Only One of Two Analog Channels is Shown
ENC
ENC
A
A
IN +
IN –
+
–
LTC2195
10
10Ω
1.2V
1.2V
10k
10k
V
DD
V
DD
V
DD
DD
/2.
C
1.8pF
C
1.8pF
PARASITIC
PARASITIC
Single-Ended Input
For applications less sensitive to harmonic distortion, the
A
centered around V
to V
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
IN
+
CM
input can be driven singled ended with a 1V
. With a singled-ended input the harmonic distortion
R
15
R
15
ON
ON
C
C
SAMPLE
SAMPLE
5pF
5pF
CM
CM
219543 F02
LTC2194/LTC2193
. The A
, setting the A/D input at its optimal
IN
–
input should be connected
LTC2195
P-P
17
signal
219543p