Datasheet
LTC2482
24
2482fc
APPLICATIONS INFORMATION
resistance is 1.1MΩ and the resulting full-scale error is
0.46ppm for each ohm of source resistance driving the
V
REF
pin. When f
O
is driven by an external oscillator with
a frequency f
EOSC
(external conversion clock operation),
the typical differential reference resistance is 0.33 • 1012/
f
EOSC
Ω and each ohm of source resistance driving the V
REF
pin will result in 1.53 • 10–6 • f
EOSC
ppm gain error. The typ-
ical +FS and –FS errors for various combinations of source
resistance seen by the V
REF
pin and external capacitance
connected to that pin are shown in Figures 14-17.
In addition to this gain error, the converter INL performance
is degraded by the reference source impedance. The INL
is caused by the input dependent terms –V
IN2
/(V
REF
•
R
EQ
) – (0.5 • V
REF
• D
T
)/R
EQ
in the reference pin current
as expressed in Figure 10. When using internal oscillator
with 50Hz/60Hz rejection, every 100Ω of reference source
resistance translates into about 0.61ppm additional INL
error. When f
O
is driven by an external oscillator with a
frequency f
EOSC
, every 100Ω of source resistance driving
V
REF
translates into about 1.99 • 10
–6
• f
EOSC
ppm addi-
tional INL error. Figure 18 shows the typical INL error due
to the source resistance driving the V
REF
pin when large
C
REF
values are used. The user is advised to minimize the
source impedance driving the V
REF
pin.
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2482 F14
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
f
O
= GND
T
A
= 25°C
C
REF
= 0.01μF
C
REF
= 0.001μF
C
REF
= 100pF
C
REF
= 0pF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–30
–10
10
10k
2482 F15
–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
f
O
= GND
T
A
= 25°C
C
REF
= 0.01μF
C
REF
= 0.001μF
C
REF
= 100pF
C
REF
= 0pF
Figure 14. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
) Figure 15. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2482 F16
200
100
0
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 1μF, 10μF
C
REF
= 0.1μF
C
REF
= 0.01μF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–200
–100
0
800
2482 F17
–300
–400
–500
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 1μF, 10μF
C
REF
= 0.1μF
C
REF
= 0.01μF
Figure 16. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
) Figure 17. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)