Datasheet

LTC2488
24
2488fa
APPLICATIONS INFORMATION
Normal Mode Rejection and Anti-aliasing
One of the advantages delta-sigma ADCs offer over
conventional ADCs is on-chip digital fi ltering. Combined
with a large oversample ratio, the LTC2488 signifi cantly
simplifi es anti-aliasing lter requirements. Additionally,
the input current cancellation feature allows external low
pass fi ltering without degrading the DC performance of
the device.
The SINC
4
digital fi lter provides excellent normal mode
rejection at all frequencies except DC and integer multiples
of the modulator sampling frequency (f
S
). 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 LTC2488 is designed
to reject line frequencies. As shown in Figure 16, rejection
nulls occur at multiples of frequency f
N
, where f
N
= 55Hz
for simultaneous 50Hz/60Hz rejection. Multiples of the
modulator sampling rate (f
S
= f
N
• 256) only reject noise
to 15dB (see Figure 17), if noise sources are present at
these frequencies anti-aliasing will reduce their effects.
The user can expect to achieve this level of performance
using the internal oscillator, as shown in Figure 18.
Measured values of normal mode rejection are shown
superimposed over the theoretical values.
Traditional high order delta-sigma modulators suffer
from potential instabilities at large input signal levels. The
R
SOURCE
(Ω)
0
+FS ERROR (ppm)
300
400
500
800
2488 F13
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
2488 F14
–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
Figure 13. +FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
Figure 14. –FS Error vs R
SOURCE
at V
REF
(Large C
REF
)
V
IN
/V
REF
–0.5
INL (ppm OF V
REF
)
2
6
10
0.3
2488 F15
–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
Figure 15. INL vs Differential Input Voltage and Reference
Source Resistance for C
REF
> 1μF
INPUT SIGNAL FREQUENCY (Hz)
INPUT NORMAL MODE REJECTION (dB)
2488 F16
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
N
0 2f
N
3f
N
4f
N
5f
N
6f
N
7f
N
8f
N
f
N
= f
EOSC/5120
Figure 16. Input Normal Mode Rejection at DC