Datasheet

LTC2433-1
21
24331fa
by Figures 14 and 15. For simplicity, two distinct situa-
tions can be considered.
For relatively small values of input capacitance (C
IN
<
0.01µF), the voltage on the sampling capacitor settles
almost completely and relatively large values for the
source impedance result in only small errors. Such values
for C
IN
will deteriorate the converter offset and gain
performance without significant benefits of signal filtering
and the user is advised to avoid them. Nevertheless, when
small values of C
IN
are unavoidably present as parasitics
of input multiplexers, wires, connectors or sensors, the
LTC2433-1 can maintain its accuracy while operating with
relative large values of source resistance as shown in
Figure 16. +FS Error vs R
SOURCE
at IN
+
or IN
–
(Large C
IN
)
Figure 17. –FS Error vs R
SOURCE
at IN
+
or IN
–
(Large C
IN
)
R
SOURCE
(Ω)
0
100 200 300 400 500 600 700 800 9001000
+FS ERROR (LSB)
24331 F16
8
4
0
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 3.75V
IN
–
= 1.25V
F
O
= GND
T
A
= 25°C
C
IN
= 1µF
C
IN
= 0.1µF
C
IN
= 0.01µF
C
IN
= 10µF
R
SOURCE
(Ω)
0
100 200 300 400 500 600 700 800 9001000
–FS ERROR (ppm OF V
REF
)
24331 F17
–8
–4
0
C
IN
= 0.1µF
C
IN
= 0.01µF
C
IN
= 10µF
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 1.25V
IN
–
= 3.75V
F
O
= GND
T
A
= 25°C
C
IN
= 1µF
Figure 15. –FS Error vs R
SOURCE
at IN
+
or IN
–
(Small C
IN
)
R
SOURCE
(Ω)
1 10 100 1k 10k 100k
–FS ERROR (LSB)
24331 F15
0
–1
–2
–3
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= GND
IN
–
= 2.5V
F
O
= GND
T
A
= 25°C
C
IN
= 0pF
C
IN
= 0.001µF
C
IN
= 100pF
C
IN
= 0.01µF
Figures 14 and 15. These measured results may be slightly
different from the first order approximation suggested
earlier because they include the effect of the actual second
order input network together with the nonlinear settling
process of the input amplifiers. For small C
IN
values, the
settling on IN
+
and IN
–
occurs almost independently and
there is little benefit in trying to match the source imped-
ance for the two pins.
Larger values of input capacitors (C
IN
> 0.01µF) may be
required in certain configurations for antialiasing or gen-
eral input signal filtering. Such capacitors will average the
input sampling charge and the external source resistance
will see a quasi constant input differential impedance.
When F
O
= LOW (internal oscillator and 50Hz/60Hz notch),
the typical differential input resistance is 6MΩ which will
generate a gain error of approximately 1LSB at full scale
for each 180Ω of source resistance driving IN
+
or IN
–
.
When F
O
is driven by an external oscillator with a fre-
quency f
EOSC
(external conversion clock operation), the
typical differential input resistance is 0.84 • 10
12
/f
EOSC
Ω
and each ohm of source resistance driving IN
+
or IN
–
will
result in 3.7 • 10
–8
• f
EOSC
LSB gain error at full scale. The
effect of the source resistance on the two input pins is
additive with respect to this gain error. The typical +FS and
–FS errors as a function of the sum of the source resis-
tance seen by IN
+
and IN
–
for large values of C
IN
are shown
in Figures 16 and 17.
APPLICATIO S I FOR ATIO
WUUU