LTC2365/LTC2366
19
23656fb
For more information www.linear.com/LTC2365
applications inForMation
Figure 18. RC Input Filter
Input Filtering and Source Impedance
The noise and the distortion of the input amplifier and other
circuitry must be considered since they will add to the
LTC2365/LTC2366 noise and distortion. The small-signal
bandwidth of the sample-and-hold circuit is 50MHz. Any
noise or distortion products that are present at the analog
inputs will be summed over this entire bandwidth. Noisy
input circuitry should be filtered prior to the analog inputs
to minimize noise. A simple 1-pole RC filter is sufficient for
many applications. For example, Figure 18 shows a 47pF
capacitor from A
IN
to ground and a 51Ω source resistor to
limit the input bandwidth to 47MHz. The 47pF capacitor also
acts as a charge reservoir for the input sample-and-hold
and isolates the ADC input from sampling-glitch sensitive
circuitry. High quality capacitors and resistors should be
used since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
are much less susceptible to both problems. When high
amplitude unwanted signals are close in frequency to the
desired signal frequency, a multiple pole filter is required.
High external source resistance, combined with the 20pF of
input capacitance, will reduce the rated 50MHz bandwidth
and increase acquisition time beyond 56ns.
Reference Input
On the TS8 package of the LTC2365/LTC2366, the voltage
on the V
REF
pin defines the full-scale range of the ADC.
The reference voltage can range from V
DD
down to 1.4V.
Input Range
The analog input of the LTC2365/LTC2366 is driven
single-ended with respect to GND from a single supply.
The input may swing up to V
DD
for the S6 package and
to V
REF
for the TS8 package. The 0V to 2.5V range is also
ideally suited for single-ended input use with V
DD
or V
REF
= 2.5V for single supply applications. If the difference
between the A
IN
input and GND exceeds V
DD
for the S6
package or V
REF
for the TS8 package, the output code will
stay fixed at all ones, and if this difference goes below 0V,
the output code will stay fixed at all zeros.
Figure 19 shows the ideal input/output characteristics for
the LTC2365/LTC2366. The code transitions occur mid-
way between successive integer LSB values (i.e. 0.5LSB,
1.5LSB, 2.5LSB, …, FS –1.5LSB). The output code is
straight binary with 1LSB = V
DD
/4096 for the S6 package
and 1LSB = V
REF
/4096 for the TS8 package.
V
DD
GND
LTC2366
23656 F18
10µF
47pF
51Ω
6
5
4
1
2
3
CS
SD0
SCK
A
IN
Figure 19. LTC2365/LTC2366 Transfer Characteristics
INPUT VOLTAGE (V)
UNIPOLAR OUTPUT CODE
23656 F19
111...111
111...110
000...000
000...001
FS – 1LSB1LSB0
