Datasheet
LTC2411/LTC2411-1
24
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. For
the LTC2411, when F
O
= LOW (internal oscillator and 60Hz
notch), the typical differential input resistance is 5.4MΩ
which will generate a gain error of approximately 0.093ppm
for each ohm of source resistance driving IN
+
or IN
–
.
When F
O
= HIGH (internal oscillator and 50Hz notch), the
typical differential input resistance is 6.5MΩ which will
generate a gain error of approximately 0.077ppm for each
ohm of source resistance driving IN
+
or IN
–
. For the
LTC2411-1, the typical differential input resistance is
6MΩ which will generate a gain error of approximately
0.084ppm for each ohm of source resistance driving IN
+
or IN
–
(F
O
= LOW). When F
O
is driven by an external
oscillator with a frequency f
EOSC
(external conversion
clock operation), the typical differential input resistance is
0.83 • 10
12
/f
EOSC
Ω and each ohm of source resistance
driving IN
+
or IN
–
will result in 0.59 • 10
–6
• f
EOSC
ppm gain
error. 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
resistance seen by IN
+
and IN
–
for large values of C
IN
are
shown in Figure 15.
In addition to this gain error, an offset error term may also
appear. The offset error is proportional with the mismatch
between the source impedance driving the two input pins
IN
+
and IN
–
and with the difference between the input and
reference common mode voltages. While the input drive
circuit nonzero source impedance combined with the
converter average input current will not degrade the INL
performance, indirect distortion may result from the modu-
lation of the offset error by the common mode component
of the input signal. Thus, when using large C
IN
capacitor
values, it is advisable to carefully match the source imped-
ance seen by the IN
+
and IN
–
pins. For the LTC2411, when
F
O
= LOW (internal oscillator and 60Hz notch), every 1Ω
mismatch in source impedance transforms a full-scale
common mode input signal into a differential mode input
signal of 0.093ppm. When F
O
= HIGH (internal oscillator
and 50Hz notch), every 1Ω mismatch in source imped-
ance transforms a full-scale common mode input signal
into a differential mode input signal of 0.077ppm. For the
LTC2411-1, when internal oscillator is used (F
O
= LOW),
every 1Ω mismatch in source impedance transforms a
full-scale common mode input signal into a differential
mode input signal of 0.084ppm. When F
O
is driven by an
external oscillator with a frequency f
EOSC
, every 1Ω mis-
match in source impedance transforms a full-scale com-
mon mode input signal into a differential mode input
signal of 0.59 • 10
–6
• f
EOSC
ppm. Figure 16 shows the
typical offset error due to input common mode voltage for
various values of source resistance imbalance between
the IN
+
and IN
–
pins when large C
IN
values are used.
If possible, it is desirable to operate with the input signal
common mode voltage very close to the reference signal
common mode voltage as is the case in the ratiometric
measurement of a symmetric bridge. This configuration
APPLICATIO S I FOR ATIO
WUUU
R
SOURCE
(Ω)
0
100 200 300 400 500 600 700 800 9001000
+FS ERROR (ppm OF V
REF
)
2411 F15a
120
100
80
60
40
20
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
Figure 15a. +FS Error vs R
SOURCE
at IN
+
or IN
–
(Large C
IN
)
Figure 15b. –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 (ppm OF V
REF
)
2411 F15b
–120
–100
–80
–60
–40
–20
0
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 1.25V
IN
–
= 3.75V
F
O
= GND
T
A
= 25°C
C
IN
=
1µF
C
IN
=
0.1µF
C
IN
=
0.01µF
C
IN
=
10µF