Datasheet

LTC2499
26
2499fd
op amp output enabling stable operation. The LTC6078
can also be biased at supply rails beyond those used by
the LTC2499. This allows the external sensor to swing rail-
to-rail (–0.3V to V
CC
+ 0.3V) without the need of external
level-shift circuitry.
Reference Current
Similar to the analog inputs, the LTC2499 samples the
differential reference pins (REF
+
and REF
–
) transferring
small amounts of charge to and from these pins, thus
producing a dynamic reference current. If incomplete set-
tling occurs (as a function the reference source resistance
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
> .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 15
and 16)). If the input common mode voltage is equal to
the reference common mode voltage, a linearity error of
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
)
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
)
applications inForMation
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
50
70
90
10k
2499 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
2499 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
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2499 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
2499 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