Datasheet

25
LTC2420
APPLICATIO S I FOR ATIO
WUU
U
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
0.015ppm/Ω independent of capacitance at V
REF
, see
Figure 26.
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.
ANTIALIASING
One of the advantages delta-sigma ADCs offer over con-
ventional ADCs is on-chip digital filtering. Combined with
a large oversampling ratio, the LTC2420 significantly
simplifies antialiasing filter requirements.
The digital filter provides very high rejection except at
integer multiples of the modulator sampling frequency
(f
S
), see Figure 27. The modulator sampling frequency is
256 • F
O
, where F
O
is the notch frequency (typically 50Hz
or 60Hz). The bandwidth of signals not rejected by the
digital filter is narrow (≈0.2%) compared to the bandwidth
of the frequencies rejected.
RESISTANCE AT V
REF
(Ω)
0
40
50
60
600 800
2420 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
2420 F24
200
100
10 100 100k
0
–100
–200
VOLTAGE
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
RESISTANCE AT V
REF
(Ω)
1
30
40
50
1k 10k
2420 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
RESISTANCE AT V
REF
(Ω)
0
6
8
10
600 800
2420 F26
4
2
–2
–4
–6
–8
–10
200 400 1000
0
INL 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
T
A
= 25°C
Figure 23. Full-Scale Error vs R
VREF
(Large C)
Figure 24. Full-Scale Error vs R
VREF
(Small C)
Figure 25. INL Error vs R
VREF
(Small C)
Figure 26. INL Error vs R
VREF
(Large C)