Datasheet

24
LTC2421/LTC2422
24212f
In addition to the input current spikes, the input ESD pro-
tection diodes have a temperature dependent leakage cur-
rent. This leakage current, nominally 1nA (±100nA 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 19 and 22. 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 dy-
namic input current. This current has negligible effect on
the offset. However, the reference current at V
IN
= V
REF
is
similar to the input current at full-scale. For large values of
reference capacitance (C
VREF
> 0.01µF), the full-scale er-
ror shift is 0.03ppm/Ω of external reference resistance
independent of the capacitance at V
REF
, see Figure 23. If
the capacitance tied to V
REF
is small (C
VREF
< 0.01µF), an
input resistance of up to 80k (20pF parasitic capacitance
at V
REF
) may be tolerated, see Figure 24.
Unlike the analog input, the integral nonlinearity of the
device can be degraded with excessive external RC time
constants tied to the reference input. If the capacitance at
node V
REF
is small (C
VREF
< 0.01µF), the reference input
can tolerate large external resistances without reduction
in INL, see Figure 25. If the external capacitance is large
(C
VREF
> 0.01µF), the linearity will be degraded by
APPLICATIO S I FOR ATIO
WUUU
R
SOURCE
(Ω)
1
–50
FULL-SCALE ERROR (ppm)
–40
–30
–20
–10
0
10
10 100 1k 10k
24212 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 23. Full-Scale Error vs R
VREF
(Large C)
Figure 24. Full-Scale Error vs R
VREF
(Small C)
RESISTANCE AT V
REF
(Ω)
0
40
50
60
600 800
24212 F23
30
20
200 400 1000
10
0
–10
FULL-SCALE ERROR (ppm)
C
VREF
= 22µF
C
VREF
= 10µF
C
VREF
= 1µF
C
VREF
= 0.1µF
C
VREF
= 0.01µF
C
VREF
= 0.001µF
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
RESISTANCE AT V
REF
(Ω)
1
300
400
500
1k 10k
24212 F24
200
100
10 100 100k
0
–100
–200
FULL-SCALE ERROR (ppm)
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
VREF
= 0.01µF
C
VREF
= 100pF
C
VREF
= 1000pF
C
VREF
= 0pF
Figure 25. INL Error vs R
VREF
(Small C)
RESISTANCE AT V
REF
(Ω)
1
30
40
50
1k 10k
24212 F25
20
10
10 100 100k
0
–10
–20
INL ERROR (ppm)
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
C
VREF
= 0.01µF
C
VREF
= 1000pF
C
VREF
= 0pF
C
VREF
= 100pF