LTC2485 LINER [Linear Technology], LTC2485 Datasheet - Page 23

• Manufacturer Part Number: LTC2485
• Description: 24-Bit ?? ADC with Easy Drive Input Current Cancellation and I2C Interface
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
voltage). Table 6 summarizes the effects of mismatched
source impedance and differences in reference/input com-
mon mode voltages.
Table 6. Suggested Input Configuration for LTC2485
Constant
V
Varying
V
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 specification 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
IN
will be insignificant (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 sufficient.
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.
IN(CM)
IN(CM)
–
, the expected drift of the dynamic current and offset
– V
– V
REF(CM)
REF(CM)
BALANCED INPUT
RESISTANCES
C
IN
Large Source Resistance Source Resistance.
with Negligible Error
C
and IN
Source Resistance with
Negligible Error
EXT
EXT
+
U
and IN
> 1nF at Both
> 1nF at Both IN
–
. Can Take Large
–
. Can Take
U
+
W
UNBALANCED INPUT
RESISTANCES
C
and IN
Unbalanced Resistance
Results in an Offset
Which Can be Calibrated
Minimize IN
Capacitors and Avoid
Large Source Impedance
(< 5k Recommended)
EXT
> 1nF at Both IN
–
. Can Take Large
U
+
and IN
+
–
and
+
V
V
INCM
INCM
Figure 14. +FS Error vs R
Figure 15. –FS Error vs R
+ 0.5V
– 0.5V
Figure 13. An RC Network at IN
–20
–40
–80
–20
–40
–80
–60
–60
80
60
40
20
80
60
40
20
IN
IN
0
0
1
1
V
V
V
V
T
V
V
V
V
T
CC
REF
IN
IN
A
CC
REF
IN
IN
A
= 25°C
= 25°C
+
–
+
–
R
R
= 5V
= 5V
= 3.75V
= 1.25V
= 1.25V
= 3.75V
= 5V
= 5V
SOURCE
SOURCE
10
10
C
EXT
R
100
R
100
C
SOURCE
SOURCE
EXT
C
C
= 1nF, 0.1µF, 1µF
EXT
EXT
SOURCE
= 1nF, 0.1µF, 1µF
SOURCE
C
C
(Ω)
(Ω)
EXT
1k
1k
EXT
C
EXT
= 100pF
= 100pF
C
C
≅ 20pF
C
≅ 20pF
at IN
at IN
EXT
PAR
PAR
= 0pF
+
10k
10k
and IN
= 0pF
+
+
LTC2485
2485 F14
2485 F15
and IN
and IN
IN
IN
LTC2485
100k
100k
+
–
–
2485 F13
–
–
23
2485fa