Datasheet

LTC2498
31
2498ff
applications inForMation
The SINC
4
digital filter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
), see Figures 19
and 20. The modulator sampling frequency is f
S
=
15,360Hz while operating with its internal oscillator and
f
S
= F
EOSC
/20 when operating with an external oscillator
of frequency F
EOSC
.
When using the internal oscillator, the LTC2498 is designed
to reject line frequencies. As shown in Figure 21, rejec-
tion nulls occur at multiples of frequency f
N
, where f
N
is
determined by the input control bits FA and FB (f
N
= 50Hz
or 60Hz or 55Hz for simultaneous rejection). Multiples
of the modulator sampling rate (f
S
= f
N
• 256) only reject
Figure 16. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 17. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 18. INL vs Differential Input Voltage and Reference
Source Resistance for C
REF
> 1µF
Figure 19. Input Normal Mode Rejection, Internal Oscillator
and 50Hz Rejection Mode
Figure 20. Input Normal Mode Rejection, Internal Oscillator
and 60Hz Rejection Mode
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2498 F16
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
2498 F17
–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
V
IN
/V
REF
(V)
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2498 F18
–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
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
S
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S
11f
S
12f
S
INPUT NORMAL MODE REJECTION (dB)
2498 F19
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 f
S
INPUT NORMAL MODE REJECTION (dB)
2498 F20
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2f
S
3f
S
4f
S
5f
S
6f
S
7f
S
8f
S
9f
S
10f
S