Datasheet
23
LTC2400
APPLICATIONS INFORMATION
WUU
U
C
IN
2400 F17
INTPUT
SIGNAL
SOURCE
R
SOURCE
V
IN
LTC2400
C
PAR
≅20pF
Figure 17. An RC Network at V
IN
R
SOURCE
(Ω)
1
OFFSET ERROR (ppm)
30
40
50
10k
2400 F18
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 18. Offset vs R
SOURCE
(Small C)
R
SOURCE
(Ω)
1
FULL-SCALE ERROR (ppm)
–20
–10
0
10k
2400 F19
–30
–40
–50
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
IN
= 0pF
C
IN
= 100pF
C
IN
= 1000pF
C
IN
= 0.01µF
Figure 19. Full-Scale Error vs R
SOURCE
(Small C)
R
SOURCE
(Ω)
0
OFFSET ERROR (ppm)
100
200
300
50
150
250
200 400 600 800
2400 F20
10001000 300 500 700 900
V
CC
= 5V
V
REF
= 5V
V
IN
= 0V
T
A
= 25°C
C
IN
= 1µF
C
IN
= 10µF
C
IN
= 0.1µF
C
IN
= 0.01µF
Figure 20. Offset vs R
SOURCE
(Large C)
0
TUE
V
REF
/2
V
IN
2400 F16
V
REF
Figure 16. Offset/Full-Scale Shift
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 16. 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 17) is small
(<0.01µF), relatively large external source resistances (up
to 20k for 20pF parasitic capacitance) can be tolerated
without any offset/full-scale error. Figures 18 and 19 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
resistance independent of input capacitance, see Figures
20 and 21. The equivalent input impedance is 1.66MΩ.
This results in ±1.5µA of input dynamic current at the
extreme values of V
IN
(V
IN
= 0V and V
IN
= V
REF
, when