Datasheet
LTC2481
25
2481fc
APPLICATIONS INFORMATION
the REF
+
and REF
–
pins. For 50Hz mode, the related differ-
ence resistance is 1.2M and the resulting full-scale error
is 0.42ppm 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
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 16-19.
In addition to this gain error, the converter INL per-
formance is degraded by the reference source imped-
ance. The INL is caused by the input dependent terms
–V
IN
2
/(V
REF
• R
EQ
) – (0.5 • V
REF
• D
T
)/R
EQ
in the reference
Figure 16. +FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
REF
)
Figure 17. –FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
REF
)
Figure 18. +FS Error vs R
SOURCE
at REF
+
or REF
–
(Large C
REF
)
Figure 19. –FS Error vs R
SOURCE
at REF
+
or REF
–
(Large C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2481 F18
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
R
SOURCE
()
0
+FS ERROR (ppm)
50
70
90
10k
2481 F16
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
R
SOURCE
()
0
–FS ERROR (ppm)
–200
–100
0
800
2481 F19
–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
R
SOURCE
()
0
–FS ERROR (ppm)
–30
–10
10
10k
2481 F17
–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
pin current as expressed in Figure 12. When using internal
oscillator and 60Hz mode, every 100 of reference source
resistance translates into about 0.67ppm additional INL
error. When using internal oscillator and 50Hz/60Hz mode,
every 100 of reference source resistance translates into
about 0.61ppm additional INL error. When using internal
oscillator and 50Hz mode, every 100 of reference source
resistance translates into about 0.56ppm additional INL
error. When CA0/f
0
is driven by an external oscillator
with a frequency f
EOSC
, every 100 of source resistance
driving REF
+
or REF
–
translates into about 2.18 • 10
–6
•
f
EOSC
ppm additional INL error. Figure 20 shows the typical
INL error due to the source resistance driving the REF
+
or REF
–
pins when large C
REF
values are used. The user
is advised to minimize the source impedance driving the
REF
+
and REF
–
pins.