
26
LTC2424/LTC2428
APPLICATIONS INFORMATION
WUU
U
Input Current (V
IN
)
If complete settling occurs on the input, conversion re-
sults will be unaffected 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 19. 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 20) 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 21 and 22 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
23 and 24. 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.
While large capacitance applied to one of the multiplexer
channel inputs may result in offset/full-scale shifts, large
capacitance applied to the MUXOUT/ADCIN results in
linearity errors. The 75Ω on-resistance of the multiplexer
switch is nonlinear with input voltage. If the capacitance
at node MUXOUT/ADCIN is less than 0.01µF, the linearity
is not degraded. On the other hand, excessive capaci-
tance (>0.01µF) results in incomplete settling as a func-
tion of the multiplexer on-resistance. Hence, the
R
SOURCE
(Ω)
1
OFFSET ERROR (ppm)
30
40
50
10k
24248 F21
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 21. Offset vs R
SOURCE
(Small C)
R
SOURCE
(Ω)
1
–50
FULL-SCALE ERROR (ppm)
–40
–30
–20
–10
0
10
10 100 1k 10k
24248 F22
100k
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
IN
= 0.01µF
C
IN
= 100pF
C
IN
= 1000pF
C
IN
= 0pF
Figure 22. Full-Scale Error vs R
SOURCE
(Small C)
Figure 19. Offset/Full-Scale Shift
Figure 20. An RC Network at CH0 to CH7
0
TUE
V
REF
= FS
SET
–ZS
SET
V
REF
/2
V
IN
24248 F19
V
REF
C
IN
24248 F20
INTPUT
SIGNAL
SOURCE
R
SOURCE
CH0 TO
CH7
LTC2424/
LTC2428
C
PAR
≅20pF
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