LTC2498
31
2498ff
applications inForMation
The SINC4 digital filter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (fS), see Figures 19
and 20. The modulator sampling frequency is fS =
15,360Hz while operating with its internal oscillator and
fS = FEOSC/20 when operating with an external oscillator
of frequency FEOSC.
Whenusingtheinternaloscillator,theLTC2498isdesigned
to reject line frequencies. As shown in Figure 21, rejec-
tion nulls occur at multiples of frequency fN, where fN is
determined by the input control bits FA and FB (fN = 50Hz
or 60Hz or 55Hz for simultaneous rejection). Multiples
of the modulator sampling rate (fS = fN 256) only reject
Figure 16. +FS Error vs RSOURCE at VREF (Large CREF)
Figure 17. –FS Error vs RSOURCE at VREF (Large CREF)
Figure 18. INL vs Differential Input Voltage and Reference
Source Resistance for CREF > 1F
Figure 19. Input Normal Mode Rejection, Internal Oscillator
and 50Hz Rejection Mode
Figure 20. Input Normal Mode Rejection, Internal Oscillator
and 60Hz Rejection Mode
RSOURCE ()
0
+FS
ERROR
(ppm) 300
400
500
800
2498 F16
200
100
0
200
400
600
1000
VCC = 5V
VREF = 5V
VIN+ = 3.75V
VIN– = 1.25V
fO = GND
TA = 25°C
CREF = 1F, 10F
CREF = 0.1F
CREF = 0.01F
RSOURCE ()
0
–FS
ERROR
(ppm) –200
–100
0
800
2498 F17
–300
–400
–500
200
400
600
1000
VCC = 5V
VREF = 5V
VIN+ = 1.25V
VIN– = 3.75V
fO = GND
TA = 25°C
CREF = 1F, 10F
CREF = 0.1F
CREF = 0.01F
VIN/VREF (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
VCC = 5V
VREF = 5V
VIN(CM) = 2.5V
TA = 25°C
CREF = 10F
R = 1k
R = 100
R = 500
DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS10fS11fS12fS
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
fS
INPUT
NORMAL
MODE
REJECTION
(dB)
2498 F20
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS