Datasheet

LTC2492
29
2492fd
APPLICATIONS INFORMATION
In cases where large bypass capacitors are required on
the reference inputs (C
REF
> 0.01μF) full-scale and linearity
errors are proportional to the value of the reference
resistance. Every ohm of reference resistance produces a
full-scale error of approximately 0.5ppm (while operating
in simultaneous 50Hz/60Hz mode) (see Figures 15 and 16).
If the input common mode voltage is equal to the reference
common mode voltage, a linearity error of approximately
0.67ppm per 100Ω of reference resistance results (see
Figure 17). In applications where the input and reference
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2492 F15
200
100
0
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 1μF, 10μF
C
REF
= 0.1μF
C
REF
= 0.01μF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–200
–100
0
800
2492 F16
–300
–400
–500
200
400
600
1000
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 1μF, 10μF
C
REF
= 0.1μF
C
REF
= 0.01μF
Figure 15. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 16. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2492 F17
–2
–6
0
4
8
–4
–8
–10
–0.3
–0.1
0.1
0.5
V
CC
= 5V
V
REF
= 5V
V
IN(CM)
= 2.5V
T
A
= 25°C
C
REF
= 10μF
R = 1k
R = 100Ω
R = 500Ω
Figure 17. INL vs Differential Input Voltage and Reference
Source Resistance for C
REF
> 1μF
common mode voltages are different, the errors increase.
A 1V difference in between common mode input and
common mode reference results in a 6.7ppm INL error
for every 100Ω of reference resistance.
In addition to the reference sampling charge, the reference
ESD protection diodes have a temperature dependent leak-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small gain error. A 100Ω reference
resistance will create a 0.5μV full-scale error.
Normal Mode Rejection and Anti-aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversample ratio, the LTC2492 signifi cantly
simplifi es anti-aliasing fi lter requirements. Additionally,
the input current cancellation feature allows external low
pass fi ltering without degrading the DC performance of
the device.