Datasheet

24
LTC2400
V
REF
= 5V). This corresponds to a 0.3ppm shift in offset
and full-scale readings for every 1Ω of input source
resistance.
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.
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 22. 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 23.
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 24. 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 25.
APPLICATIONS INFORMATION
WUU
U
RESISTANCE AT V
REF
(Ω)
0
0
FULL-SCALE ERROR (ppm)
100
200
300
400
500
600
200 400 600 800
2400 F22
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
Figure 22. Full-Scale Error vs R
VREF
(Large C)
RESISTANCE AT V
REF
(Ω)
1
30
40
50
1k
2400 F23
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
Figure 23. Full-Scale Error vs R
VREF
(Small C)
RESISTANCE AT V
REF
(Ω)
1
–10
INL ERROR (ppm)
0
10
20
30
40
50
10 100 1k 10k
2400 F24
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 24. INL Error vs R
VREF
(Small C)
R
SOURCE
(Ω)
0
–300
FULL-SCALE ERROR (ppm)
–250
–200
–150
–100
–50
0
200 400 600 800
2400 F21
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
Figure 21. Full-Scale Error vs R
SOURCE
(Large C)