Datasheet

LTC2436-1
23
24361f
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. When F
O
= LOW
(internal oscillator and 50Hz/60Hz notch), the typical
differential reference resistance is 1.4M which will gen-
erate a gain error of approximately 1LSB full scale for each
40 of source resistance driving REF
+
or REF
. When F
O
is driven by an external oscillator with a frequency f
EOSC
(external conversion clock operation), the typical differen-
tial reference resistance is 0.20 • 10
12
/f
EOSC
and each
ohm of source resistance drving REF
+
or REF
will result
in 1.54 • 10
–7
• f
EOSC
LSB gain error at full scale. The effect
APPLICATIO S I FOR ATIO
WUUU
Figure 19. +FS Error vs R
SOURCE
at REF
+
or REF
(Small C
IN
) Figure 20. –FS Error vs R
SOURCE
at REF
+
or REF
(Small C
IN
)
Figure 21. +FS Error vs R
SOURCE
at REF
+
and REF
(Large C
REF
) Figure 22. –FS Error vs R
SOURCE
at REF
+
and REF
(Large C
REF
)
R
SOURCE
()
1 10 100 1k 10k 100k
+FS ERROR (LSB)
24361 F19
0
–3
–2
–1
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
()
1 10 100 1k 10k 100k
FS ERROR (LSB)
2412 F19
3
0
1
2
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
()
0
100 200 300 400 500 600 700 800 9001000
+FS ERROR (LSB)
24361 F21
0
6
11
17
22
30
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
()
0
100 200 300 400 500 600 700 800 9001000
FS ERROR (LSB)
24361 F22
30
22
17
11
6
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
of the source resistance on the two reference pins is
additive with respect to this gain error. The typical +FS and
–FS errors for various combinations of source resistance
seen by the REF
+
and REF
pins and external capacitance
C
REF
connected to these pins are shown in Figures 19, 20,
21 and␣ 22.
In addition to this gain error, the converter INL perfor-
mance is degraded by the reference source impedance.
When F
O
= LOW (internal oscillator and 50Hz/60Hz notch),
every 1000 of source resistance driving REF
+
or REF
translates into about 1LSB additional INL error. When F
O
is driven by an external oscillator with a frequency f
EOSC
,
every 100 of source resistance driving REF
+
or REF