Datasheet

LTC2485
23
2485fc
APPLICATIONS INFORMATION
voltage). Table 6 summarizes the effects of mismatched
source impedance and differences in reference/input
common mode voltages.
Table 6. Suggested Input Confi guration for LTC2485
BALANCED INPUT
RESISTANCES
UNBALANCED INPUT
RESISTANCES
Constant
V
IN(CM)
– V
REF(CM)
C
EXT
> 1nF at Both
IN
+
and IN
–
. Can
Take Large Source
Resistance with
Negligible Error
C
EXT
> 1nF at Both IN
+
and
IN
–
. Can Take Large Source
Resistance. Unbalanced
Resistance Results in an Offset
Which Can be Calibrated
Varying
V
IN(CM)
– V
REF(CM)
C
EXT
> 1nF at Both
IN
+
and IN
–
. Can
Take Large Source
Resistance with
Negligible Error
Minimize IN
+
and IN
–
Capacitors and Avoid Large
Source Impedance
(<5k Recommended)
The magnitude of the dynamic input current depends upon
the size of the very stable internal sampling capacitors and
upon the accuracy of the converter sampling clock. The
accuracy of the internal clock over the entire temperature
and power supply range is typically better than 0.5%. Such
a specifi cation can also be easily achieved by an external
clock. When relatively stable resistors (50ppm/°C) are
used for the external source impedance seen by IN
+
and
IN
–
, the expected drift of the dynamic current and offset
will be insignifi cant (about 1% of their respective values
over the entire temperature and voltage range). Even for
the most stringent applications, a one-time calibration
operation may be suffi cient.
In addition to the input sampling charge, the input ESD
protection diodes have a temperature dependent leakage
current. This current, nominally 1nA (±10nA max), results
in a small offset shift. A 1k source resistance will create a
1µV typical and 10µV maximum offset voltage.
Figure 13. An RC Network at IN
+
and IN
–
Figure 14. +FS Error vs R
SOURCE
at IN
+
and IN
–
Figure 15. –FS Error vs R
SOURCE
at IN
+
and IN
–
C
EXT
2485 F13
V
INCM
+ 0.5V
IN
R
SOURCE
IN
+
LTC2485
C
PAR
20pF
C
EXT
V
INCM
– 0.5V
IN
R
SOURCE
IN
–
C
PAR
20pF
R
SOURCE
()
1
+FS ERROR (ppm)
–20
0
20
1k
100k
2485 F14
–40
–60
–80
10 100 10k
40
60
80
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
T
A
= 25°C
C
EXT
= 0pF
C
EXT
= 100pF
C
EXT
= 1nF, 0.1µF, 1µF
R
SOURCE
()
1
–FS ERROR (ppm)
–20
0
20
1k
100k
2485 F15
–40
–60
–80
10 100 10k
40
60
80
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
T
A
= 25°C
C
EXT
= 0pF
C
EXT
= 100pF
C
EXT
= 1nF, 0.1µF, 1µF