Datasheet
23
LTC2421/LTC2422
24212f
Input Current (V
IN
)
If complete settling occurs on the input, conversion re-
sults will be uneffected by the dynamic input current. If the
settling is incomplete, it does not degrade the linearity
performance of the device. It simply results in an offset/
full-scale shift, see Figure 17. To simplify the analysis of
input dynamic current, two separate cases are assumed:
large capacitance at V
IN
(C
IN
> 0.01µF) and small capaci-
tance at V
IN
(C
IN
< 0.01µF).
If the total capacitance at V
IN
(see Figure 18) is small
(<0.01µF), relatively large external source resistances (up
to 80k for 20pF parasitic capacitance) can be tolerated
without any offset/full-scale error. Figures 19 and 20 show
a family of offset and full-scale error curves for various
small valued input capacitors (C
IN
< 0.01µF) as a function
of input source resistance.
For large input capacitor values (C
IN
> 0.01µF), the input
spikes are averaged by the capacitor into a DC current. The
gain shift becomes a linear function of input source resis-
tance independent of input capacitance, see Figures 21
and 22. The equivalent input impedance is 16.6MΩ. This
results in ±150nA of input dynamic current at the extreme
values of V
IN
(V
IN
= 0V and V
IN
= V
REF
, when V
REF
= 5V).
This corresponds to a 0.3ppm shift in offset and full-scale
readings for every 10Ω of input source resistance.
APPLICATIO S I FOR ATIO
WUUU
Figure 18. An RC Network at V
IN
Figure 19. Offset vs R
SOURCE
(Small C)
Figure 17. Offset/Full-Scale Shift
ZS
SET
TUE
V
IN
24212 F17
FS
SET
C
IN
24212 F17
INTPUT
SIGNAL
SOURCE
R
SOURCE
V
IN
LTC2421/
LTC2422
C
PAR
≅20pF
R
SOURCE
(Ω)
1
OFFSET ERROR (ppm)
30
40
50
10k
24212 F19
20
10
0
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
= 0V
T
A
= 25°C
C
IN
= 100pF
C
IN
= 1000pF
C
IN
= 0pF
C
IN
= 0.01µF
Figure 20. Offset vs R
SOURCE
(Large C)
Figure 21. Full-Scale Error vs R
SOURCE
(Large C)
R
SOURCE
(Ω)
0
25
30
35
600 800
24212 F20
20
15
200 400 1000
10
5
0
OFFSET ERROR (ppm)
C
IN
= 22µF
C
IN
= 10µF
C
IN
= 1µF
C
IN
= 0.1µF
C
IN
= 0.01µF
C
IN
= 0.001µF
V
CC
= 5V
V
REF
= 5V
V
IN
= 0V
T
A
= 25°C
R
SOURCE
(Ω)
0
FULL-SCALE ERROR (ppm)
–20
–15
–10
600
1000
24212 F21
–25
–30
–35
200 400 800
–5
0
5
C
IN
= 22µF
C
IN
= 10µF
C
IN
= 1µF
C
IN
= 0.1µF
C
IN
= 0.01µF
C
IN
= 0.001µF
V
CC
= 5V
V
REF
= 5V
V
IN
= 0V
T
A
= 25°C