LTC1865 LINER [Linear Technology], LTC1865 Datasheet - Page 15

• Manufacturer Part Number: LTC1865
• Description: Ultra-Tiny, Differential, 16-Bit ADC with SPI Interface SPI Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The LT6660 reference is an ideal match for driving the
LTC2452’s REF pin. The LTC6660 is available in a 2mm
× 2mm DFN package with 2.5V, 3V, 3.3V and 5V options.
A 0.1µF , high quality, ceramic capacitor in parallel with a
10µF ceramic capacitor should be connected between the
REF and GND pins, as close as possible to the package. The
0.1µF capacitor should be placed closest to the ADC.
Driving V
The input drive requirements can best be analyzed using
the equivalent circuit of Figure 16. The input signal V
connected to the ADC input pins (IN
equivalent source resistance R
the actual generator source resistance and any additional
optional resistors connected to the input pins. Optional
input capacitors C
pins. This capacitor is placed in parallel with the ADC
input parasitic capacitance C
layout, C
addition, the equivalent circuit of Figure 16 includes the
converter equivalent internal resistor R
capacitor C
SIG
SIG
+
–
+
–
+
–
Figure 16. LTC2452 Input Drive Equivalent Circuit
R
R
PAR
S
S
IN
+
EQ
has typical values between 2pF and 15pF . In
and V
.
C
C
IN
IN
IN
IN
C
C
PAR
PAR
are also connected to the ADC input
–
IN
IN
+
–
I
I
I
I
LEAK
LEAK
LEAK
LEAK
PAR
S
. This resistor includes both
V
V
CC
CC
. Depending on the PCB
+
(TYP)
(TYP)
R
R
15k
15k
0.35pF
0.35pF
SW
SW
and IN
(TYP)
(TYP)
C
C
EQ
EQ
SW
and sampling
–
) through an
2452 F16
I
I
CONV
CONV
SIG
is
There are some immediate trade-offs in R
needing a full circuit analysis. Increasing R
give the following benefits:
1) Due to the LTC2452’s input sampling algorithm, the input
2) The bandwidth from V
3) Switching transients generated by the ADC are attenu-
4) A large C
5) Increasing R
There is a limit to how large R
application. Increasing R
the voltage drop across R
to the point that significant measurement errors exist.
Additionally, for some applications, increasing the R
product too much may unacceptably attenuate the signal
at frequencies of interest.
For most applications, it is desirable to implement C
a high-quality 0.1µF ceramic capacitor and R
capacitor should be located as close as possible to the
actual V
by this circuit path, as well as the path length, should be
minimized.
In the case of a 2-wire sensor that is not remotely
grounded, it is desirable to split R
resistors in the ADC input line as well as in the sensor
ground return line, which should be tied to the ADC GND
pin using a star connection topology.
current drawn by either V
cycle is typically 50nA. A high R
high frequency components of the input current, and
R
(IN
from high frequency signals, and as such provides
simple antialiasing and input noise reduction.
ated before they go back to the signal source.
helping reduce reflections back to the signal source.
during an outside-the-rails fault condition.
S
+
values up to 1k result in <1LSB error.
, IN
IN
package pin. Furthermore, the area encompassed
–
). This bandwidth reduction isolates the ADC
IN
gives a better AC ground at the input pins,
S
protects the ADC by limiting the current
S
SIG
beyond a given point increases
IN
S
is reduced at the input pins
S
+
due to the input current,
• C
or V
IN
S
IN
should be for a given
• C
S
–
over a conversion
LTC2452
and place series
IN
S
and C
attenuates the
S
and C
S
≤ 1k. This
IN
without
15
IN
S
IN
• C
2452fc
can
as
IN