Datasheet
24
LTC2401/LTC2402
resistance independent of input capacitance, see Figures
20 and 21. The equivalent input impedance is 6.25MΩ.
This results in ±400µA 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 0.8ppm shift in offset
and full-scale readings for every 10Ω of input source
resistance.
C
IN
24012 F17
INTPUT
SIGNAL
SOURCE
R
SOURCE
V
IN
LTC2401/
LTC2402
C
PAR
≅20pF
APPLICATIO S I FOR ATIO
WUUU
Figure 17. An RC Network at V
IN
Figure 18. Offset vs R
SOURCE
(Small C)
Figure 19. Offset vs R
SOURCE
(Large C)
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.
The effect of input leakage current is evident for C
IN
= 0 in
Figures 18 and 21. A leakage current of 3nA results in a
150µV (30ppm) error for a 50k source resistance. As
R
SOURCE
gets larger, the switched capacitor input current
begins to dominate.
Reference Current (V
REF
)
Similar to the analog input, the reference input has a
dynamic input current. This current has negligible effect
Figure 20. Full-Scale Error vs R
SOURCE
(Large C)
Figure 21. Full-Scale Error vs R
SOURCE
(Small C)
R
SOURCE
(Ω)
1
–10
OFFSET ERROR (ppm)
0
10
20
30
40
50
10 100 1k 10k
24012 F18
100k
V
CC
= 5V
V
REF
= 5V
V
IN
= 0V
T
A
= 25°C
C
IN
= 100pF
C
IN
= 0.01µF
C
IN
= 1000pF
C
IN
= NO CAP
R
SOURCE
(Ω)
0
OFFSET ERROR (ppm)
40
60
800
24012 F19
20
0
200
400
600
1000
80
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
–80
FULL-SCALE ERROR (ppm)
–70
–50
–40
–30
400
800
1000
10
24012 F20
–60
200 600
–20
–10
0
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
= 5V
T
A
= 25°C
R
SOURCE
(Ω)
0
FULL-SCALE (ppm)
–10
10
10k
24012 F21
–30
–50
10
100
1k
100k
30
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
IN
= NO CAP
C
IN
= 0.01µF
C
IN
= 1000pF
C
IN
= 100pF