ltc2428 Linear Technology Corporation, ltc2428 Datasheet - Page 26

• Manufacturer Part Number: ltc2428
• Description: 4-/8-channel 20-bit ?power No Latency Delta-sigma Adc
• Manufacturer: Linear Technology Corporation

APPLICATIONS
LTC2424/LTC2428
Input Current (V
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
tance at V
If the total capacitance at V
(< 0.01 F), 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
of input source resistance.
For large input capacitor values (C
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 16.6M .
26
TUE
0
SOURCE
SIGNAL
INTPUT
IN
V
REF
Figure 20. An RC Network at CH0 to CH7
(C
= FS
Figure 19. Offset/Full-Scale Shift
IN
SET
IN
< 0.01 F).
R
)
–ZS
SOURCE
U
SET
IN
(C
INFORMATION
U
V
IN
REF
V
C
IN
IN
> 0.01 F) and small capaci-
IN
/2
(see Figure 20) is small
IN
< 0.01 F) as a function
IN
C
W
PAR
20pF
> 0.01 F), the input
CH0 TO
CH7
LTC2424/
LTC2428
24248 F20
U
24248 F19
V
REF
This results in 150nA of input dynamic current at the
extreme values of V
V
and full-scale readings for every 10
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 75 on-resistance of the multiplexer
switch is nonlinear with input voltage. If the capacitance
at node MUXOUT/ADCIN is less than 0.01 F, the linearity
is not degraded. On the other hand, excessive capaci-
tance (>0.01 F) results in incomplete settling as a func-
tion of the multiplexer on-resistance. Hence, the
REF
= 5V). This corresponds to a 0.3ppm shift in offset
Figure 22. Full-Scale Error vs R
–10
–20
–30
–40
–50
10
50
40
20
10
30
Figure 21. Offset vs R
0
0
1
1
V
V
V
T
V
V
V
T
A
CC
REF
IN
A
CC
REF
IN
= 25 C
= 25 C
= 5V
= 0V
= 5V
= 5V
= 5V
= 5V
10
10
C
IN
C
C
IN
C
IN
IN
C
C
= 1000pF
IN
IN
C
= 1000pF
IN
= 100pF
(V
C
IN
= 100pF
100
= 0.01 F
R
100
R
= 0pF
IN
SOURCE
IN
= 0pF
SOURCE
= 0.01 F
= 0V and V
( )
SOURCE
1k
( )
1k
SOURCE
10k
10k
(Small C)
24248 F22
24248 F21
IN
of input source
(Small C)
100k
100k
= V
REF
, when