Datasheet

LTC2483
20
2483fc
gain performance without significant benefits of reference
filtering and the user is advised to avoid them.
Larger values of reference capacitors (C
REF
> 1nF) may be
required as reference filters in certain configurations. Such
capacitors will average the reference sampling charge and
the external source resistance will see a quasi constant
reference differential impedance.
In the following discussion, it is assumed the input and
reference common mode are the same. For the internal
oscillator, the related difference resistance is 1.1MΩ and
applicaTions inFormaTion
Figure 11. +FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2483 F11
30
10
40
60
80
20
0
–10
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.001µF
C
REF
= 100pF
C
REF
= 0pF
the resulting full-scale error is 0.46ppm for each ohm of
source resistance driving the REF
+
and REF
–
pins. When
CA0/F
0
is driven by an external oscillator with a frequency
f
EOSC
(external conversion clock operation), the typical
differential reference resistance is 0.30 • 10
12
/f
EOSC
Ω
and each ohm of source resistance driving the REF
+
or
REF
–
pins will result in 1.67 • 10
–6
• f
EOSC
ppm gain error.
The typical +FS and –FS errors for various combinations
of source resistance seen by the REF
+
or REF
–
pins and
external capacitance connected to that pin are shown in
Figures 11-14.
Figure 12. –FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
REF
)
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–30
–10
10
10k
2483 F12
–50
–70
–40
–20
0
–60
–80
–90
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.001µF
C
REF
= 100pF
C
REF
= 0pF
Figure 13. +FS Error vs R
SOURCE
at REF
+
or REF
–
(Large C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2483 F13
200
100
0
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF
Figure 14. –FS Error vs R
SOURCE
at REF
+
or REF
–
(Large C
REF
)
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–200
–100
0
800
2483 F14
–300
–400
–500
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
T
A
= 25°C
C
REF
= 1µF, 10µF
C
REF
= 0.1µF
C
REF
= 0.01µF