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.