Datasheet

LTC2484
28
2484fd
APPLICATIONS INFORMATION
Reference Current
In a similar fashion, the LTC2484 samples the differential
reference pins V
REF
+
and GND transferring small amount
of charge to and from the external driving circuits thus
producing a dynamic reference current. This current does
not change the converter offset, but it may degrade the
gain and INL performance. The effect of this current can
be analyzed in two distinct situations.
For relatively small values of the external reference capaci-
tors (C
REF
< 1nF), the voltage on the sampling capacitor
settles almost completely and relatively large values for
the source impedance result in only small errors. Such
values for C
REF
will deteriorate the converter offset and
gain performance without signifi cant benefi ts of reference
fi ltering and the user is advised to avoid them.
Larger values of reference capacitors (C
REF
> 1nF) may be
required as reference fi lters in certain confi gurations. Such
capacitors will average the reference sampling charge and
the external source resistance will see a quasi constant
reference differential impedance.
In the following discussion, it is assumed the input and
reference common mode are the same. Using internal
oscillator for 60Hz mode, the typical differential reference
resistance is 1MΩ which generates a full-scale (V
REF
/2) gain
error of 0.51ppm for each ohm of source resistance driving
the V
REF
pin. For 50Hz/60Hz mode, the related difference
resistance is 1.1MΩ and the resulting full-scale error is
0.46ppm for each ohm of source resistance driving the
V
REF
pin. For 50Hz mode, the related difference resistance
is 1.2MΩ and the resulting full-scale error is 0.42ppm for
each ohm of source resistance driving the V
REF
pin. When
f
O
is driven by an external oscillator with a frequency f
EOSC
(external conversion clock operation), the typical differen-
tial reference resistance is 0.30 • 10
12
/f
EOSC
Ω and each
ohm of source resistance driving the V
REF
pin will result
C
IN
2484 F12
V
INCM
+ 0.5V
IN
R
SOURCE
IN
+
LTC2484
C
PAR
20pF
C
IN
V
INCM
– 0.5V
IN
R
SOURCE
IN
–
C
PAR
20pF
Figure 12. An RC Network at IN
+
and IN
–
R
SOURCE
(Ω)
1
+FS ERROR (ppm)
–20
0
20
1k
100k
2484 F13
–40
–60
–80
10 100 10k
40
60
80
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 3.75V
V
IN
–
= 1.25V
f
O
= GND
T
A
= 25°C
C
IN
= 0pF
C
IN
= 100pF
C
IN
= 1nF, 0.1μF, 1μF
Figure 13. +FS Error vs R
SOURCE
at IN
+
or IN
–
R
SOURCE
(Ω)
1
–FS ERROR (ppm)
–20
0
20
1k
100k
2484 F14
–40
–60
–80
10 100 10k
40
60
80
V
CC
= 5V
V
REF
= 5V
V
IN
+
= 1.25V
V
IN
–
= 3.75V
f
O
= GND
T
A
= 25°C
C
IN
= 0pF
C
IN
= 100pF
C
IN
= 1nF, 0.1μF, 1μF
Figure 14. –FS Error vs R
SOURCE
at IN
+
or IN
–