Datasheet

LTC2451
14
2451fg
There are some immediate trade-offs in R
S
and C
IN
without
needing a full circuit analysis. Increasing R
S
and C
IN
can
provide the following benefits:
1. Due to the LTC2451’s input sampling algorithm, the
input current drawn by the input pin (IN) over a con-
version cycle is 50nA. A high R
S
• C
IN
attenuates the
high frequency components of the input current, and
R
S
values up to 1k result in <1LSB additional INL.
2. The bandwidth from V
SIG
is reduced at the input pin.
This bandwidth reduction isolates the ADC from high
frequency signals, and as such provides simple anti-
aliasing and input noise reduction.
3. Switching transients generated by the ADC are attenu-
ated before they go back to the signal source.
4. A large C
IN
gives a better AC ground at the input pin,
helping reduce reflections back to the signal source.
5. Increasing R
S
protects the ADC by limiting the current
during an outside-the-rails fault condition.
There is a limit to how large R
S
• C
IN
should be for a given
application. Increasing R
S
beyond a given point increases
Figure 12. Measured INL vs Input Voltage,
C
IN
= 0.1µF, V
CC
= 5V, T
A
= 25°C
the voltage drop across R
S
due to the input current,
to the point that significant measurement errors exist.
Additionally, for some applications, increasing the R
S
• C
IN
product too much may unacceptably attenuate the signal
at frequencies of interest.
For most applications, it is desirable to implement C
IN
as a high quality 0.1µF ceramic capacitor and R
S
≤ 1k.
This capacitor should be located as close as possible to
the input pin. Furthermore, the area encompassed 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
S
and place series
resistors in the ADC input line and in the sensor ground
return line, which should be tied to the ADC GND pin using
a star connection topology.
Figure 12 shows the measured LTC2451 INL versus the
input voltage as a function of R
S
value with an input ca-
pacitor C
IN
= 0.1µF.
In some cases, R
S
can be increased above these guide-
lines. The input current is negligible when the ADC is
APPLICATIONS INFORMATION
INPUT VOLTAGE (V)
0
INL(LSB)
–4
0
4
3
5
2451 F12
–8
–12
–16
1 2 4
8
12
16
R
S
= 10k
R
S
= 1k
R
S
= 0