Datasheet
24
LTC2404/LTC2408
APPLICATIONS INFORMATION
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Figure 15. Offset vs R
SOURCE
(Small C)
Figure 16. Full-Scale Error vs R
SOURCE
(Small C)
R
SOURCE
(Ω)
1
OFFSET ERROR (ppm)
30
40
50
10k
24048 F15
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
R
SOURCE
(Ω)
1
FULL-SCALE ERROR (ppm)
–20
–10
0
10k
24048 F16
–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 18. Full-Scale Error vs R
SOURCE
(Large C)
Figure 17. Offset vs R
SOURCE
(Large C)
R
SOURCE
(Ω)
0
OFFSET ERROR (ppm)
100
200
300
50
150
250
200 400 600 800
24048 F17
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
R
SOURCE
(Ω)
0
–300
FULL-SCALE ERROR (ppm)
–250
–200
–150
–100
–50
0
200 400 600 800
24048 F18
1000
C
IN
= 0.01µF
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
IN
= 0.1µF
C
IN
= 1µF
C
IN
= 10µF
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
17 and 18. 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
V
REF
= 5V). This corresponds to a 0.3ppm shift in offset
and full-scale readings for every 1Ω 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 capacitance
(>0.01µF) results in incomplete settling as a function of
the multiplexer on-resistance. Hence, the nonlinearity of
the multiplexer switch is seen in the overall transfer
characteristic.
In addition to the input current spikes, the input ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA
max), results in a fixed offset shift of 10µV for a 10k source
resistance.