Datasheet

25
LTC2404/LTC2408
APPLICATIONS INFORMATION
WUU
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Reference Current (V
REF
)
Similar to the analog input, the reference input has a
dynamic 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
error shift is 0.3ppm/Ω of external reference resistance
independent of the capacitance at V
REF
, see Figure 19. If
the capacitance tied to V
REF
is small (C
VREF
< 0.01µF), an
input resistance of up to 20k (20pF parasitic capacitance
at V
REF
) may be tolerated, see Figure 20.
Unlike the analog input, the integral nonlinearity of the
device can be degraded with excessive external RC time
RESISTANCE AT V
REF
(Ω)
1
–10
INL ERROR (ppm)
0
10
20
30
40
50
10 100 1k 10k
24048 F21
100k
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
C
VREF
= 0pF
C
VREF
= 100pF
C
VREF
= 1000pF
C
VREF
= 0.01µF
Figure 22. INL Error vs R
VREF
(Large C)
Figure 21. INL Error vs R
VREF
(Small C)
RESISTANCE AT V
REF
(Ω)
0
–20
INL ERROR (ppm)
0
40
60
80
400
800
1000
160
24048 F22
20
200 600
100
120
140
C
VREF
= 0.01µF
C
VREF
= 0.1µF
C
VREF
= 1µF
C
VREF
= 10µF
V
CC
= 5V
V
REF
= 5V
T
A
= 25°C
Figure 20. Full-Scale Error vs R
VREF
(Small C)
Figure 19. Full-Scale Error vs R
VREF
(Large C)
RESISTANCE AT V
REF
(Ω)
0
0
FULL-SCALE ERROR (ppm)
100
200
300
400
500
600
200 400 600 800
24048 F19
1000
C
VREF
= 10µF
C
VREF
= 0.01µF
C
VREF
= 0.1µF
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
VREF
= 1µF
RESISTANCE AT V
REF
(Ω)
1
30
40
50
1k
24048 F20
20
10
10 100 100k10k
0
–10
–20
FULL-SCALE ERROR (ppm)
V
CC
= 5V
V
REF
= 5V
V
IN
= 5V
T
A
= 25°C
C
VREF
= 100pF
C
VREF
= 1000pF
C
VREF
= 0.01µF
C
VREF
= 0pF
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 21. If the external capacitance is large
(C
VREF
> 0.01µF), the linearity will be degraded by
0.15ppm/Ω independent of capacitance at V
REF
, see
Figure 22.
In addition to the dynamic reference current, the V
REF
ESD
protection diodes have a temperature dependent leakage
current. This leakage current, nominally 1nA (±10nA max),
results in a fixed full-scale shift of 10µV for a 10k source
resistance.