Datasheet

LTC2415/LTC2415-1
30
sn2415 24151fs
APPLICATIO S I FOR ATIO
WUU
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Figure 25. +FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
IN
) Figure 26. –FS Error vs R
SOURCE
at REF
+
or REF
–
(Small C
IN
)
Figure 27. +FS Error vs R
SOURCE
at REF
+
and REF
–
(Large C
REF
) Figure 28. –FS Error vs R
SOURCE
at REF
+
and REF
–
(Large C
REF
)
R
SOURCE
(Ω)
1 10 100 1k 10k 100k
+FS ERROR (ppm OF V
REF
)
2415 F25
0
–10
–20
–30
–40
–50
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 5V
IN
–
= 2.5V
F
O
= GND
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.001µF
C
REF
= 100pF
C
REF
= 0pF
R
SOURCE
(Ω)
1 10 100 1k 10k 100k
–FS ERROR (ppm OF V
REF
)
2415 F26
50
40
30
20
10
0
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= GND
IN
–
= 2.5V
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
100 200 300 400 500 600 700 800 9001000
+FS ERROR (ppm OF V
REF
)
2415 F27
0
–90
–180
–270
–360
–450
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 3.75V
IN
–
= 1.25V
F
O
= GND
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.1µF
C
REF
= 1µF, 10µF
R
SOURCE
(Ω)
0
100 200 300 400 500 600 700 800 9001000
–FS ERROR (ppm OF V
REF
)
2415 F28
450
360
270
180
90
0
V
CC
= 5V
REF
+
= 5V
REF
–
= GND
IN
+
= 1.25V
IN
–
= 3.75V
F
O
= GND
T
A
= 25°C
C
REF
= 0.01µF
C
REF
= 0.1µF
C
REF
= 1µF, 10µF
Reference Current
In a similar fashion, the LTC2415/LTC2415-1 sample the
differential reference pins REF
+
and REF
–
transferring
small amount of charge to and from the external driving
circuits thus producing a dynamic reference current. This
current does not change the converter offset, but it may
degrade the gain and INL performance. The effect of this
current can be analyzed in the same two distinct situa-
tions.
For relatively small values of the external reference capaci-
tors (C
REF
< 0.01µF), the voltage on the sampling capacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for C
REF
will deteriorate the converter offset and
gain performance without significant benefits of reference
filtering and the user is advised to avoid them.
Larger values of reference capacitors (C
REF
> 0.01µF) 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 con-
stant reference differential impedance. For the LTC2415,
when F
O
= LOW (internal oscillator and 60Hz notch), the
typical differential reference resistance is 1.3MΩ which
will generate a gain error of approximately 0.38ppm for
each ohm of source resistance driving REF
+
or REF
–
.
When F
O
= HIGH (internal oscillator and 50Hz notch), the
typical differential reference resistance is 1.56MΩ which
will generate a gain error of approximately 0.32ppm for
each ohm of source resistance driving REF
+
or REF
–
. For
the LTC2415-1, the typical differential reference resis-