Datasheet

LTC2486
28
2486fd
APPLICATIONS INFORMATION
and reference bypass capacitance) linearity and gain errors
are introduced.
For relatively small values of external reference capacitance
(C
REF
< 1nF), the voltage on the sampling capacitor settles
for reference impedances of many kΩ (if C
REF
= 100pF up
to 10kΩ will not degrade the performance) (see Figures
13 and 14).
In cases where large bypass capacitors are required on
the reference inputs (C
REF
> 0.01μF) full-scale and linear-
ity 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
common mode voltages are different, the errors increase.
Figure 13. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
Figure 14. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2486 F13
30
10
40
60
80
20
0
–10
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
REF
= 0.01μF
C
REF
= 0.001μF
C
REF
= 100pF
C
REF
= 0pF
R
SOURCE
(Ω)
0
–FS ERROR (ppm)
–30
–10
10
10k
2486 F14
–50
–70
–40
–20
0
–60
–80
–90
10
100
1k
100k
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
REF
= 0.01μF
C
REF
= 0.001μF
C
REF
= 100pF
C
REF
= 0pF
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
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2486 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
2486 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 17. INL vs Differential Input Voltage and Reference
Source Resistance for C
REF
> 1μF
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2486 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