Datasheet

LTC2487
24
2487fd
Figure 14. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 15. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2487 F14
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
2487 F15
–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
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
12 and 13)
).
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 operat-
ing in simultaneous 50Hz/60Hz mode (see Figures 14
and 15)
). 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 16). In applications where the input
and reference 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.
Figure 12. +FS Error vs R
SOURCE
at V
REF
(Small C
REF
) Figure 13. –FS Error vs R
SOURCE
at V
REF
(Small C
REF
)
APPLICATIONS INFORMATION
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2487 F12
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
2487 F13
–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