Datasheet

24
LTC2420
APPLICATIO S I FOR ATIO
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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
21 and 22. The equivalent input impedance is 16.6MΩ.
This results in ±150nA 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 10Ω 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.
R
SOURCE
(Ω)
1
OFFSET ERROR (ppm)
30
40
50
10k
2420 F19
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
Figure 19. Offset vs R
SOURCE
(Small C)
Figure 20. Full-Scale Error vs R
SOURCE
(Small C)
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 0.03ppm/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
R
SOURCE
(Ω)
0
25
30
35
600 800
2420 F21
20
15
200 400 1000
10
5
0
OFFSET ERROR (ppm)
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
FULL-SCALE ERROR (ppm)
–20
–15
–10
600
1000
2420 F22
–25
–30
–35
200 400 800
–5
0
5
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
Figure 21. Offset vs R
SOURCE
(Large C)
Figure 22. Full-Scale Error vs R
SOURCE
(Large C)
R
SOURCE
(Ω)
1
–50
FULL-SCALE ERROR (ppm)
–40
–30
–20
–10
0
10
10 100 1k 10k
2420 F20
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